We present a new heterogeneous biocatalyst based on the grafting of Bilirubin Oxidase from Bacillus pumilus into macrocellular Si(HIPE) materials dedicated to water treatment. Due to the host intrinsic high porosity and monolithic character, on-line catalytic process is reached. We thus used this biocatalyst toward uni-axial flux decolorization of Congo Red and Remazol Brilliant Blue (RBBR) at two different pH (4 and 8.2), both in presence or absence of redox mediator. In absence of redox mediators, 40% decolorization efficiency was reached within 24 h at pH 4 for Congo Red and 80% for RBBR at pH 8.2 in 24 h. In presence of 10 mu M ABTS, it respectively attained 100% efficiency after 2h and 12h. We have also demonstrated that non-toxic species were generated upon dyes decolorization. These results show that unlike laccases, this new biocatalyst exhibits excellent decolorization properties over a wide range of pH. Beyond, this enzymatic-based heterogeneous catalyst can be reused during two months being simply stored at room temperature while maintaining its decolorization efficiency. This study shows that this biocatalyst is a promising and robust candidate toward wastewater treatments, both in acidic and alkaline conditions where in the latter efficient decolorization strategies were still missing.

VL - 120 UR - https://linkinghub.elsevier.com/retrieve/pii/S0141022918303466 JO - Enzyme and Microbial Technology ER - TY - JOUR T1 - Bulk Photodriven CO2 Conversion through TiO2@Si(HIPE) Monolithic Macrocellular Foams JF - Advanced Functional Materials Y1 - 2019 A1 - Bernadet, Sophie A1 - Tavernier, Eugénie A1 - Ta, Duc-Minh A1 - Vallée, Renaud A. L. A1 - Ravaine, Serge A1 - Fécant, Antoine A1 - Backov, Renal AB -Operating photo‐induced reactions exclusively on catalyst surfaces while not exploiting the full catalyst volume generates a major footprint penalty for the photocatalytic reactor and leads to an inefficient use of the catalytic material. Photonic investigations clearly show that the solid foams have a strongly multidiffusive character, with photons being significantly trapped within the sample cores while addressing a photon mean free path *l*_{t} = 20.1 ± 1.3 µm. This 3D process both greatly limits back‐reactions and promotes outstanding selectivity toward methane (around 80%) generation, and even ethane (around 18%) through C‐C coupling reaction, with residual carbon monoxide and dihydrogen contents (around 2%). Silica–titania TiO_{2}@Si(HIPE) self‐standing macrocellular catalysts lead to optimal efficient thicknesses up to 20 times those of powders, thereby enhancing the way for real 3D‐photodriven catalytic processes above the millimeter scale and up to a 6 mm thickness. A rather simple Langmuir–Hinshelwood based kinetic model is proposed which highlights the strong dependence of photocatalytic reaction rates on light scattering and the crucial role on oxidation back‐reactions. In addition, a strong correlation between light attenuation coefficient and photon mean free path and median pore aperture diameter is demonstrated, offering thus a tool for photocatalytic behavior prediction.

The mechanical properties of the cell depend crucially on the tension of its cytoskeleton, a biopolymer network that is put under stress by active motor proteins. While the fibrous nature of the network is known to strongly affect the transmission of these forces to the cellular scale, our understanding of this process remains incomplete. Here we investigate the transmission of forces through the network at the individual filament level, and show that active forces can be geometrically amplified as a transverse motor-generated force "plucks'' the fiber and induces a nonlinear tension. In stiff and densely connected networks, this tension results in large network-wide tensile stresses that far exceed the expectation drawn from a linear elastic theory. This amplification mechanism competes with a recently characterized network-level amplification due to fiber buckling, suggesting that that fiber networks provide several distinct pathways for living systems to amplify their molecular forces.

VL - 15 UR - http://xlink.rsc.org/?DOI=C8SM00979Ahttp://pubs.rsc.org/en/content/articlepdf/2019/SM/C8SM00979A IS - 7 JO - Soft Matter ER - TY - JOUR T1 - Grains3D, a flexible DEM approach for particles of arbitrary convex shape—Part III: extension to non-convex particles modelled as glued convex particles JF - Computational Particle Mechanics Y1 - 2019 A1 - Rakotonirina, Andriarimina Daniel A1 - Jean-Yves Delenne A1 - Farhang Radjaï A1 - Wachs, Anthony KW - Granular flow; Discrete element method; Non-convex shape; GJK algorithm; Glued convex AB -Large-scale numerical simulation using the discrete element method (DEM) contributes to improving our understanding of granular flow dynamics involved in many industrial processes and geophysical flows. In industry, it leads to an enhanced design and an overall optimization of the corresponding equipment and process. Most of the DEM simulations in the literature have been performed using spherical particles. A limited number of studies dealt with non-spherical particles, even less with non-convex particles. Even convex bodies do not always represent the real shape of certain particles. In fact, more complex-shaped particles are found in many industrial applications, for example, catalytic pellets in chemical reactors or crushed glass debris in recycling processes. In Grains3D-Part I (Wachs et al. in Powder Technol 224:374-389, 2012), we addressed the problem of convex shape in granular simulations, while in Grains3D-Part II (Rakotonirina and Wachs in Powder Technol 324:18-35, 2018), we suggested a simple though efficient parallel strategy to compute systems with up to a few hundreds of millions of particles. The aim of the present study is to extend even further the modelling capabilities of Grains3D towards non-convex shapes, as a tool to examine the flow dynamics of granular media made of non-convex particles. Our strategy is based on decomposing a non-convex-shaped particle into a set of convex bodies, called elementary components. We call our method glued or clumped convex method, as an extension of the popular glued sphere method. Essentially, a non-convex particle is constructed as a cluster of convex particles, called elementary components. At the level of these elementary components of a glued convex particle, we employ the same contact detection strategy based on a Gilbert-Johnson-Keerthi algorithm and a linked-cell spatial sorting that accelerates the resolution of the contact, that we introduced in [39]. Our glued convex model is implemented as a new module of our code Grains3D and is therefore automatically fully parallel. We illustrate the new modelling capabilities of Grains3D in two test cases: (1) the filling of a container and (2) the flow dynamics in a rotating drum.

VL - 106 UR - http://link.springer.com/10.1007/s40571-018-0198-3 IS - 1 JO - Comp. Part. Mech. ER - TY - JOUR T1 - Investigation of mixed ionic/nonionic building blocks for the dual templating of macro-mesoporous silica JF - Journal of Colloid and Interface Science Y1 - 2019 A1 - Roucher, Armand A1 - Emo, Melanie A1 - Vibert, Francois A1 - Stebe, Marie-Jose A1 - Schmitt, Veronique A1 - Jonas, Florian A1 - Backov, Renal A1 - Blin, Jean-Luc AB -Traditional porous monoliths Si(HIPE) (High Internal Phase Emulsion), prepared from the Tetradecyltrimethylammonium Bromide (TTAB)/dodecane/water system, offer high specific surface area, mainly due to microporosity. Aside, mesoporous materials SBA-15, prepared from Pluronic P123, have a high specific surface area, but are obtained as powder, which limits their applications. Starting from the mixed TTAB-P123 surfactant, it is expected to tune the mesoporosity of Si(HIPE), while keeping their monolithic character.

The ternary TTAB/P123/water phase diagram was established by varying the weight ratio between these two surfactants. The micellar structure as well as the structural parameters of the liquid crystal domains were determined by SAXS (Small Angle X-ray Scattering). The effect of dodecane solubilization was also investigated and concentrated emulsions were formulated from the (P123/TTAB)/dodecane/ water systems. After this soft matter dedicated study, the acquired knowledge was transferred toward the hierarchical porous silica generations, where the sol-gel process is involved.

Mixing P123 with TTAB, macro-mesoporous monolithic silica with an enhanced contribution of the specific surface area due to mesoporosity can be prepared. The variation of the TTAB/P123 weight ratio allows controlling the porosity at the mesoscale. Moreover, the macroporosity can be tuned by changing the preparation method, by mixing either the two micellar solutions or directly the two surfactants prior the emulsification process. (C) 2018 Elsevier Inc. All rights reserved.

VL - 533 UR - https://linkinghub.elsevier.com/retrieve/pii/S0021979718309883 JO - Journal of Colloid and Interface Science ER - TY - JOUR T1 - Parallel implicit contact algorithm for soft particle systems JF - Computer Physics Communications Y1 - 2019 A1 - Saeid Nezamabadi A1 - Frank, Xavier A1 - Jean-Yves Delenne A1 - Julien Averseng A1 - Farhang Radjaï AB -This paper presents a numerical technique to model soft particle materials in which the particles can undergo large deformations. It combines an implicit finite strain formalism of the Material Point Method and the Contact Dynamics method. In this framework, the large deformations of individual particles as well as their collective interactions are treated consistently. In order to reduce the computational cost, this method is parallelised using the Message Passing Interface (MPI) strategy. Using this approach, we investigate the uniaxial compaction of 2D packings composed of particles governed by a Neo-Hookean material behaviour. We consider compressibility rates ranging from fully compressible to incompressible particles. The packing deformation mechanism is a combination of both particle rearrangements and large deformations, and leads to high packing fractions beyond the jamming state. We show that the packing strength declines when the particle compressibility decreases, and the packing can deform considerably. We also discuss the evolution of the connectivity of the particles and particle deformation distributions in the packing. (C) 2018 Elsevier B.V. All rights reserved.

VL - 237 UR - https://linkinghub.elsevier.com/retrieve/pii/S0010465518303904 JO - Computer Physics Communications ER - TY - JOUR T1 - Stress-dependent amplification of active forces in nonlinear elastic media JF - Soft Matter Y1 - 2019 A1 - Ronceray, Pierre A1 - Broedersz, Chase P. A1 - Lenz, Martin AB -The production of mechanical stresses in living organisms largely relies on localized, force-generating active units embedded in filamentous matrices. Numerical simulations of discrete fiber networks with fixed boundaries have shown that buckling in the matrix dramatically amplifies the resulting active stresses. Here we extend this result to a continuum elastic medium prone to buckling subjected to an arbitrary external stress, and derive analytical expressions for the active, nonlinear constitutive relations characterizing the full active medium. Inserting these relations into popular active gel descriptions of living tissues and the cytoskeleton will enable investigations into nonlinear regimes previously inaccessible due to the phenomenological nature of these theories.

VL - 15 UR - http://xlink.rsc.org/?DOI=C8SM00949J IS - 2 JO - Soft Matter ER - TY - JOUR T1 - Virus-templated Pt–Ni(OH)2 nanonetworks for enhanced electrocatalytic reduction of water JF - Nano Energy Y1 - 2019 A1 - Records, William C. A1 - Yoon, Youngmin A1 - Ohmura, Jacqueline F. A1 - Chanut, Nicolas A1 - Belcher, Angela M. AB -Clean hydrogen production via water electrolysis is incumbent upon the development of high-performing hydrogen evolution reaction electrocatalysts. Despite decades of commercial maturity, however, alkaline water electrolyzers continue to suffer from limitations in electrocatalytic activity and stability, even with noble metal catalysts. In recent years, combining platinum with oxophilic materials, such as metal hydroxides, has shown great promise for improving performance potentially by enabling stronger water dissociation at the surface of electrocatalysts. In this work, we leveraged the nanoscopic proportions and surface programmability of the filamentous M13 bacteriophage in the design, synthesis, and exceptional performance of 3D nanostructured biotemplated electrocatalysts for alkaline hydrogen evolution. We developed a facile synthesis method for phage-templated, Pt–Ni(OH)2 nanonetworks, relying on scalable techniques like electroless deposition. After optimization of the platinum content, our materials display –4.9 A mg–1Pt at −70 mV versus the reversible hydrogen electrode, the highest reported mass activity in 1 M KOH to date, and undergo minimal changes in overpotential under galvanostatic operation at −10 mA cm–2geo. Looking forward, the performance of these catalysts suggests that biotemplating nanostructures with M13 bacteriophage offers an interesting new route for developing high-performing electrocatalysts.

VL - 58 UR - https://linkinghub.elsevier.com/retrieve/pii/S2211285518309960 JO - Nano Energy ER - TY - JOUR T1 - Inertial shear flow of assemblies of frictionless polygons: Rheology and microstructure JF - The European Physical Journal E Y1 - 2018 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Jean-Noel Roux KW - CONTACT DYNAMICS METHOD; DENSE GRANULAR FLOWS; GEOMETRIC ORIGIN; BEHAVIOR; PACKING; MEDIA; SHAPE AB -Motivated by the understanding of shape effects in granular materials, we numerically investigate the macroscopic and microstructural properties of anisotropic dense assemblies of frictionless polydisperse rigid pentagons in shear flow, and compare them with similar systems of disks. Once subjected to large cumulative shear strains their rheology and microstructure are investigated in uniform steady states, depending on inertial number I, which ranges from the quasistatic limit (I similar to 10(-5)) to 0.2. In the quasistatic limit both systems are devoid of Reynolds dilatancy, i.e., flow at their random close packing density. Both macroscopic friction angle., an increasing function of I, and solid fraction., a decreasing function of I, are larger with pentagons than with disks at small I, but the differences decline for larger I and, remarkably, nearly vanish for I similar to 0.2. Under growing I, the depletion of contact networks is considerably slower with pentagons, in which increasingly anisotropic, but still well-connected force-transmitting structures are maintained throughout the studied range. Whereas contact anisotropy and force anisotropy contribute nearly equally to the shear strength in disk assemblies, the latter effect dominates with pentagons at small I, while the former takes over for I of the order of 10(-2). The size of clusters of grains in side-to-side contact, typically comprising more than 10 pentagons in the quasistatic limit, very gradually decreases for growing I.

VL - 4 UR - http://link.springer.com/10.1140/epje/i2018-11608-9 IS - 1 JO - Eur. Phys. J. E ER - TY - JOUR T1 - Mechanical strength of wet particle agglomerates JF - Mechanics Research Communications Y1 - 2018 A1 - Vo, Thanh-Trung A1 - Patrick Mutabaruka A1 - Saeid Nezamabadi A1 - Jean-Yves Delenne A1 - Izard, Edouard A1 - Roland Jean-Marc Pellenq A1 - Farhang Radjaï AB -Using particle dynamics simulations, we investigate the strength and microstructure of agglomerates of wet frictional particles subjected to axial compression. The numerical model accounts for the cohesive and viscous effects of the binding liquid up to a debonding distance with the liquid assumed to be distributed homogeneously inside the agglomerate. We show that wet agglomerates undergo plastic deformation due to the rearrangements of primary particles during compression. The compressive strength is thus characterized by the plastic threshold before the onset of failure by the irreversible loss of wet contacts between primary particles. We find that the agglomerate plastic threshold is proportional to the characteristic cohesive stress defined from the liquid-vapor surface tension and the mean diameter of primary particles, with a prefactor that is a nearly linear function of the debonding distance and increases with size span. We analyze the agglomerate microstructure and, considering only the cohesive capillary forces at all bonds between primary particles, we propose an expression of the plastic strength as a function of the texture parameters such as the wet coordination number and packing fraction. This expression is shown to be consistent with our simulations up to a multiplicative factor reflecting the distribution of the capillary bridges. (C) 2018 Published by Elsevier Ltd.

VL - 92 UR - https://www-sciencedirect-com.libproxy.mit.edu/science/article/pii/S0093641318301216 JO - Mechanics Research Communications ER - TY - JOUR T1 - A methodology to calibrate and to validate effective solid potentials of heterogeneous porous media from computed tomography scans and laboratory-measured nanoindentation data JF - Acta Geotechnica Y1 - 2018 A1 - Monfared, Siavash A1 - Hadrien Laubie A1 - Farhang Radjaï A1 - Hubler, Mija H. A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm AB -Built on the framework of effective interaction potentials using lattice element method, a methodology to calibrate and to validate the elasticity of solid constituents in heterogeneous porous media from experimentally measured nanoindentation moduli and imported scans from advanced imaging techniques is presented. Applied to computed tomography (CT) scans of two organic-rich shales, spatial variations of effective interaction potentials prove instrumental in capturing the effective elastic behavior of highly heterogeneous materials via the first two cumulants of experimentally measured distributions of nanoindentation moduli. After calibration and validation steps while implicitly accounting for mesoscale texture effects via CT scans, Biot poroelastic coefficients are simulated. Analysis of stress percolation suggests contrasting pathways for load transmission, a reflection of microtextural differences in the studied cases. This methodology to calibrate elastic energy content of real materials from advanced imaging techniques and experimental measurements paves the way to study other phenomena such as wave propagation and fracture while providing a platform to fine-tune effective behavior of materials given advancements in additive manufacturing and machine learning algorithms .

VL - 13 UR - http://link.springer.com/10.1007/s11440-018-0687-9 IS - 6 JO - Acta Geotech. ER - TY - JOUR T1 - Multi-periodic boundary conditions and the Contact Dynamics method JF - Comptes Rendus Mécanique Y1 - 2018 A1 - Farhang Radjaï AB -For investigating the mechanical behavior of granular materials by means of the discrete element approach, it is desirable to be able to simulate representative volume elements with macroscopically homogeneous deformations. This can be achieved by means of fully periodic boundary conditions such that stresses or displacements can be applied in all space directions. We present a general framework for periodic boundary conditions in granular materials and its implementation more specifically in the Contact Dynamics method. (c) 2017 Academie des sciences. Published by Elsevier Masson SAS.

VL - 346 UR - https://linkinghub.elsevier.com/retrieve/pii/S1631072117302346 IS - 3 JO - Comptes Rendus Mécanique ER - TY - JOUR T1 - Multiscale modeling for bioresources and bioproducts JF - Innovative Food Science & Emerging Technologies Y1 - 2018 A1 - Barnabe, M. A1 - Blanc, Nicolas A1 - Chabin, T. A1 - Jean-Yves Delenne A1 - Duri, A. A1 - Frank, Xavier A1 - Hugouvieux, V. A1 - Lutton, E. A1 - Mabille, F. A1 - Saeid Nezamabadi A1 - Perrot, N. A1 - Farhang Radjaï A1 - Ruiz, T. A1 - Tonda, A. AB -Designing and processing complex matter and materials are key objectives of bioresource and bioproduct research. Modeling approaches targeting such systems have to account for their two main sources of complexity: their intrinsic multi-scale nature; and the variability and heterogeneity inherent to all living systems. Here we provide insight into methods developed at the Food & Bioproduct Engineering division (CEPIA) of the French National Institute of Agricultural Research (INRA). This brief survey focuses on innovative research lines that tackle complexity by mobilizing different approaches with complementary objectives. On one hand cognitive approaches aim to uncover the basic mechanisms and laws underlying the emerging collective properties and macroscopic behavior of soft-matter and granular systems, using numerical and experimental methods borrowed from physics and mechanics. The corresponding case studies are dedicated to the structuring and phase behavior of biopolymers, powders and granular materials, and to the evolution of these structures caused by external constraints. On the other hand machine learning approaches can deal with process optimizations and outcome predictions by extracting useful information and correlations from huge datasets built from experiments at different length scales and in heterogeneous conditions. These predictive methods are illustrated in the context of cheese ripening, grape maturity prediction and bacterial production.

VL - 46 UR - https://linkinghub.elsevier.com/retrieve/pii/S1466856417302230 IS - Special Issue: SI JO - Innovative Food Science & Emerging Technologies ER - TY - JOUR T1 - A nonequilibrium force can stabilize 2D active nematics JF - Proceedings of the National Academy of Sciences Y1 - 2018 A1 - Maitra, Ananyo A1 - Srivastava, Pragya A1 - Marchetti, M. Cristina A1 - Lintuvuori, Juho S. A1 - Ramaswamy, Sriram A1 - Lenz, Martin AB -Suspensions of actively driven anisotropic objects exhibit distinctively nonequilibrium behaviors, and current theories predict that they are incapable of sustaining orientational order at high activity. By contrast, here we show that nematic suspensions on a substrate can display order at arbitrarily high activity due to a previously unreported, potentially stabilizing active force. This force moreover emerges inevitably in theories of active orientable fluids under geometric confinement. The resulting nonequilibrium ordered phase displays robust giant number fluctuations that cannot be suppressed even by an incompressible solvent. Our results apply to virtually all experimental assays used to investigate the active nematic ordering of self-propelled colloids, bacterial suspensions, and the cytoskeleton and have testable implications in interpreting their nonequilibrium behaviors.

VL - 115 UR - http://www.pnas.org/lookup/doi/10.1073/pnas.1720607115 IS - 27 JO - Proc Natl Acad Sci USA ER - TY - JOUR T1 - Rheology of granular materials composed of crushable particles JF - The European Physical Journal E Y1 - 2018 A1 - Duc-Hanh Nguyen A1 - Emilien Azéma A1 - Philippe Sornay A1 - Farhang Radjaï KW - CONTACT DYNAMICS METHOD; DISCRETE ELEMENT METHOD; SHEAR BANDS; NUMERICAL-SIMULATION; BED COMMINUTION; FRAGMENTATION; BREAKAGE; ROCK; DEM; MODEL AB -We investigate sheared granular materials composed of crushable particles by means of contact dynamics simulations and the bonded-cell model for particle breakage. Each particle is paved by irregular cells interacting via cohesive forces. In each simulation, the ratio of the internal cohesion of particles to the confining pressure, the relative cohesion, is kept constant and the packing is subjected to biaxial shearing. The particles can break into two or more fragments when the internal cohesive forces are overcome by the action of compressive force chains between particles. The particle size distribution evolves during shear as the particles continue to break. We find that the breakage process is highly inhomogeneous both in the fragment sizes and their locations inside the packing. In particular, a number of large particles never break whereas a large number of particles are fully shattered. As a result, the packing keeps the memory of its initial particle size distribution, whereas a power-law distribution is observed for particles of intermediate size due to consecutive fragmentation events whereby the memory of the initial state is lost. Due to growing polydispersity, dense shear bands are formed inside the packings and the usual dilatant behavior is reduced or cancelled. Hence, the stress-strain curve no longer passes through a peak stress, and a progressive monotonic evolution towards a pseudo-steady state is observed instead. We find that the crushing rate is controlled by the confining pressure. We also show that the shear strength of the packing is well expressed in terms of contact anisotropies and force anisotropies. The force anisotropy increases while the contact orientation anisotropy declines for increasing internal cohesion of the particles. These two effects compensate each other so that the shear strength is nearly independent of the internal cohesion of particles.

VL - 41 UR - http://link.springer.com/10.1140/epje/i2018-11656-1 IS - 4 JO - Eur. Phys. J. E ER - TY - JOUR T1 - Sol–gel process and complex fluids: sculpting porous matter at various lengths scales towards the Si(HIPE), Si(PHIPE), and SBA-15-Si(HIPE) series JF - Journal of Sol-Gel Science and Technology Y1 - 2018 A1 - Roucher, Armand A1 - Schmitt, Veronique A1 - Blin, Jean-Luc A1 - Backov, Renal AB -Taking inspiration from diatom hierarchically organized porosity, synthetic porous monoliths have been synthesized through rational designs while combining sol–gel chemistry, emulsions, Pickering-based emulsions, and lyotropic mesophases. We have described the synthesis of the Si(HIPE) series employing traditional emulsions to tune the macroscopic void diameters and cationic surfactant molecules to both stabilize the oil/water interface at the macroscopic length scale and create vermicular poorly organized porosity at the mesoscopic length scale. In a second part, while substituting surfactant molecules with modified silica nanoparticles acting as emulsion-stabilizers, we depict the possibility of tuning independently the macroscopic cell diameters and the cell junctions via Pickering emulsions employed as templates, leading to the Si(PHIPE) series presenting both monodiperse macroscopic void and cell junction diameters. Finally taking the synergetic effect of nonionic surfactants and a kosmotrope salt effect we show that it is possible generating self-standing Si(HIPE) bearing high organized mesoporosity while addressing the SBA-15-Si(HIPE) series. Open image in new window Taking inspiration from diatom hierarchically organized porosity, synthetic porous self-standing silica foams have been obtained through rational designs where sol–gel chemistry, emulsions, Pickering-based emulsions and lyotropic mesophases are advantageously combined.

VL - 18 UR - http://link.springer.com/10.1007/s10971-018-4794-8 JO - J Sol-Gel Sci Technol ER - TY - JOUR T1 - Synthesis, characterization, and catalytic application in aldehyde hydrosilylation of half-sandwich nickel complexes bearing (κ1-C)- and hemilabile (κ 2-C, S)-thioether-functionalised NHC ligands JF - Dalton Transactions Y1 - 2018 A1 - Franz-Josef Ulm A1 - Poblador-Bahamonde, Amalia I. A1 - Choppin, Sabine A1 - Bellemin-Laponnaz, Stéphane A1 - Chetcuti, Michael J. A1 - Achard, Thierry A1 - Ritleng, Vincent AB -Neutral nickel-N-heterocyclic carbene complexes, ((1)-C)-[NiCpBr{R-NHC-(CH2)(2)SR}] [Cp = (5)-C5H5; R-NHC-(CH2)(2)SR = 1-mesityl-3-[2-(tert-butylthio)ethyl]- (1a), 1-mesityl-3-[2-(phenylthio)ethyl]- (1b), 1-benzyl-3-[2-(tert-butylthio)ethyl]- (1c), 1-benzyl-3-[2-(phenylthio)ethyl]-imidazol-2-ylidene (1d)], which bear a N-bound thioether side arm, were prepared by the reaction of nickelocene with the corresponding imidazolium bromides [R-NHC-(CH2)(2)SRHBr] (a-d), via conventional or microwave heating. The H-1 NMR spectra of the benzyl-substituted species 1c and 1d showed signals for diastereotopic NCH2CH2S protons at room temperature. However, structural studies established the absence of coordination of the sulphur atom in the solid state, and solvent DFT calculations showed that bromide displacement by sulphur is an unfavourable process (G = +13.5 kcal mol(-1) for 1d), thereby suggesting that the observed disatereotopicity is more likely due to significant steric congestion rather than to a possible C,S-chelation in solution. Treatment of these complexes with KPF6 in tetrahydrofuran (THF) led to bromide abstraction to afford the cationic complexes [NiCp{R-NHC-(CH2)(2)SR}](PF6) (2a-c). Alternatively, 2a-c could also be prepared by the direct reaction of nickelocene with the corresponding imidazolium hexafluorophosphate salts [R-NHC-(CH2)(2)SRHPF6]. Inversely to the neutral species, whereas X-ray crystallography established C,S-chelation in the solid state, the H-1 NMR spectra (CDCl3, CD2Cl2, or thf-d(8)) at room temperature showed no diastereotopic NCH2CH2S protons, thus suggesting the possible displacement of the sulphur atom by the respective solvents and/or very fast sulphur inversion. DFT calculations established a low energy inversion process in all cases (+9 G(double dagger) +13 kcal mol(-1)) as well as a favourable solvent coordination process (G(double dagger) approximate to +11 kcal mol(-1); G approximate to -7 kcal mol(-1)) with a solvent such as THF, thus suggesting that sulphur inversion and/or solvent coordination can both account for the absence of diastereotopy at room temperature, depending on the solvent. While all complexes catalysed the hydrosilylation of benzaldehyde in the absence of any additive, the cationic C,S-chelated complexes 2 proved more active than the sterically constrained neutral species 1. In particular, 2c proved to be the most active pre-catalyst and its catalytic charge could be lowered down to 2 mol% with PhSiH3 as the hydrogen source.

VL - 47 UR - http://xlink.rsc.org/?DOI=C8DT03882A IS - 47 JO - Dalton Trans. ER - TY - JOUR T1 - Two-dimensional numerical simulation of chimney fluidization in a granular medium using a combination of discrete element and lattice Boltzmann methods JF - Physical Review E Y1 - 2018 A1 - Jeff Ngoma A1 - Pierre Philippe A1 - Stéphane Bonelli A1 - Farhang Radjaï A1 - Jean-Yves Delenne AB -We present here a numerical study dedicated to the fluidization of a submerged granular medium induced by a localized fluid injection. To this end, a two-dimensional (2D) model is used, coupling the lattice Boltzmann method (LBM) with the discrete element method (DEM) for a relevant description of fluid-grains interaction. An extensive investigation has been carried out to analyze the respective influences of the different parameters of our configuration, both geometrical (bed height, grain diameter, injection width) and physical (fluid viscosity, buoyancy). Compared to previous experimental works, the same qualitative features are recovered as regards the general phenomenology including transitory phase, stationary states, and hysteretic behavior. We also present quantitative findings about transient fluidization, for which several dimensionless quantities and scaling laws are proposed, and about the influence of the injection width, from localized to homogeneous fluidization. Finally, the impact of the present 2D geometry is discussed, by comparison to the real three-dimensional (3D) experiments, as well as the crucial role of the prevailing hydrodynamic regime within the expanding cavity, quantified through a cavity Reynolds number, that can presumably explain some substantial differences observed regarding upward expansion process of the fluidized zone when the fluid viscosity is changed.

VL - 97 UR - https://link.aps.org/doi/10.1103/PhysRevE.97.052902 IS - 5 JO - Phys. Rev. E ER - TY - JOUR T1 - Cohesive strength of iron ore granules JF - EPJ Web of Conferences Y1 - 2017 A1 - Contreras, Rafael Jaimes A1 - Berger, Nicolas A1 - Izard, Edouard A1 - Douce, Jean-François A1 - Koltsov, Alexey A1 - Jean-Yves Delenne A1 - Emilien Azéma A1 - Saeid Nezamabadi A1 - van Loo, Frédéric A1 - Roland Jean-Marc Pellenq A1 - Farhang Radjaï ED - Luding, S. AB -We present an experimental and numerical investigation of the mechanical strength of crude iron ore (Hematite) granules in which capillary bonds between primary particles are the source of internal cohesion. The strength is measured by subjecting the granules to vertical compression between two plates. We show that the behavior of the granules is ductile with a well-defined plastic threshold which increases with the amount of water. It is found that the compressive strength scales with capillary cohesion with a pre-factor that is nearly independent of size polydispersity for the investigated range of parameters but increases with friction coefficient between primary particles. This weak dependence may be attributed to the class of fine particles which, due to their large number, behaves as a cohesive matrix that controls the strength of the granule.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714008020 IS - 10141146 JO - EPJ Web Conf. ER - TY - JOUR T1 - Compaction of granular materials composed of deformable particles JF - EPJ Web of Conferences Y1 - 2017 A1 - Thanh Hai Nguyen A1 - Saeid Nezamabadi A1 - Jean-Yves Delenne A1 - Farhang Radjaï ED - Luding, S. AB -In soft particle materials such as metallic powders the particles can undergo large deformations without rupture. The large elastic or plastic deformations of the particles are expected to strongly affect the mechanical properties of these materials compared to hard particle materials more often considered in research on granular materials. Herein, two numerical approaches are proposed for the simulation of soft granular systems: (i) an implicit formulation of the Material Point Method (MPM) combined with the Contact Dynamics (CD) method to deal with contact interactions, and (i) Bonded Particle Model (BPM), in which each deformable particle is modeled as an aggregate of rigid primary particles using the CD method. These two approaches allow us to simulate the compaction of an assembly of elastic or plastic particles. By analyzing the uniaxial compaction of 2D soft particle packings, we investigate the effects of particle shape change on the stress-strain relationship and volume change behavior as well as the evolution of the microstructure.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714005013 JO - EPJ Web Conf. ER - TY - JOUR T1 - Disorder-induced stiffness degradation of highly disordered porous materials JF - Journal of the Mechanics and Physics of Solids Y1 - 2017 A1 - Hadrien Laubie A1 - Monfared, Siavash A1 - Farhang Radjaï A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm AB -The effective mechanical behavior of multiphase solid materials is generally modeled by means of homogenization techniques that account for phase volume fractions and elastic moduli without considering the spatial distribution of the different phases. By means of extensive numerical simulations of randomly generated porous materials using the lattice element method, the role of local textural properties on the effective elastic properties of disordered porous materials is investigated and compared with different continuum micromechanics-based models. It is found that the pronounced disorder-induced stiffness degradation originates from stress concentrations around pore clusters in highly disordered porous materials. We identify a single disorder parameter, φ*sa*, which combines a measure of the spatial disorder of pores (the clustering index, *sa*) with the pore volume fraction (the porosity, φ) to scale the disorder-induced stiffness degradation. Thus, we conclude that the classical continuum micromechanics models with one spherical pore phase, due to their underlying homogeneity assumption fall short of addressing the clustering effect, unless additional texture information is introduced, e.g. in form of the shift of the percolation threshold with disorder, or other functional relations between volume fractions and spatial disorder; as illustrated herein for a differential scheme model representative of a two-phase (solid–pore) composite model material.

VL - 106 ER - TY - JOUR T1 - Effect of particle size distribution on 3D packings of spherical particles JF - EPJ Web of Conferences Y1 - 2017 A1 - Taiebat, Mahdi A1 - Patrick Mutabaruka A1 - Roland Jean-Marc Pellenq A1 - Farhang Radjaï ED - Saeid Nezamabadi ED - Luding, S. ED - Jean-Yves Delenne AB -

We use molecular dynamics simulations of frictionless spherical particles to investigate a class of polydisperse granular materials in which the particle size distribution is uniform in particle volumes. The particles are assembled in a box by uniaxial compaction under the action of a constant stress. Due to the absence of friction and the nature of size distribution, the generated packings have the highest packing fraction at a given size span, defined as the ratio *α* of the largest size to the smallest size. We find that, up to *α* = 5, the packing fraction is a nearly linear function of *α*. While the coordination number is nearly constant due to the isostatic nature of the packings, we show that the connectivity of the particles evolves with *α*. In particular, the proportion of particles with 4 contacts represents the largest proportion of particles mostly of small size. We argue that this particular class of particles occurs as a result of the high stability of local configurations in which a small particle is stuck by four larger particles.

Lattice approaches have emerged as a powerful tool to capture the effective mechanical behavior of heterogeneous materials using harmonic interactions inspired from beam-type stretch and rotational interactions between a discrete number of mass points. In this paper, the lattice element method (LEM) is reformulated within the conceptual framework of empirical force fields employed at the lattice scale. Within this framework, because classical harmonic formulations are but a Taylor expansion of nonharmonic potential expressions, they can be used to model both the linear and the nonlinear response of discretized material systems. Specifically, closed-form calibration procedures for such interaction potentials are derived for both the isotropic and the transverse isotropic elastic cases on cubic lattices, in the form of linear relations between effective elasticity properties and energy parameters that define the interactions. The relevance of the approach is shown by an application to the classical Griffith crack problem. In particular, it is shown that continuum-scale quantities of linear-elastic fracture mechanics, such as stress intensity factors (SIFs), are well captured by the method, which by its very discrete nature removes geometric discontinuities that provoke stress singularities in the continuum case. With its strengths and limitations thus defined, the proposed LEM is well suited for the study of multiphase materials whose microtextural information is obtained by, e.g., X-ray micro-computed tomography. (c) 2017 American Society of Civil Engineers.

VL - 7 UR - http://ascelibrary.org/doi/10.1061/%28ASCE%29NM.2153-5477.0000125 IS - 3 JO - J. Nanomech. Micromech. ER - TY - JOUR T1 - From cellulose to kerogen: molecular simulation of a geological process JF - Chemical Science Y1 - 2017 A1 - Atmani, Lea A1 - Christophe Bichara A1 - Roland Jean-Marc Pellenq A1 - Henri Van Damme A1 - Adri CT Van Duin A1 - Raza, Zamaan A1 - Truflandier, Lionel A. A1 - Amaël Obliger A1 - Kralert, Paul G. A1 - Franz-Josef Ulm A1 - Jean-Marc Leyssale AB -The process by which organic matter decomposes deep underground to form petroleum and its underlying kerogen matrix has so far remained a no man's land to theoreticians, largely because of the geological (Myears) timescale associated with the process. Using reactive molecular dynamics and an accelerated simulation framework, the replica exchange molecular dynamics method, we simulate the full transformation of cellulose into kerogen and its associated fluid phase under prevailing geological conditions. We observe in sequence the fragmentation of the cellulose crystal and production of water, the development of an unsaturated aliphatic macromolecular phase and its aromatization. The composition of the solid residue along the maturation pathway strictly follows what is observed for natural type III kerogen and for artificially matured samples under confined conditions. After expulsion of the fluid phase, the obtained microporous kerogen possesses the structure, texture, density, porosity and stiffness observed for mature type III kerogen and a microporous carbon obtained by saccharose pyrolysis at low temperature. As expected for this variety of precursor, the main resulting hydrocarbon is methane. The present work thus demonstrates that molecular simulations can now be used to assess, almost quantitatively, such complex chemical processes as petrogenesis in fossil reservoirs and, more generally, the possible conversion of any natural product into bio-sourced materials and/or fuel.

VL - 8 UR - http://xlink.rsc.org/?DOI=C7SC03466K IS - 12 JO - Chem. Sci. ER - TY - JOUR T1 - Inorganic, Hybridized and Living Macrocellular Foams: “Out of the Box” Heterogeneous Catalysis JF - The Chemical Record Y1 - 2017 A1 - Roucher, Armand A1 - Depardieu, Martin A1 - Pekin, Deniz A1 - Morvan, Mickaël A1 - Backov, Renal KW - catalysts; emulsion-templating; integrative chemistry; porous matter; sol-gel process AB -With this personal account we show how the Integrative Chemistry, when combining the sol-gel process and concentrated emulsions, allows to trigger inorganic, hybrid or living materials when dedicated toward heterogeneous catalysis applications. In here we focus on 3D-macrocellular monolithic foams bearing hierarchical porosities and applications thereof toward heterogeneous catalysis where both activities and mass transport are enhanced. We thereby first depict the general background of emulsions, focusing on concentrated ones, acting as soft templates for the design of solid (HIPE) foams, HIPE being the acronym for High Internal Phase Emulsions while encompassing both sol-gel and polymer chemistry. Secondly we extend this approach toward the design of inorganic cellular materials labeled Si(HIPE) and hybrid organic-inorganic foams, labeled Organo-Si(HIPE), where heterogeneous catalysis applications are addressed considering acidic, metallic, enzymatic and bacterial-based modified Si-HIPE. Along, we will show how the fluid hydrodynamic within the macrocellular foams is offering advanced "out of the box" heterogeneous catalytic capabilities.

VL - 18 UR - http://doi.wiley.com/10.1002/tcr.v18.7-8 IS - 7-8 JO - Chem. Rec. ER - TY - JOUR T1 - Mesoscale Poroelasticity of Heterogeneous Media JF - Journal of Nanomechanics and Micromechanics Y1 - 2017 A1 - Monfared, Siavash A1 - Hadrien Laubie A1 - Farhang Radjaï A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm AB -The poromechanics of heterogeneous media is reformulated in a discrete framework using the lattice element method (LEM) that accounts for the presence of interfaces as well as local microtextural and elastic variations. The exchange of mechanical information between pore and solid(s) is captured by means of force field potentials for these domains, which eliminate the requirement of scale separability of continuum-based poromechanics approaches. In congruence with mu VT and NPT ensembles of statistical mechanics, discrete expressions for Biot poroelastic coefficients are derived. Considering harmonic-type interaction potentials for each link, analytical expressions for both isotropic and transversely isotropic effective elasticity are presented. The theory is validated against continuum-based expressions of Biot poroelastic coefficients for porous media with isotropic and transversely isotropic elastic solid behavior. (C) 2017 American Society of Civil Engineers.

VL - 7 UR - http://ascelibrary.org/doi/10.1061/%28ASCE%29NM.2153-5477.0000136 IS - 4 JO - J. Nanomech. Micromech. ER - TY - JOUR T1 - Modeling Granular Materials: Century-Long Research across Scales JF - Journal of Engineering Mechanics Y1 - 2017 A1 - Farhang Radjaï A1 - Jean-Noel Roux A1 - Daouadji, Ali AB -Granular materials are the most recurrent form of solid-state matter on Earth. They challenge researchers and engineers in various fields not only because they occur with a broad variety of grain sizes, shapes and interactions in nature and industry, but also because they show a rich panoply of mechanical states. Despite this polymorphism, all these different types of soils, powders, granules, ores, pharmaceutical products, etc., are instances of the granular matter with the same least common denominator of being sandlike (psammoid in Greek), i.e., solid grains interacting via frictional contacts. This review describes milestone contributions to the field of granular materials since the early elastic-plastic models developed for soils in the 1950s. The research on granular materials has grown into a vast multidisciplinary field in the 1980s with increasing focus on the microstructure and owing to new experimental tools and discrete simulation methods. It turns out that the granular texture, particle-scale kinematics, and force transmission are far more complex than presumed in early micromechanical models of granular materials. Hence, constitutive relations cannot easily be derived from the particle-scale behavior although advanced continuum models have been developed to account for anisotropy, intermediate stress, and complex loading paths. The subtle elastic properties and origins of bulk friction will be discussed, as well as the effects of particle shape and size distributions. The review covers also recent developments in macroscopic modeling such as the thermomechanical approach, anisotropic critical state theory, nonlocal modeling approach, inertial flows, and material instabilities. Finally, a brief account is given of open issues and some new frontiers and challenges in the field.

VL - 143 IS - 4 JO - J. Eng. Mech. ER - TY - JOUR T1 - Modeling root growth in granular soils: effects of root stiffness and packing fraction JF - EPJ Web of Conferences Y1 - 2017 A1 - Fakih, Mahmoud A1 - Jean-Yves Delenne A1 - Farhang Radjaï A1 - Fourcaud, Thierry ED - Saeid Nezamabadi ED - Luding, S. AB -We use molecular dynamics simulations to investigate the effects of root bending stiffness and packing fraction on the path followed by a growing root in 2D packings of grains representing a soil. The root is modeled as a chain of elements that can grow in length and change their direction depending on the forces exerted by soil grains. We show that the root shape is mainly controlled by the bending stiffness of its apex. At low stiffness, the root randomly explores the pore space whereas at sufficiently high stiffness, of the order of soil hardness multiplied by mean grain size, the root follows a straight path across the soil. Between these two limits, the root shape can be characterized by the standard deviation of its re-directions at the scale of soil grains. We find that this shape parameter varies as a power-law function of the normalized bending stiffness.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714014013 JO - EPJ Web Conf. ER - TY - JOUR T1 - Modeling soft granular materials JF - Granular Matter Y1 - 2017 A1 - Saeid Nezamabadi A1 - Thanh Hai Nguyen A1 - Jean-Yves Delenne A1 - Farhang Radjaï AB -Soft-grain materials such as clays and other colloidal pastes share the common feature of being composed of grains that can undergo large deformations without rupture. For the simulation of such materials, we present two alternative methods: (1) an implicit formulation of the material point method (MPM), in which each grain is discretized as a collection of material points, and (2) the bonded particle model (BPM), in which each soft grain is modeled as an aggregate of rigid particles using the contact dynamics method. In the MPM, a linear elastic behavior is used for the grains. In order to allow the aggregates in the BPM to deform without breaking, we use long-range center-to-center attraction forces between the primary particles belonging to each grain together with steric repulsion at their contact points. We show that these interactions lead to a plastic behavior of the grains. Using both methods, we analyze the uniaxial compaction of 2D soft granular packings. This process is nonlinear and involves both grain rearrangements and large deformations. High packing fractions beyond the jamming state are reached as a result of grain shape change for both methods. We discuss the stress-strain and volume change behavior as well as the evolution of the connectivity of the grains. Similar textures are observed at large deformations although the BPM requires higher stress than the MPM to reach the same level of packing fraction.

VL - 19 IS - 1 JO - Granular Matter ER - TY - Generic T1 - Modelling Transient Dynamics of Granular Slopes: MPM and DEM T2 - PROCEEDINGS OF THE 1ST INTERNATIONAL CONFERENCE ON THE MATERIAL POINT METHOD (MPM 2017) Book Series: Procedia Engineering Y1 - 2017 A1 - Krishna Kumar A1 - Kenichi Soga A1 - Jean-Yves Delenne A1 - Farhang Radjaï ED - Rohe, A ED - Kenichi Soga ED - Teunissen, H AB -Transient granular flows, such as rock falls, debris flows, and aerial and submarine avalanches, occur very often in nature. In the geotechnical context, transient movements of large granular slopes are a substantial factor of risk due to their destructive force and the transformations the y may produce in the landscape. This paper investigates the ability of MPM, a continuum approach, to reproduce the evolution of a granular slope destabilised by an external energy source. In particular, a central issue is whether the power-law dependence of run-out distance and time observed with respect to the initial geometry or energy can be reproduced by a simple Mohr-Coulomb plastic behaviour. The effect of base friction on the run-out kinematics is studied by comparing the data obtained from the DEM and MPM simulations. The mechanism of energy dissipation is primarily through friction and the MPM is able to predict the run-out response in good agreement with the DEM simulations. At very low excitation energies, the DEM simulations show longer run-out in comparison to the MPM due to local destabilization at the flow front. At low input energies, a larger fraction of the energy is consumed in the destabilisation process, hence the amount energy available for flow is less. However, at higher input energy, where most of the energy is dissipated during the spreading phase, the run-out distance has a weak dependence on the distribution of velocity in the granular mass.

JF - PROCEEDINGS OF THE 1ST INTERNATIONAL CONFERENCE ON THE MATERIAL POINT METHOD (MPM 2017) Book Series: Procedia Engineering PB - Elsevier Ltd. CY - JAN 10-13, 2017, Delft, NETHERLANDS VL - 175 JO - Procedia Engineering ER - TY - Generic T1 - MPM with Frictional Contact for Application to Soft Particulate Materials T2 - PROCEEDINGS OF THE 1ST INTERNATIONAL CONFERENCE ON THE MATERIAL POINT METHOD (MPM 2017) Book Series: Procedia Engineering Y1 - 2017 A1 - Saeid Nezamabadi A1 - Thanh Hai Nguyen A1 - Jean-Yves Delenne A1 - Julien Averseng A1 - Frank, Xavier A1 - Farhang Radjaï ED - Rohe, A ED - Kenichi Soga ED - Teunissen, H AB -Soft particle materials are composed of discrete particles that can undergo large deformations without rupture. Most food products, many powders, colloidal pastes, vesicles and biological cells are soft particle systems. In order to model such materials, we present an efficient numerical approach combining an implicit formulation of the Material Point Method (MPM) and Contact Dynamics (CD) method. The MPM deals with bulk variables of an individual particle by discretizing it as a collection of material points, whereas the CD allows for the treatment of frictional contacts between particles. This model is applied for the simulation of the uniaxial compression of 2D soft-particle packings. The compaction is a nonlinear process in which new contacts are formed between particles and the contact areas increase. The change of particle shapes allows these materials to reach high packing fraction. We find that the contact specific surface, the orientation anisotropy and the aspect ratio of particles increase as a function of the packing fraction but at different rates. We also evidence the effect of friction, which favors strong stress chains and thus the elongation of particles, leading to larger values of the orientation anisotropy and the aspect ratio at a given level of packing fraction as compared to a frictionless particle packing.

JF - PROCEEDINGS OF THE 1ST INTERNATIONAL CONFERENCE ON THE MATERIAL POINT METHOD (MPM 2017) Book Series: Procedia Engineering PB - Elsevier Ltd. CY - JAN 10-13, 2017, Delft, NETHERLANDS VL - 175 JO - Procedia Engineering ER - TY - JOUR T1 - Nano-granular texture of cement hydrates JF - EPJ Web of Conferences Y1 - 2017 A1 - Katerina Ioannidou A1 - Franz-Josef Ulm A1 - Pierre E. Levitz A1 - Emanuela Del Gado A1 - Roland Jean-Marc Pellenq ED - Farhang Radjaï ED - Saeid Nezamabadi ED - Luding, S. ED - Jean-Yves Delenne AB -Mechanical behavior of concrete crucially depends on cement hydrates, the “glue” of cement. The design of high performance and more environmentally friendly cements demands a deeper understanding of the formation of the multiscale structure of cement hydrates, when they precipitate and densify. We investigate the precipitation and setting of nano-grains of cement hydrates using a combination of Monte Carlo and Molecular Dynamics numerical simulations and study their texture from nano up to the micron scale. We characterize the texture of cement hydrates using the local volume fraction distribution, the pore size distribution, the scattering intensity and the chord length distribution and we compare them with experiments. Our nano-granular model provides cement structure with realistic texture and mechanics and can be further used to investigate degradation mechanisms.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714015027 JO - EPJ Web Conf. ER - TY - JOUR T1 - Numerical insight into the micromechanics of jet erosion of a cohesive granular material JF - EPJ Web of Conferences Y1 - 2017 A1 - Cuellar, Pablo A1 - Benseghier, Zeyd A1 - Luu, Li-Hua A1 - Stéphane Bonelli A1 - Jean-Yves Delenne A1 - Farhang Radjaï A1 - Pierre Philippe ED - Saeid Nezamabadi ED - Luding, S. AB -Here we investigate the physical mechanisms behind the surface erosion of a cohesive granular soil induced by an impinging jet by means of numerical simulations coupling fluid and grains at the microscale. The 2D numerical model combines the Discrete Element and Lattice Boltzmann methods (DEM-LBM) and accounts for the granular cohesion with a contact model featuring a paraboloidal yield surface. Here we review first the hydrodynamical conditions imposed by the fluid jet on a solid granular packing, turning then the attention to the impact of cohesion on the erosion kinetics. Finally, the use of an additional subcritical debonding damage model based on the work of Silvani and co-workers provides a novel insight into the internal solicitation of the cohesive granular sample by the impinging jet.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714015017 JO - EPJ Web Conf. ER - TY - JOUR T1 - Numerical modeling of the tensile strength of a biological granular aggregate: Effect of the particle size distribution JF - EPJ Web of Conferences Y1 - 2017 A1 - Heinze, Karsta A1 - Frank, Xavier A1 - Valérie Lullien-Pellerina A1 - George, Matthieu A1 - Farhang Radjaï A1 - Jean-Yves Delenne ED - Saeid Nezamabadi ED - Luding, S. AB -Wheat grains can be considered as a natural cemented granular material. They are milled under high forces to produce food products such as flour. The major part of the grain is the so-called starchy endosperm. It contains stiff starch granules, which show a multi-modal size distribution, and a softer protein matrix that surrounds the granules. Experimental milling studies and numerical simulations are going hand in hand to better understand the fragmentation behavior of this biological material and to improve milling performance. We present a numerical study of the effect of granule size distribution on the strength of such a cemented granular material. Samples of bi-modal starch granule size distribution were created and submitted to uniaxial tension, using a peridynamics method. We show that, when compared to the effects of starch-protein interface adhesion and voids, the granule size distribution has a limited effect on the samples’ yield stress.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714008013 JO - EPJ Web Conf. ER - TY - JOUR T1 - Numerical study of the failure of materials embedding soft to hard particles JF - EPJ Web of Conferences Y1 - 2017 A1 - Frank, Xavier A1 - Jean-Yves Delenne A1 - Farhang Radjaï ED - Saeid Nezamabadi ED - Luding, S. AB -In this study, we use a bond-based peridynamic approach to investigate the mechanical strength and cracking of composite materials with spherical inclusions. The total volume fraction of particles and the particle-matrix toughness ratio were varied to cover a range of soft to hard inclusions. The mean particle damage was characterized together with crack patterns at a sub-particle scale. Three types of crack patterns are identified depending on the toughness ratio.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714002029 JO - EPJ Web Conf. ER - TY - JOUR T1 - Peridynamics simulation of the comminution of particles containing microcraks JF - EPJ Web of Conferences Y1 - 2017 A1 - Blanc, Nicolas A1 - Frank, Xavier A1 - Mayer-Laigle, Claire A1 - Farhang Radjaï A1 - Jean-Yves Delenne ED - Saeid Nezamabadi ED - Luding, S. ED - Jean-Yves Delenne AB -In this study, we rely on a ’bond-based’ peridynamic approach to investigate the strength and failure of 2D particles containing a collection of 1D microcracks. The mechanical tests were performed on disks under diametral compression. In an extensive parametric study, the distribution of microcracks was varied for different particle sizes. The evolution of yield stress with diameter and the probability of failure in terms of Weibull distributions are investigated in detail. Finally, by means of a floodfill algorithm, we analyze the variation of the mean fragment size as a function of the density of defects.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714007018 JO - EPJ Web Conf. ER - TY - JOUR T1 - The Potential of Mean Force concept for bridging (length and time) scales in the modeling of complex porous materials JF - EPJ Web of Conferences Y1 - 2017 A1 - Katerina Ioannidou A1 - Benoit Carrier A1 - Matthieu Vandamme A1 - Roland Jean-Marc Pellenq ED - Farhang Radjaï ED - Saeid Nezamabadi ED - Luding, S. ED - Jean-Yves Delenne AB -We introduce the concept of Potential of Mean Force, PMF, as a way to implement upscaling modeling from the nano-scale to micron-scale. A PMF is a free energy function representing in an effective way the interactions between objects (cement hydrates, clay platelets, etc.) at thermodynamics conditions. The PMF is therefore the key piece of information allowing to coarse-grained Physical-Chemistry information in a meso-scale model formulation. The use of PMF offers a huge computational advantage as it allows a straight up-scaling to the meso-scale while keeping essential interactions information that are the hallmark of Physical-Chemistry processes. Such a coarse-grained modeling integrates atomistic response into inter-particle potentials that fully propagate molecular scale information all the way to the meso-scale.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714001009 JO - EPJ Web Conf. ER - TY - JOUR T1 - A potential-of-mean-force approach for fracture mechanics of heterogeneous materials using the lattice element method JF - Journal of the Mechanics and Physics of Solids Y1 - 2017 A1 - Hadrien Laubie A1 - Farhang Radjaï A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm AB -Fracture of heterogeneous materials has emerged as a critical issue in many engineering applications, ranging from subsurface energy to biomedical applications, and requires a rational framework that allows linking local fracture processes with global fracture descriptors such as the energy release rate, fracture energy and fracture toughness. This is achieved here by means of a local and a global potential-of-mean-force (PMF) inspired Lattice Element Method (LEM) approach. In the local approach, fracture-strength criteria derived from the effective interaction potentials between mass points are shown to exhibit a scaling commensurable with the energy dissipation of fracture processes. In the global PMF-approach, fracture is considered as a sequence of equilibrium states associated with minimum potential energy states analogous to Griffith’s approach. It is found that this global approach has much in common with a Grand Canonical Monte Carlo (GCMC) approach, in which mass points are randomly removed following a maximum dissipation criterion until the energy release rate reaches the fracture energy. The duality of the two approaches is illustrated through the application of the PMF-inspired LEM for fracture propagation in a homogeneous linear elastic solid using different means of evaluating the energy release rate. Finally, by application of the method to a textbook example of fracture propagation in a heterogeneous material, it is shown that the proposed PMF-inspired LEM approach captures some well-known toughening mechanisms related to fracture energy contrast, elasticity contrast and crack deflection in the considered two-phase layered composite material.

VL - 105 JO - Journal of the Mechanics and Physics of Solids ER - TY - JOUR T1 - Scaling behavior of immersed granular flows JF - EPJ Web of Conferences Y1 - 2017 A1 - L. Amarsid A1 - Jean-Yves Delenne A1 - Patrick Mutabaruka A1 - Yann Monerie A1 - Perales, F. A1 - Farhang Radjaï ED - Saeid Nezamabadi ED - Luding, S. AB -The shear behavior of granular materials immersed in a viscous fluid depends on fluid properties (viscosity, density), particle properties (size, density) and boundary conditions (shear rate, confining pressure). Using computational fluid dynamics simulations coupled with molecular dynamics for granular flow, and exploring a broad range of the values of parameters, we show that the parameter space can be reduced to a single parameter that controls the packing fraction and effective friction coefficient. This control parameter is a modified inertial number that incorporates viscous effects.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714009044 JO - EPJ Web Conf. ER - TY - JOUR T1 - Small solar system bodies as granular systems JF - EPJ Web of Conferences Y1 - 2017 A1 - Hestroffer, Daniel A1 - Adriano Campo Bagatín A1 - Losert, Wolfgang A1 - Opsomer, Eric A1 - Sánchez, Paul A1 - Scheeres, Daniel J. A1 - Staron, Lydie A1 - Taberlet, Nicolas A1 - Yano, Hajime A1 - Eggl, Siegfried A1 - Lecomte, Charles-Edouard A1 - Murdoch, Naomi A1 - Farhang Radjaï A1 - Richardson, Derek C. A1 - Salazar, Marcos A1 - Schwartz, Stephen R. A1 - Tanga, Paolo ED - Saeid Nezamabadi ED - Luding, S. ED - Jean-Yves Delenne AB -Asteroids and other Small Solar System Bodies (SSSBs) are currently of great scientific and even industrial interest. Asteroids exist as the permanent record of the formation of the Solar System and therefore hold many clues to its understanding as a whole, as well as insights into the formation of planetary bodies. Additionally, SSSBs are being investigated in the context of impact risks for the Earth, space situational awareness and their possible industrial exploitation (asteroid mining). In all these aspects, the knowledge of the geophysical characteristics of SSSB surface and internal structure are of great importance. Given their size, constitution, and the evidence that many SSSBs are not simple monoliths, these bodies should be studied and modelled as self-gravitating granular systems in general, or as granular systems in micro-gravity environments in particular contexts. As such, the study of the geophysical characteristics of SSSBs is a multi-disciplinary effort that lies at the crossroads between Granular Mechanics, Celestial Mechanics, Soil Mechanics, Aerospace Engineering and Computer Sciences.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714014011 JO - EPJ Web Conf. ER - TY - JOUR T1 - Strength of wet agglomerates of spherical particles: effects of friction and size distribution JF - EPJ Web of Conferences Y1 - 2017 A1 - Vo, Thanh-Trung A1 - Patrick Mutabaruka A1 - Jean-Yves Delenne A1 - Saeid Nezamabadi A1 - Farhang Radjaï ED - Luding, S. AB -We investigate the mechanical behavior of wet granular agglomerates composed of spherical particles by means of molecular dynamics simulations. The capillary cohesion force is modeled as an attraction force at the contact between two particles and expressed as an explicit function of the gap and volume of the liquid bridge. We are interested in the effect of the friction coefficient between primary particles. The agglomerates are subjected to diametrical compression tests. We find that the deformation is ductile involving particle rearrangements. However, a well-defined stress peak is observed and the peak stress is used as a measure of the compressive strength of the agglomerate. The strength increases with friction coefficient but levels off at friction coefficients above 0.4. Furthermore, the compressive strength is an increasing function of particle size span.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714008021 JO - EPJ Web Conf. ER - TY - JOUR T1 - Stress Transmission and Failure in Disordered Porous Media JF - PHYSICAL REVIEW LETTERS Y1 - 2017 A1 - Hadrien Laubie A1 - Farhang Radjaï A1 - Roland Jean-Marc Pellenq A1 - et al AB -By means of extensive lattice-element simulations, we investigate stress transmission and its relation with failure properties in increasingly disordered porous systems. We observe a non-Gaussian broadening of stress probability density functions under tensile loading with increasing porosity and disorder, revealing a gradual transition from a state governed by single-pore stress concentration to a state controlled by multipore interactions and metric disorder. This effect is captured by the excess kurtosis of stress distributions and shown to be nicely correlated with the second moment of local porosity fluctuations, which appears thus as a (dis)order parameter for the system. By generating statistical ensembles of porous textures with varying porosity and disorder, we derive a general expression for the fracture stress as a decreasing function of porosity and disorder. Focusing on critical sites where the local stress is above the global fracture threshold, we also analyze the transition to failure in terms of a coarse-graining length. These findings provide a general framework which can also be more generally applied to multiphase and structural heterogeneous materials.

VL - 119 IS - 7 ER - TY - JOUR T1 - Stress Transmission and Failure in Disordered Porous Media JF - Physical Review Letters Y1 - 2017 A1 - Hadrien Laubie A1 - Farhang Radjaï A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm AB -

By means of extensive lattice-element simulations, we investigate stress transmission and its relation with failure properties in increasingly disordered porous systems. We observe a non-Gaussian broadening of stress probability density functions under tensile loading with increasing porosity and disorder, revealing a gradual transition from a state governed by single-pore stress concentration to a state controlled by multipore interactions and metric disorder. This effect is captured by the excess kurtosis of stress distributions and shown to be nicely correlated with the second moment of local porosity fluctuations, which appears thus as a (dis)order parameter for the system. By generating statistical ensembles of porous textures with varying porosity and disorder, we derive a general expression for the fracture stress as a decreasing function of porosity and disorder. Focusing on critical sites where the local stress is above the global fracture threshold, we also analyze the transition to failure in terms of a coarse-graining length. These findings provide a general framework which can also be more generally applied to multiphase and structural heterogeneous materials.

VL - 119 UR - https://link.aps.org/doi/10.1103/PhysRevLett.119.075501 IS - 7 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Three-dimensional bonded-cell model for grain fragmentation JF - Computational Particle Mechanics Y1 - 2017 A1 - Cantor, D A1 - Emilien Azéma A1 - Philippe Sornay A1 - Farhang Radjaï KW - Bonded-cell model; Fragmentation; Discrete element method; Contact dynamics method; Voronoi cell; Weibull statistics AB -We present a three-dimensional numerical method for the simulation of particle crushing in 3D. This model is capable of producing irregular angular fragments upon particle fragmentation while conserving the total volume. The particle is modeled as a cluster of rigid polyhedral cells generated by a Voronoi tessellation. The cells are bonded along their faces by a cohesive Tresca law with independent tensile and shear strengths and simulated by the contact dynamics method. Using this model, we analyze the mechanical response of a single particle subjected to diametral compression for varying number of cells, their degree of disorder, and intercell tensile and shear strength. In particular, we identify the functional dependence of particle strength on the intercell strengths. We find that two different regimes can be distinguished depending on whether intercell shear strength is below or above its tensile strength. In both regimes, we observe a power-law dependence of particle strength on both intercell strengths but with different exponents. The strong effect of intercell shear strength on the particle strength reflects an interlocking effect between cells. In fact, even at low tensile strength, the particle global strength can still considerably increase with intercell shear strength. We finally show that the Weibull statistics describes well the particle strength variability.

VL - 4 UR - http://link.springer.com/10.1007/s40571-016-0129-0 IS - 4 JO - Comp. Part. Mech. ER - TY - JOUR T1 - Understanding rheological hysteresis in soft glassy materials JF - Soft Matter Y1 - 2017 A1 - Radhakrishnan, Rangarajan A1 - Divoux, Thibaut A1 - Manneville, Sébastien A1 - Fielding, Suzanne M. AB -Motivated by recent experimental studies of rheological hysteresis in soft glassy materials, we study numerically strain rate sweeps in simple yield stress fluids and viscosity bifurcating yield stress fluids. Our simulations of downward followed by upward strain rate sweeps, performed within fluidity models and the soft glassy rheology model, successfully capture the experimentally observed monotonic decrease of the area of the rheological hysteresis loop with sweep time in simple yield stress fluids, and the bell shaped dependence of hysteresis loop area on sweep time in viscosity bifurcating fluids. We provide arguments explaining these two different functional forms in terms of differing tendencies of simple and viscosity bifurcating fluids to form shear bands during the sweeps, and show that the banding behaviour captured by our simulations indeed agrees with that reported experimentally. We also discuss the difference in hysteresis behaviour between inelastic and viscoelastic fluids. Our simulations qualitatively agree with the experimental data discussed here for four different soft glassy materials.

VL - 13 UR - http://xlink.rsc.org/?DOI=C6SM02581A IS - 9 JO - Soft Matter ER - TY - JOUR T1 - Viscoinertial regime of immersed granular flows JF - Physical Review E Y1 - 2017 A1 - L. Amarsid A1 - Jean-Yves Delenne A1 - Patrick Mutabaruka A1 - Yann Monerie A1 - Perales, F. A1 - Farhang Radjaï AB -By means of extensive coupled molecular dynamics–lattice Boltzmann simulations, accounting for grain dynamics and subparticle resolution of the fluid phase, we analyze steady inertial granular flows sheared by a viscous fluid. We show that, for a broad range of system parameters (shear rate, confining stress, fluid viscosity, and relative fluid-grain density), the frictional strength and packing fraction can be described by a modified inertial number incorporating the fluid effect. In a dual viscous description, the effective viscosity diverges as the inverse square of the difference between the packing fraction and its jamming value, as observed in experiments. We also find that the fabric and force anisotropies extracted from the contact network are well described by the modified inertial number, thus providing clear evidence for the role of these key structural parameters in dense suspensions.

VL - 96 IS - 1 JO - Phys. Rev. E ER - TY - JOUR T1 - Wall roughness and nonlinear velocity profiles in granular shear flows JF - EPJ Web of Conferences Y1 - 2017 A1 - Schuhmacher, Paul A1 - Farhang Radjaï A1 - Stéphane Roux ED - Saeid Nezamabadi ED - Luding, S. ED - Jean-Yves Delenne AB -Inhomogeneous velocity profiles in granular flows are well known from both experiments and simulations, and considered as a hallmark of nonlocal behavior. By means of extensive contact dynamics simulations, we show that the sigmoidal velocity profiles in 2D flows of rigid disks are controlled by the roughness of driving boundary walls. We find that the velocity profile becomes linear for a critical value of wall roughness up to an exponential decay close to the walls with a characteristic length that does not depend on the flow thickness and rate. We describe the velocity profiles by introducing a state parameter that carries wall perturbation. By assuming that the local shear rate is a linear function of the state parameter, we obtain an analytical expression that fits velocity profiles. In this model, the nonlinear velocity profiles are explained in terms of the effects of wall roughness as boundary condition for the state parameter.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714003090 JO - EPJ Web Conf. ER - TY - JOUR T1 - Binary mixtures of disks and elongated particles: Texture and mechanical properties JF - Physical Review E Y1 - 2016 A1 - Emilien Azéma A1 - Preechawuttipong, Itthichai A1 - Farhang Radjaï AB -We analyze the shear strength and microstructure of binary granular mixtures consisting of disks and elongated particles by varying systematically both the mixture ratio and degree of homogeneity (from homogeneous to fully segregated). The contact dynamics method is used for numerical simulations with rigid particles interacting by frictional contacts. A counterintuitive finding of this work is that the shear strength, packing fraction, and, at the microscopic scale, the fabric, force, and friction anisotropies of the contact network are all nearly independent of the degree of homogeneity. In other words, homogeneous mixtures have the same strength properties as segregated packings of the two particle shapes. In contrast, the shear strength increases with the proportion of elongated particles correlatively with the increase of the corresponding force and fabric anisotropies. By a detailed analysis of the contact network topology, we show that various contact types contribute differently to force transmission and friction mobilization.

VL - 94 IS - 4 JO - Phys. Rev. E ER - TY - JOUR T1 - Bottom-up model for understanding the effects of wheat endosperm microstructure on its mechanical strength JF - Journal of Food Engineering Y1 - 2016 A1 - Chichti, Emna A1 - Valérie Lullien-Pellerina A1 - George, Matthieu A1 - Farhang Radjaï A1 - Rafik Affès A1 - Jean-Yves Delenne AB -Wheat flours are essential ingredients of daily food products like bread, cookies or pastries. Their quality depends on the milling process and mechanical strength of wheat grains. Although it is well known that the strength and rupture of grains are strongly controlled by the endosperm microstructure, the respective roles of the starch and polymer volume fractions and their adhesion are not yet fully understood. This typical biological microstructure can be modeled as a cemented granular material, where the two size populations of starch granules (large:A-type, small:B-type) are the particles, and the protein matrix, which partially fills the space between granules, plays the role of a cement. This structural model of wheat endosperm is used, together with mechanical characteristics of starch and proteins obtained by means of Atomic Force Microscopy (AFM) measurements, to simulate the mechanical behavior and breakage of wheat endosperm in milling process. We find that the porosity outweighs the effect of other parameters for the elastic modulus, which declines as a nearly linear function of porosity. We also show that the tensile strength is an increasing function of the amount and connectivity of starch granules with increasing concentration of stresses along chains of granules. This effect is more significant at low porosity where stress distribution is mainly controlled by the contact network between starch granules. This effect explains why the protein content is not fully correlated to vitreousness, and samples of similar protein content can be different in vitreosity. Finally, we find that the starch-granule adhesion strongly affects the tensile strength whereas the effect of starch volume fraction appears mainly at high interface adhesion, which is the case of hard type wheat grains.

VL - 190 JO - Journal of Food Engineering ER - TY - Generic T1 - Contribution of mechanical factors to the variability of root architecture: Quantifying the past history of interaction forces between growing roots and soil grains T2 - 2016 IEEE INTERNATIONAL CONFERENCE ON FUNCTIONAL-STRUCTURAL PLANT GROWTH MODELING, SIMULATION, VISUALIZATION AND APPLICATIONS (FSPMA) Y1 - 2016 A1 - Fakih, Mahmoud A1 - Jean-Yves Delenne A1 - Farhang Radjaï A1 - Fourcaud, Thierry AB -The relation between a growing root and the soil movement has often been under-estimated. The present work aims to determine how grains in granular soils are reorganized by the action of growing roots, and in turn how the resulting forces acting on root tips modify their development. For this purpose, we have developed a 2D Discrete Element Model (DEM) able to compute a numerical growth of a single root inside a granular medium, taking into account the grain-grain and the root-grain contact forces during the growth. First in silico simulations were carried out in order to : 1-quantify the influence of the granular structure (grain diameter distribution and gaps) and root mechanical properties (root bending stiffness) on the evolution of reaction forces applied to a single root during its growth; 2-highlight “group effects”, e.g. how the reorganization of grains and their interaction forces due to a given growing root can affect the mechanical signal perceived by its near neighbours; 3-investigate how the presence of initial channels within the granular medium can effect the growth trajectory and minimize the resistance to penetration. All simulations were carried out assuming that root growth direction was only driven by external forces. Simlation results allowed the extraction of general physical laws that will be used further to provide mechanoperceptive indicators and analyze experimental data provided by phenotyping platforms. The final objective will be to quantify the response of plants to mechanical stresses in terms of root elongation rate, root straightness and ramification.

JF - 2016 IEEE INTERNATIONAL CONFERENCE ON FUNCTIONAL-STRUCTURAL PLANT GROWTH MODELING, SIMULATION, VISUALIZATION AND APPLICATIONS (FSPMA) PB - I E E E CY - NOV 07-11, 2016, Qingdao, PEOPLES R CHINA ER - TY - JOUR T1 - An equation of state for granular media at the limit state of isotropic compression JF - EPL (Europhysics Letters) Y1 - 2016 A1 - Oquendo, W. F. A1 - Muñoz, J. D. A1 - Farhang Radjaï AB -It is well believed that the volumetric entropy of Edwards captures part of the physics of granular media, but it is still unclear whether it can be applied to granular systems under mechanical stress. By working out a recent proposal by Aste, Di Matteo *et al.* to measure Edwards' compactivity from the volume distribution of Voronoï or Delaunay tessellations (*Phys. Rev. E*, **77** (2008) 021309), and assuming that the total volume divides into elementary cells of fixed minimal volume, we derive an equation of state relating the compactivity to the packing fraction, and we show by extensive molecular-dynamics simulations that this equation and its underlying assumption describe well the volumetric aspects of both the limit state of isotropic compression and the limit state of shear (also called *critical state* in soil mechanics) for three-dimensional ensembles of mono-disperse spheres, for a broad range of the sliding and rolling friction coefficients. In addition, by using the limit state of isotropic compression as testing ground, we find that the compactivity, the entropy per elementary cell and the number of elementary cells per grain computed by this method are the same within statistical precision, either by using Voronoï, Delaunay, or centroidal Voronoï tessellations, allowing thus for an objective definition. This means that not only Aste's cell method is robust and suitable to measure Edwards' compactivity of granular systems under mechanical stress but also the actual nature of the elementary cells might be unimportant.

We use capillary condensation simulated by a multiphase Lattice Boltzmann model as a means to generate homogeneous distributions of liquid clusters in 2D granular media. Liquid droplets condense from the vapour phase between and on the grains, and they transform into capillary bonds and liquid clusters as thermodynamic equilibrium is approached. As the amount of condensed liquid is increased, liquid clusters of increasing connectivity are formed and the distribution of liquid undergoes topological transitions until the whole pore space is filled by the liquid. We investigate the cluster statistics and local grain environments. From extensive simulations, we also obtain the mean Laplace pressure as a function of the amount of liquid, which is found to be quite similar to the well-known experimental retention curve in soil mechanics. The tensile stress carried by the grains increases as a function of the amount of condensed liquid up to a peak in the funicular state beyond which the stress falls off as a result of pressure drop inside the merging clusters.

VL - 80 IS - SI JO - Computers and Geotechnics ER - TY - JOUR T1 - Nanoporous chalcogenides for adsorption and gas separation JF - Physical Chemistry Chemical Physics Y1 - 2016 A1 - Guido Ori A1 - Carlo Massobrio A1 - Pradel, Annie A1 - Ribes, Miche A1 - Benoit A. Coasne AB -The adsorption and gas separation properties of amorphous porous chalcogenides such as GeS2 are investigated using statistical mechanics molecular simulation. Using a realistic molecular model of such amorphous adsorbents, we show that they can be used efficiently to separate different gases relevant to environmental and energy applications (H-2, CO2, CH4, N-2). In addition to shedding light on the microscopic adsorption mechanisms, we show that coadsorption in this novel class of porous materials can be described using the ideal adsorbed solution theory (IAST). Such a simple thermodynamic model, which allows avoiding complex coadsorption measurements, describes the adsorption of mixture from pure component adsorption isotherms. Our results, which are found to be in good agreement with available experimental data, paves the way for the design of gas separation membranes using the large family of porous chalcogenides.

VL - 18 UR - http://xlink.rsc.org/?DOI=C6CP00467A IS - 19 JO - Phys. Chem. Chem. Phys. ER - TY - JOUR T1 - Probing Interconnectivity in Hierarchical Microporous/Mesoporous Materials Using Adsorption and Nuclear Magnetic Resonance Diffusion JF - The Journal of Physical Chemistry C Y1 - 2016 A1 - Anne Galarneau A1 - Guenneau, Flavien A1 - Gedeon, Antoine A1 - Mereib, Diaa A1 - Rodriguez, Jeremy A1 - Fajula, François A1 - Benoit A. Coasne AB -Adsorption and transport in hierarchical materials are investigated by means of adsorption and nuclear magnetic resonance experiments. Using micro/mesoporous zeolites with well-defined mesoporosity, we show that adsorption at a given pressure can be described as a simple linear combination of the adsorbed amounts taken at the same pressure for the pure microporous (zeolite FAU-Y) and mesoporous (Al-MCM-41) solids. Such a quantitative decomposition allows us to demonstrate the ability of diffusion measurements by Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) to probe interconnectivity in hierarchical solids. On the one hand, transport in the mechanical mixtures can be described as the superimposition of diffusion in pure microporous and mesoporous solids. On the other hand, PFG NMR for the hierarchical sample provides an effective diffusivity that is intermediate between those for the pure zeolite and mesoporous silica. Furthermore, this effective diffusivity is slower than the linear combination of the two diffusivities weighted by the number of molecules present in each phase (used in the independent domain and fast-exchange theories) clearly showing interconnectivities and transfer limitations between the microporous and mesoporous domains. We also discuss the ability of combining theories such as the fast exchange model and the effective medium theory to quantitatively predict diffusion in such microporous/mesoporous materials.

VL - 120 UR - http://pubs.acs.org/doi/10.1021/acs.jpcc.5b10129 IS - 3 JO - J. Phys. Chem. C ER - TY - JOUR T1 - Scaling behaviour of cohesive granular flows JF - EPL (Europhysics Letters) Y1 - 2016 A1 - Berger, Nicolas A1 - Emilien Azéma A1 - Douce, Jean-François A1 - Farhang Radjaï AB -The shear strength of dense granular flows is generally described by an effective friction coefficient, ratio of shear to normal stress, as a function of the inertial number *I*. However, this ratio depends on the normal stress when the particles interact via both friction and adhesion forces, and in this sense it does not properly represent a Coulomb-like friction. For the same reason, it is not a unique function of *I*. We used extensive contact dynamics simulations to isolate the cohesive strength from the purely frictional strength in dense inertial flows for a broad range of shear rates and adhesion forces between particles. Remarkably, while the frictional part of the strength increases with *I*, the cohesive strength is found to be a decreasing function of *I*. We show that a single dimensionless parameter, combining interparticle adhesion with *I*, controls not only the cohesive strength but also the packing fraction and granular texture in inertial flows.

This special issue of the *European Physical Journal Special Topics* is dedicated to selected papers from the symposium “High surface area porous and granular materials” organized in the frame of the conference “Matériaux 2014”, held on November 24–28, 2014 in Montpellier, France.

Porous materials and granular materials gather a wide variety of heterogeneous, isotropic or anisotropic media made of inorganic, organic or hybrid solid skeletons, with open or closed porosity, and pore sizes ranging from the centimeter scale to the sub-nanometer scale. Their technological and industrial applications cover numerous areas from building and civil engineering to microelectronics, including also metallurgy, chemistry, health, waste water and gas effluent treatment. Many emerging processes related to environmental protection and sustainable development also rely on this class of materials. Their functional properties are related to specific transfer mechanisms (matter, heat, radiation, electrical charge), to pore surface chemistry (exchange, adsorption, heterogeneous catalysis) and to retention inside confined volumes (storage, separation, exchange, controlled release). The development of innovative synthesis, shaping, characterization and modeling approaches enables the design of advanced materials with enhanced functional performance. The papers collected in this special issue offer a good overview of the state-of-the-art and science of these complex media.

We would like to thank all the speakers and participants for their contribution to the success of the symposium. We also express our gratitude to the organization committee of “Mat´eriaux 2014”. We finally thank the reviewers and the staff of the European Physical Journal Special Topics who made the publication of this special issue possible.

VL - 224 IS - 9 JO - Eur. Phys. J. Spec. Top. ER - TY - JOUR T1 - Bonded-cell model for particle fracture JF - Physical Review E Y1 - 2015 A1 - Duc-Hanh Nguyen A1 - Emilien Azéma A1 - Philippe Sornay A1 - Farhang Radjaï AB -Particle degradation and fracture play an important role in natural granular flows and in many applications of granular materials. We analyze the fracture properties of two-dimensional disklike particles modeled as aggregates of rigid cells bonded along their sides by a cohesive Mohr-Coulomb law and simulated by the contact dynamics method. We show that the compressive strength scales with tensile strength between cells but depends also on the friction coefficient and a parameter describing cell shape distribution. The statistical scatter of compressive strength is well described by the Weibull distribution function with a shape parameter varying from 6 to 10 depending on cell shape distribution. We show that this distribution may be understood in terms of percolating critical intercellular contacts. We propose a random-walk model of critical contacts that leads to particle size dependence of the compressive strength in good agreement with our simulation data.

VL - 91 IS - 2 JO - Phys. Rev. E ER - TY - Generic T1 - Creep of Bulk C-S-H: Insights from Molecular Dynamics Simulations T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 Y1 - 2015 A1 - Mathieu Bauchy A1 - Enrico Masoero A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -Understanding the physical origin of creep in calcium–silicate–hydrate (C–S–H) is of primary importance, both for fundamental and practical interest. Here, we present a new method, based on molecular dynamics simulation, allowing us to simulate the long-term visco-elastic deformations of C–S–H. Under a given shear stress, C–S–H features a gradually increasing shear strain, which follows a logarithmic law. The computed creep modulus is found to be independent of the shear stress applied and is in excellent agreement with nanoindentation measurements, as extrapolated to zero porosity.

JF - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 PB - American Society of Civil Engineers CY - September 21–23, 2015, Vienna, AustriaReston, VA VL - CONCREEP 10: MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES ER - TY - Generic T1 - Creep of Clay: Numerical Results at the Scale of a Layer and Experimental Results at the Scale of Thin Self-Standing Films T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 Y1 - 2015 A1 - Benoit Carrier A1 - Matthieu Vandamme A1 - Roland Jean-Marc Pellenq A1 - Henri Van Damme ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -This work focuses on the creep of clay-based materials, which exhibit significant analogies with cement-based materials. Here, we studied the creep of clay at two scales and with two techniques: numerically (with molecular simulations) at the scale of a clay layer (nm), and experimentally at the scale of thin self-standing clay films (few dozen *μ*m). At the scale of the clay layer, numerical simulations showed that the shear rate was constant over time and an affine function of the shear stress. Creep experiments showed that, after a transient period, the creep function of our thin self-standing clay films was a logarithmic function of time. A comparison of the results obtained at the two scales shows that the origin of the logarithmic feature of clay creep must at least partly originate from a scale greater than that of an individual clay layer. By analogy, such result is likely to hold for cementitious materials, which are also known to creep logarithmically with respect to time in the long term: the origin of this logarithmic feature is likely to stem at least partly from a scale greater than the scale of an individual C-S-H layer.

Despite their impact on longevity, serviceability, and environmental footprint of our built infrastructure, the chemo-physical origins of nanoscale properties of cementitious materials, and their link to macroscale properties still remain rather obscure. Here, we discuss a multi-scale approach that describes different aspects of physical properties of C-S-H at the nano- and meso-scales. These include dynamics of water, thermal properties and mechanical behavior of C-S-H and its effect on properties of cement paste at different scales.

JF - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 PB - American Society of Civil Engineers CY - September 21–23, 2015, Vienna, AustriaReston, VA VL - CONCREEP 10: MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES UR - http://ascelibrary.org/doi/book/10.1061/9780784479346http://ascelibrary.org/doi/pdf/10.1061/9780784479346http://ascelibrary.org/doi/10.1061/9780784479346.006http://ascelibrary.org/doi/pdf/10.1061/9780784479346.006 ER - TY - JOUR T1 - Effect of curvature and confinement on the Casimir-Polder interaction JF - Physical Review A Y1 - 2015 A1 - Rodriguez-Lopez, Pablo A1 - Emig, Thorsten A1 - Noruzifar, Ehsan A1 - Zandi, Roya AB -Modifications of Casimir-Polder interactions due to confinement inside a cylindrical cavity and due to curvature in- and outside the cavity are studied. We consider a perfectly conducting cylindrical shell with a single particle (atom or macroscopic sphere) located next to its interior or exterior surface, or two atoms placed inside the shell. By employing the scattering approach, we obtain the particle-cavity interaction and the modification of the two-particle interaction due to the cavity. We consider both retardation and thermal effects. While for the atoms a dipole description is sufficient, for the macroscopic sphere we sum (numerically) over many multipole fluctuations to compute the interaction at short separations. In the latter limit we compare to the proximity approximation and a gradient expansion and find agreement. Our results indicate an confinement induced suppression of the force between atoms. General criteria for suppression and enhancement of Casimir interactions due to confinement are discussed.

VL - 91 IS - 1 JO - Phys. Rev. A ER - TY - JOUR T1 - Effects of shape and size polydispersity on strength properties of granular materials. JF - Phys Rev E Stat Nonlin Soft Matter Phys Y1 - 2015 A1 - Duc-Hanh Nguyen A1 - Emilien Azéma A1 - Philippe Sornay A1 - Farhang Radjaï AB -By means of extensive contact dynamics simulations, we analyze the combined effects of polydispersity both in particle size and in particle shape, defined as the degree of shape irregularity, on the shear strength and microstructure of sheared granular materials composed of pentagonal particles. We find that the shear strength is independent of the size span, but unexpectedly, it declines with increasing shape polydispersity. At the same time, the solid fraction is an increasing function of both the size span and the shape polydispersity. Hence, the densest and loosest packings have the same shear strength. At the scale of the particles and their contacts, we analyze the connectivity of particles, force transmission, and friction mobilization as well as their anisotropies. We show that stronger forces are carried by larger particles and propped by an increasing number of small particles. The independence of shear strength with regard to size span is shown to be a consequence of contact network self-organization, with the falloff of contact anisotropy compensated by increasing force anisotropy.

VL - 91 IS - 3 ER - TY - Generic T1 - Evolution of particle size distributions in crushable granular materials T2 - 3rd International Symposium on Geomechanics from Micro to Macro Y1 - 2015 A1 - Duc-Hanh Nguyen A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Philippe Sornay ED - Kenichi Soga ED - Krishna Kumar ED - Giovanna Biscontin ED - Kuo, Matthew AB -

By means of the contact dynamics method together with a particle fracture model, in which the particles are cohesive aggregates of irreducible polygonal fragments, we investigate the evolution of particle size distribution in the process of uniaxial compaction of granular materials. The case of single particle breakup under compressive stress is used to test the method and the influence of discretization (number of irreducible fragments). We show that the breaking threshold of the granular assembly scales with the internal cohesion of the particles but it depends also on the initial size distribution and irregularity of polygonal particle shapes. The evolution of size distribution proceeds by consecutive periods of intense particle crushing, characterized by local shattering instability, and periods of little breaking activity. Starting with either monodisperse or power-law distribution of particle sizes, the latter evolves towards a broad distribution of the fragmented particles with a nearly power-law distribution in the range of intermediate particle sizes. Interestingly, a finite number of large particles survive despite ongoing crushing process due to the more homogeneous distribution of forces in the presence of small fragmented particles filling the pores between larger particles.

JF - 3rd International Symposium on Geomechanics from Micro to Macro PB - CRC Press CY - SEP 01-03-2014 Univ Cambridge, Cambridge, ENGLAND VL - Geomechanics from Micro to Macro SN - 978-1-138-02707-7 ER - TY - JOUR T1 - First-principles prediction of kink-pair activation enthalpy on screw dislocations in bcc transition metals: V, Nb, Ta, Mo, W, and Fe JF - Physical Review B Y1 - 2015 A1 - Dezerald, Lucile A1 - Proville, L. A1 - Ventelon, Lisa A1 - Willaime, F. A1 - Rodney, D. AB -The atomistic study of kink pairs on screw dislocations in body-centered cubic (bcc) metals is challenging because interatomic potentials in bcc metals still lack accuracy and kink pairs require too many atoms to be modeled by first principles. Here, we circumvent this difficulty using a one-dimensional line tension model whose parameters, namely the line tension and Peierls barrier, are reachable to density functional theory calculations. The model parameterized in V, Nb, Ta, Mo, W, and Fe, is used to study the kink-pair activation enthalpy and spatial extension. Interestingly, we find that the atomistic line tension is more than twice the usual elastic estimates. The calculations also show interesting group tendencies with the line tension and kink-pair width larger in group V than in group VI elements. Finally, the present kink-pair activation energies are shown to compare qualitatively with experimental data and potential origins of quantitative discrepancies are discussed.

VL - 91 UR - https://link.aps.org/doi/10.1103/PhysRevB.91.094105 IS - 9 JO - Phys. Rev. B ER - TY - Generic T1 - Hydration Kinetics and Gel Morphology of C-S-H T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 Y1 - 2015 A1 - Katerina Ioannidou A1 - Enrico Masoero A1 - Pierre E. Levitz A1 - Roland Jean-Marc Pellenq A1 - Emanuela Del Gado ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -Calcium-silicate hydrate (C-S-H) is the main binder in cement and concrete. It starts forming from the early stages of cement hydration and it progressively densifies as cement sets. C-S-H nanoscale building blocks form a cohesive gel, whose structure and mechanics are still poorly understood, in spite of its practical importance. Here we review a statistical physics approach recently developed, which allows us to investigate the C-S-H gel formation under the out-of-equilibrium conditions typical of cement hydration. Our approach is based on colloidal particles, precipitating in the pore solution and interacting with effective forces associated to the ionic environment. We present the evolution of the space filling of C-S-H with different particle interactions and compare them with experimental data at different lime concentrations. Moreover, we discuss the structural features of C-S-H in the mesoscale in terms of the scattering intensity. The comparison of our early stage C-S-H structures with small angle neutron scattering (SANS) experiments shows that long range spatial correlations and structural heterogeneties that develop in that early stages of hydration persist also in the hardened paste.

JF - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 PB - American Society of Civil Engineers CY - September 21–23, 2015, Vienna, AustriaReston, VA VL - CONCREEP 10: MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES ER - TY - JOUR T1 - Implicit frictional-contact model for soft particle systems JF - Journal of the Mechanics and Physics of Solids Y1 - 2015 A1 - Saeid Nezamabadi A1 - Farhang Radjaï A1 - Julien Averseng A1 - Jean-Yves Delenne AB -We introduce a novel numerical approach for the simulation of soft particles interacting via frictional contacts. This approach is based on an implicit formulation of the Material Point Method, allowing for large particle deformations, combined with the Contact Dynamics method for the treatment of unilateral frictional contacts between particles. This approach is both precise due to the treatment of contacts with no regularization and artificial damping parameters, and robust due to implicit time integration of both bulk degrees of freedom and relative contact velocities at the nodes representing the contact points. By construction, our algorithm is capable of handling arbitrary particle shapes and deformations. We illustrate this approach by two simple 2D examples: a Hertz contact and a rolling particle on an inclined plane. We also investigate the compaction of a packing of circular particles up to a solid fraction well above the jamming limit of hard particles. We find that, for the same level of deformation, the solid fraction in a packing of frictional particles is above that of a packing of frictionless particles as a result of larger particle shape change.

VL - 83 JO - Journal of the Mechanics and Physics of Solids ER - TY - JOUR T1 - Internal friction and absence of dilatancy of packings of frictionless polygons JF - Physical Review E Y1 - 2015 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Jean-Noel Roux AB -By means of numerical simulations, we show that assemblies of frictionless rigid pentagons in slow shear flow possess an internal friction coefficient (equalto0.183±0.008 with our choice of moderately polydisperse grains) but no macroscopic dilatancy. In other words, despite side-side contacts tending to hinder relative particle rotations, the solidfraction under quasistatic shear coincides with that of isotropic random close packings of pentagonal particles. Properties of polygonal grains are thus similar to those of disks in that respect. We argue that continuous reshuffling of the force-bearing network leads to frequent collapsing events at the microscale, thereby causing the macroscopic dilatancy to vanish. Despite such rearrangements, the shear flow favors an anisotropic structure that is at the origin of the ability of the system to sustain shear stress.

VL - 91 IS - 1 JO - Phys. Rev. E ER - TY - Generic T1 - Kinetic Simulations of Cement Creep: Mechanisms from Shear Deformations of Glasses T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 Y1 - 2015 A1 - Enrico Masoero A1 - Mathieu Bauchy A1 - Emanuela Del Gado A1 - Hegoi Manzano A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Sidney Yip ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -The logarithmic deviatoric creep of cement paste is a technical and scientific challenge. Transition State Theory (TST) indicates that some nanoscale mechanisms of shear deformation, associated with a specific kind of strain hardening, can explain the type of deviatoric creep observed experimentally in mature cement pastes. To test this possible explanation, we simulate the shear deformations of a colloidal model of cement hydrates at the nanoscale. Results from quasi-static simulations indicate a strain hardening analogous to that postulated by the TST approach. Additional results from oscillatory shear (fatigue) simulations show an increase of deformation with number of loading cycles that is consistent with the observed creep. These findings indicate that nanoscale simulations can improve our current understanding of the mechanisms underlying creep, with potential to go beyond the logarithmic creep and explore the onset of failure during tertiary creep.

JF - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 PB - American Society of Civil Engineers CY - September 21–23, 2015, Vienna, AustriaReston, VA VL - CONCREEP 10: MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES ER - TY - JOUR T1 - Les matériaux poreux : un domaine scientifique et technologique pluridisciplinaire au futur radieux JF - Matériaux & Techniques Y1 - 2015 A1 - Ayral, Andre A1 - Calas-Etienne, Sylvie A1 - Benoit A. Coasne A1 - Deratani, André A1 - Grande, Daniel A1 - Quemener, Damien A1 - Rossignol, Sylvie AB -Porous materials cover a broad multidisciplinary scientific and technological domain. They are used in numerous existing and emerging devices and processes related to key societal issues both in environmental protection and for the benefit of the economy. Our choice was to illustrate advances in porous materials through three specific examples: porous materials engineering in the case of organic mesoporous materials prepared from block polymers, modeling and numerical simulation of adsorption of molecular species at the nanoscale and, finally technological applications as thermal insulators.

VL - 103 UR - http://www.mattech-journal.org/10.1051/mattech/2015050 IS - 7 JO - Matériaux & Techniques ER - TY - JOUR T1 - Liquid clustering and capillary pressure in granular media JF - Journal of Fluid Mechanics Y1 - 2015 A1 - Jean-Yves Delenne A1 - Vincent Richefeu A1 - Farhang Radjaï AB -By means of extensive lattice Boltzmann simulations, we investigate the process of growth and coalescence of liquid clusters in a granular material as the amount of liquid increases. A homogeneous grain–liquid mixture is obtained by means of capillary condensation, thus providing meaningful statistics on the liquid distribution inside the granular material. The tensile stress carried by the grains as a function of the amount of condensed liquid reveals four distinct states, with a peak stress occurring at the transition from a primary coalescence process, where the cohesive strength is carried mostly by the grains, to a secondary process governed by the increase of the liquid cluster volumes. We show that the evolution of capillary states is correctly captured by a simple model accounting for the competing effects of the Laplace pressure and grain–liquid interface.

VL - 762 JO - J. Fluid Mech. ER - TY - JOUR T1 - Magnetic nanopantograph in the SrCuMagnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu2(BO3)(2) (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the CuOCu superexchange angle that, due to the orthogonal Cu2+ dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of similar to 10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO.

VL - 112 UR - http://www.pnas.org/lookup/doi/10.1073/pnas.1421414112 IS - 7 JO - Proc Natl Acad Sci USA ER - TY - JOUR T1 - Magnetic nanopantograph in the SrCu2(BO3)2 Shastry–Sutherland lattice JF - Proceedings of the National Academy of Sciences Y1 - 2015 A1 - Guillaume Radtke A1 - Andres Saùl A1 - Dabkowska, Hanna A. A1 - Salamon, Myron B. A1 - Jaime, Marcelo AB -Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu2(BO3)2 (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the CuOCu^ superexchange angle that, due to the orthogonal Cu2+ dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of ∼10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO.

VL - 112 ER - TY - JOUR T1 - The Maya blue nanostructured material concept applied to colouring geopolymers JF - RSC Adv. Y1 - 2015 A1 - Ouellet-Plamondon, Claudiane A1 - Aranda, Pilar A1 - Favier, Aurélie A1 - Habert, Guillaume A1 - Henri Van Damme A1 - Ruiz-Hitzky, Eduardo AB -Maya blue is an ancient nanostructured pigment synthetized by assembling indigo, a natural dye, with palygorskite, a microfibrous clay mineral. The novelty of our approach is to mimic “pre-Columbian nanotechnology” and to functionalize geopolymers with a sepiolite-based hybrid organic–inorganic nanocomposite inspired from the Maya blue. It is acid- and UV-resistant, as confirmed by the stability of Maya mural paintings over time. We synthesized analogous pigments, using methylene blue (MB) and methyl red (MR) as organic dyes and sepiolite as fibrous clay mineral. We used an aqueous and a solid-state method, both leading to encapsulation of dye monomers into the clay micropores, as confirmed by UV-vis spectroscopy. This nanostructured pigment was then included into a geopolymer matrix at room temperature. The stability of the new material to UV and acid was tested. It was confirmed that it is the prior encapsulation of the dye into sepiolite that leads to the stability of the pigment in the geopolymer matrix. This first study opens the way to numerous possibilities for functionalizing inorganic binder materials with organic elements that would be otherwise sensitive to thermal treatment in conventional ceramic processing.

VL - 5 IS - 120 JO - RSC Adv. ER - TY - JOUR T1 - Mechanism of H2O Insertion and Chemical Bond Formation in AlPO 4-54·x H2O at High Pressure JF - Journal of the American Chemical Society Y1 - 2015 A1 - Alabarse, Frederico G. A1 - Rouquette, Jérôme A1 - Benoit A. Coasne A1 - Haidoux, Abel A1 - Paulmann, Carsten A1 - Cambon, Olivier A1 - Haines, Julien AB -The insertion of H2O in AlPO4-54•xH2O at high pressure was investigated by single-crystal x-ray diffraction and Monte Carlo molecular simulation. H2O molecules are concentrated, in particular near the pore walls. Upon insertion, the additional water is highly disordered. Insertion of H2O (superhydration) is found to impede pore collapse in the material, thereby strongly modifying its mechanical behavior. However, instead of stabilizing the structure with respect to amorphization, the results provide evidence for the early stages of chemical bond formation between H2O molecules and tetrahedrally coordinated aluminum, which is at the origin of the amorphization/reaction process.

VL - 137 IS - 2 JO - J. Am. Chem. Soc. ER - TY - Generic T1 - The Meso-Scale Texture of Cement Hydrate Gels: Out-of-Equilibrium Evolution and Thermodynamic Driving T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete Structures CONCREEP 10 Y1 - 2015 A1 - Emanuela Del Gado A1 - Katerina Ioannidou A1 - Enrico Masoero A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Sidney Yip ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -By the end of cement hydration calcium-silicate-hydrate (C-S-H) gels extends over tens and hundreds of nanometers. Their complex texture affects directly, and to a large extent, the macroscopic hygrothermal and mechanical behavior of cement. Here we review a statistical physics approach recently developed, which allows us to investigate the gel formation under the out-of-equilibrium conditions typical of cement hydration and the role of the nano-scale structure in C—S—H mechanics upon hardening. Our investigations have unveiled the role, in the C-S-H gels, of nano-scale structural and mechanical heterogeneities that develop due to the the far-from-equilibrium physico-chemical environment in which the material forms. A subtle interplay between the out-of-equilibrium evolution and the effective interactions emerging between the nano-scale units of the gels at different stages of the hydration process ultimately determines the mesoscale texture of cement hydrates and their material properties.

JF - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete Structures CONCREEP 10 PB - American Society of Civil Engineers CY - September 21–23, 2015 Vienna, AustriaReston, VA VL - CONCREEP 10: MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES ER - TY - Generic T1 - Micromechanical analysis of the surface erosion of a cohesive soil by means of a coupled LBM-DEM model T2 - IV International Conference on Particle-based Methods (PARTICLES 2015) Y1 - 2015 A1 - Cuellar, Pablo A1 - Pierre Philippe A1 - Stéphane Bonelli A1 - Benahmed, Nadia A1 - Brunier-Coulin, Florian A1 - Jeff Ngoma A1 - Jean-Yves Delenne A1 - Farhang Radjaï ED - Onate, E ED - Bischoff, M ED - Owen, DRJ KW - Cohesion KW - DEM KW - LBM KW - Surface erosion AB -The elementary mechanisms driving the ubiquitous surface erosion of cohesive geomaterials can be analysed from a micromechanical perspective combining well-known numerical techniques. Here, a coupled model combining the Discrete Element and Lattice Boltzmann methods (DEM-LBM) provides an insight into the solid-fluid interaction during the transient erosion caused by a vertical fluid jet impinging on the surface of a granular assembly. The brittle cementation providing cohesion between the solid grains is described here by means of a simple bond model with a single-parameter yield surface. The initial topology of the surface erosion tends to mimic the profile of fluid velocity directly above the soil surface. We find that both the rate of erosion and the magnitude of eroded mass depend directly on the micromechanical strength of the single solid bonds.

JF - IV International Conference on Particle-based Methods (PARTICLES 2015) PB - International Center for Numerical Methods in Engineering (CIMNE) CY - SEP 28-30 2015 Barcelona, SPAIN VL - PARTICLE-BASED METHODS IV-FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-01269324 ER - TY - JOUR T1 - Modeling force transmission in granular materials JF - Comptes Rendus Physique Y1 - 2015 A1 - Farhang Radjaï AB -The probability density function of contact forces in granular materials has been extensively studied and modeled as an outstanding signature of granular microstructure. Arguing that particle environments play a fundamental role in force transmission, we analyze the effects of steric constraints with respect to force balance condition and show that each force may be considered as resulting from a balance between lower and larger forces in proportions that mainly depend on steric effects. This idea leads to a general model that predicts an analytical expression of force density with a single free parameter. This expression fits well our simulation data and generically predicts the exponential fall-off of strong forces, a small peak below the mean force and the non-zero probability of vanishingly small forces.

VL - 16 IS - 1 JO - Comptes Rendus Physique ER - TY - Generic T1 - Numerical simulation of granular media composed with irregular polyhedral particles: effect of particles' angularity T2 - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) Y1 - 2015 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Dubois, Frédéric ED - Onate, E ED - Owen, DRJ KW - angularity KW - force transmission KW - Granular Materials KW - particle shape KW - texture AB -We use contact dynamic simulations to perform a systematic investigation of the effects of particles shape angularity on mechanicals response in sheared granular ma-terials. The particles are irregular polyhedra with varying numbers of face from spheres to "double pyramid" shape with a constant aspect ratio. We study the quasi-static behav-ior, structural and force anisotropies of several packings subjected to triaxial compression. An interesting finding is that the shear strength first increases with angularity up to a maximum value and then saturates as the particles become more angular. Analyzing the anisotropies induced by the angular distributions of contacts and forces orientations, we show that the saturation of the shear strength at higher angularities is a consequence of fall-off of the texture anisotropies compensated by an increase of the tangential force anisotropy. This is attributed to the fact that at higher angularity, particles are bet-ter connected (or surrounded) leading to an increase of friction mobilization in order to achieve the deformation. Moreover, the most angular particles also have very few sides so that, this effect is enhanced by the increase of the proportion of face-side and side-side contacts with angularity.

JF - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) CY - OCT 26-28 2011 Barcelona, SPAIN VL - PARTICLE-BASED METHODS II: FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-01112373 ER - TY - JOUR T1 - Predicting Adsorption on Bare and Modified Silica Surfaces JF - The Journal of Physical Chemistry C Y1 - 2015 A1 - Lépinay, Matthieu A1 - Broussous, Lucile A1 - Licitra, Christophe A1 - Bertin, François A1 - Rouessac, Vincent A1 - Ayral, Andre A1 - Benoit A. Coasne AB -We show that Derjaguin’s theory of adsorption can be used to predict adsorption on bare and modified surfaces using parameters available to simple experiments. Using experiment and molecular simulation of adsorption of various gases on hydroxylated, methylated, and trifluoromethylated silica, this simple parametrization of Derjaguin’s model allows predicting adsorption on any functionalized surface using a minimum set of parameters such as the heat of vaporization of the adsorbate and the Henry constant of the adsorption isotherm. This general yet simple scheme constitutes a powerful tool as it avoids having to carry out tedious and complex adsorption measurements.

Ellipsometric porosimetry (EP) experiments are performed to obtain the adsorption isotherms of water, methanol, and toluene on pristine and damaged SiOCH porous materials. The use of gaseous adsorbates with different polarities, sizes, and surface tensions enables us to probe their affinity with such organosilica surfaces. Using reference t-curves obtained from Statistical Mechanics molecular simulations, we discuss the ability of the MicroPore analysis (MP) method to accurately estimate micropore sizes from EP measurements by comparing them with the mean pore sizes obtained using positron annihilation lifetime spectroscopy (PALS) and grazing incidence small angle X-ray scattering (GISAXS). We also report accessible microporous volumes estimated from the *t*-plot method used with the reference t-curves obtained by means of molecular simulation. We show that EP characterization of microporous films combined with the MP and *t*-plot methods can be improved by taking into account the effect of the chemical nature of the pore surface on the variation of the adsorbed thickness (*t*-curve).

It has long been argued that shrinkage in cementitious materials can occur even in the absence of drying phenomena. Herein we argue that such shrinkage is attributable to colloidal forces that develop between C-S-H particles during the hydration in an out-of-equilibrium process. We base our argument on meso-scale simulation results, and show via microporomechanics theory that such colloidal forces can lead to the build-up of solid eigenstresses causing macroscopic shrinkage deformation.

JF - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete Structures CONCREEP 10 PB - American Society of Civil Engineers CY - September 21–23, 2015, Vienna, AustriaReston, VA VL - CONCREEP 10: MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES ER - TY - JOUR T1 - Structure and Dynamics of Ionic Liquids Confined in Amorphous Porous Chalcogenides JF - Langmuir Y1 - 2015 A1 - Guido Ori A1 - Carlo Massobrio A1 - Pradel, Annie A1 - Ribes, Michel A1 - Benoit A. Coasne AB -

Besides the abundant literature on ionic liquids in porous silica and carbon, the confinement of such intriguing liquids in porous chalcogenides has received very little attention. Here, molecular simulation is employed to study the structural and dynamical properties of a typical ionic liquid confined in a realistic molecular model of amorphous chalcogenide with various pore sizes and surface chemistries. Using molecular dynamics in the isobaric–isothermal (NPT) ensemble, we consider confinement conditions relevant to real samples. Both the structure and self-dynamics of the confined phase are found to depend on the *surface*-to-*volume* ratio of the host confining material. Consequently, most properties of the confined ionic liquid can be written as a linear combination of surface and bulk-like contributions, arising from the ions in contact with the surface and the ions in the pore center, respectively. On the other hand, collective dynamical properties such as the ionic conductivity remain close to their bulk counterpart and almost insensitive to pore size and surface chemistry. These results, which are in fair agreement with available experimental data, provide a basis for the development of novel applications using hybrid organic–inorganic solids consisting of ionic liquids confined in porous chalcogenides.

We investigate by means of Contact Dynamics simulations the transient dynamics of a 2D granular pile set into motion by applying shear velocity during a short time interval to all particles. The spreading dynamics is directly controlled by the input energy whereas in recent studies of column collapse the dynamics scales with the initial potential energy of the column. As in column collapse, we observe a power-law dependence of the runout distance with respect to the input energy with nontrivial exponents. This suggests that the power-law behavior is a generic feature of granular dynamics, and the values of the exponents reflect the distribution of kinetic energy inside the material. We observe two regimes with different values of the exponents: the low-energy regime reflects the destabilization of the pile by the impact with a runout time independent of the input energy whereas the high-energy regime is governed by the input energy. We show that the evolution of the pile in the high-energy regime can be described by a characteristic decay time and the available energy after the pile is destabilized.

Beyond a given threshold, an upward fluid flow at constant flowrate, injected through a small size section, is able to generate a fluidization along a vertical chimney over the entire height of a granular assembly. Fluidization is first initiated in the immediate vicinity of the injection hole and then the fluidized zone grows gradually until reaching the upper surface of the granular packing. In this work, we present numerical results on the kinetics of chimney fluidization in an immersed granular bed produced with two-dimensional simulations coupling the Discrete Element and Lattice Boltzmann Methods (DEM-LBM). A parametric study is carried out with 11 different sets of physical parameters and analyzed based on spatio-temporal diagrams. Then a dimensional analysis allows finding general scaling laws for both threshold and growth rate of the fluidized zone by use of two dimensionless numbers, namely Reynolds and Archimedes numbers, while quite simple empirical relationships can also be proposed.

JF - IV International Conference on Particle-based Methods (PARTICLES 2015) PB - International Center for Numerical Methods in Engineering (CIMNE) CY - SEP 28-30 2015 Barcelona, SPAIN VL - PARTICLE-BASED METHODS IV-FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-01269325 ER - TY - JOUR T1 - An atomistic modeling of the xenon bubble behavior in the UO2 matrix JF - Journal of Nuclear Materials Y1 - 2014 A1 - Andrei Jelea A1 - Roland Jean-Marc Pellenq A1 - F. Ribeiro AB -The behavior of the xenon nanoinclusions/bubbles in the uranium dioxide (UO2) matrix and their influence on its swelling were investigated through atomistic simulation techniques. The pressure in bubbles of less than 2 nm in diameter, calculated using a virial equation that takes into account the xenon/matrix interactions, is larger than the pressure calculated in simulations of the equivalent density and temperature of super critical bulk xenon. The radial distribution function of confined xenon is characteristic of a dense (*ρ* > 4 g/cm3) glassy phase. The swelling of the UO2 induced by the intragranular bubbles is proportional to the Xe/U ratio but independent of the temperature.

Density functional calculations have been carried out to investigate the microscopic origin of the magnetic properties of β-TeVO4. Two different approaches, based either on a perturbative treatment of the multiorbital Hubbard model in the strongly correlated limit or on the calculation of supercell total energy differences, have been employed to evaluate magnetic couplings in this compound. The picture provided by these two approaches is that of weakly coupled frustrated chains with ferromagnetic nearest-neighbor and antiferromagnetic second-nearest-neighbor couplings. These results, differing substantially from previous reports, should motivate further experimental investigations of the magnetic properties of this compound.

VL - 89 UR - https://link.aps.org/doi/10.1103/PhysRevB.89.104414 IS - 10 JO - Phys. Rev. B ER - TY - JOUR T1 - A density functional approach of the magnetism of b-TeVO4 JF - Phys. Rev. B. Y1 - 2014 A1 - Andres Saùl A1 - Guillaume Radtke AB -Density functional calculations have been carried out to investigate the microscopic origin of the magnetic properties of

β-TeVO4. Two different approaches, based either on a perturbative treatment of the multiorbital Hubbard model in the strongly correlated limit or on the calculation of supercell total energy differences, have been employed to evaluate magnetic couplings in this compound. The picture provided by these two approaches is that of weakly coupled frustrated chains with ferromagnetic nearest-neighbor and antiferromagnetic second-nearest-neighbor couplings. These results, differing substantially from previous reports, should motivate further experimental investigations of the magnetic properties of this compound.

We present a detailed analysis of the morphology of granular systems composed of frictionless pentagonal particles by varying systematically both the size span and particle shape irregularity, which represent two polydispersity parameters of the system. The microstructure is characterized in terms of various statistical descriptors such as global and local packing fractions, radial distribution functions, coordination number, and fraction of floating particles. We find that the packing fraction increases with the two parameters of polydispersity, but the effect of shape polydispersity for all the investigated structural properties is significant only at low size polydispersity where the positional and/or orientational ordering of the particles prevail. We focus in more detail on the class of side/side contacts, which is the interesting feature of our system as compared to a packing of disks. We show that the proportion of such contacts has weak dependence on the polydispersity parameters. The side- side contacts do not percolate but they define clusters of increasing size as a function of size polydispersity and decreasing size as a function of shape polydispersity. The clusters have anisotropic shapes but with a decreasing aspect ratio as polydispersity increases. This feature is argued to be a consequence of strong force chains (forces above the mean), which are mainly captured by side-side contacts. Finally, the force transmission is intrinsically multiscale, with a mean force increasing linearly with particle size.

VL - 90 IS - 1 JO - Phys. Rev. E ER - TY - JOUR T1 - Erratum to “Static states of cohesive granular media” JF - Journal of Mechanical Science and Technology Y1 - 2014 A1 - Preechawuttipong, I. A1 - Peyroux, R. A1 - Farhang Radjaï A1 - Rangsri, W. AB -This paper investigates the influence of cohesion on the behavior of 2D granular media simulated by using a molecular dynamics method, involving a simple contact law with adhesion. The study considers an adhesion index which is non intrinsic but significant for interpreting the mechanical effects of applied pressure. A static state system of 4000 particles is numerically simulated by oedometrical compression under a constant force, without gravity. The results show how the geometrical texture and the network of contact forces change according to the level of adhesion. This effect is explained essentially in term of the increase in the number of the tensile contacts and by the development of the internal self-stress structure. As in the case of non cohesive granular media, a high spatial heterogeneity of the contact forces is observed.

In this work, we present experimental and numerical results on the interaction between two localized fluidization cavities in an immersed granular packing. According to the gap between the two locally injected upward fluid flows, each fluidized cavity will evolve independently of the another, or conversely, the two cavities can interact with each other: they merge and eventually behave like a single cavity. Combined optical techniques are used for visualization of particle motion inside the granular media (Refractive Index-Matching between liquid and beads and Planar Laser-Induced Fluorescence), the experimental results are compared to a two-dimensional simulation based on coupled Discrete Element and Lattice Boltzmann Methods (DEM-LBM).

JF - 3rd International Symposium on Geomechanics from Micro to Macro PB - CRC Press CY - SEP 01-03-2014 Univ Cambridge, Cambridge, ENGLAND VL - Geomechanics from Micro to Macro SN - 978-1-138-02707-7 ER - TY - JOUR T1 - Initiation of immersed granular avalanches JF - Physical Review E Y1 - 2014 A1 - Patrick Mutabaruka A1 - Jean-Yves Delenne A1 - Kenichi Soga A1 - Farhang Radjaï AB -By means of coupled molecular dynamics-computational fluid dynamics simulations, we analyze the initiation of avalanches in a granular bed of spherical particles immersed in a viscous fluid and inclined above its angle of repose. In quantitative agreement with experiments, we find that the bed is unstable for a packing fraction below 0.59 but is stabilized above this packing fraction by negative excess pore pressure induced by the effect of dilatancy. From detailed numerical data, we explore the time evolution of shear strain, packing fraction, excess pore pressures, and granular microstructure in this creeplike pressure redistribution regime, and we show that they scale excellently with a characteristic time extracted from a model based on the balance of granular stresses in the presence of a negative excess pressure and its interplay with dilatancy. The cumulative shear strain at failure is found to be ≃ 0.2, in close agreement with the experiments, irrespective of the initial packing fraction and inclination angle. Remarkably, the avalanche is triggered when dilatancy vanishes instantly as a result of fluctuations while the average dilatancy is still positive (expanding bed) with a packing fraction that declines with the initial packing fraction. Another nontrivial feature of this creeplike regime is that, in contrast to dry granular materials, the internal friction angle of the bed at failure is independent of dilatancy but depends on the inclination angle, leading therefore to a nonlinear dependence of the excess pore pressure on the inclination angle. We show that this behavior may be described in terms of the contact network anisotropy, which increases with a nearly constant connectivity and levels off at a value (critical state) that increases with the inclination angle. These features suggest that the behavior of immersed granular materials is controlled not only directly by hydrodynamic forces acting on the particles but also by the influence of the fluid on the granular microstructure.

VL - 89 IS - 5 JO - Phys. Rev. E ER - TY - JOUR T1 - Internal Structure of Inertial Granular Flows JF - Physical Review Letters Y1 - 2014 A1 - Emilien Azéma A1 - Farhang Radjaï AB -We analyze inertial granular flows and show that, for all values of the inertial number I, the effective friction coefficient μ arises from three different parameters pertaining to the contact network and force transmission: (1) contact anisotropy, (2) force chain anisotropy, and (3) friction mobilization. Our extensive 3D numerical simulations reveal that μ increases with I mainly due to an increasing contact anisotropy and partially by friction mobilization whereas the anisotropy of force chains declines as a result of the destabilizing effect of particle inertia. The contact network undergoes topological transitions, and beyond I≃0.1 the force chains break into clusters immersed in a background “soup” of floating particles. We show that this transition coincides with the divergence of the size of fluidized zones characterized from the local environments of floating particles and a slower increase of μ with I.

VL - 112 IS - 7 JO - Phys. Rev. Lett. ER - TY - Generic T1 - Lattice Boltzmann modeling of liquid clusters in granular media T2 - 3rd International Symposium on Geomechanics from Micro to Macro Y1 - 2014 A1 - Jean-Yves Delenne A1 - Vincent Richefeu A1 - Farhang Radjaï ED - Kenichi Soga ED - Krishna Kumar ED - Giovanna Biscontin ED - Kuo, Matthew AB -We use capillary condensation simulated by a multiphase Lattice Boltzmann model as a means to generate homogeneous distributions of liquid clusters in 2D granular media. Liquid droplets condense from the vapor phase between and on the grains, and they transform into capillary bonds and liquid clusters as thermodynamic equilibrium is approached. As the amount of condensed liquid is increased, liquid clusters of increasing connectivity are formed and the distribution of liquid undergoes topological transitions until the whole pore space is filled by the liquid. We investigate the cluster statistics and local grain environments. From extensive simulations, we also obtain the mean Laplace pressure as a function of the amount of liquid, which is found to be quite similar to the well-known experimental retention curve in soil mechanics.

JF - 3rd International Symposium on Geomechanics from Micro to Macro PB - CRC Press CY - SEP 01-03-2014 Univ Cambridge, Cambridge, ENGLAND VL - Geomechanics from Micro to Macro SN - 978-1-138-02707-7 ER - TY - Generic T1 - Modelling soft-particle materials T2 - 3rd International Symposium on Geomechanics from Micro to Macro Y1 - 2014 A1 - Saeid Nezamabadi A1 - Farhang Radjaï A1 - Julien Averseng ED - Kenichi Soga ED - Krishna Kumar ED - Giovanna Biscontin AB -Soft-particle materials include colloidal pastes, vesicles, many powders, microgels and suspensions. They share the common feature of being composed of particles that can undergo large deformations without rupture. For the simulation of such materials, we present a modelling approach based on an implicit formulation of the Material Point Method (MPM) interfaced with the Contact Dynamics (CD) method for the treatment of frictional contacts between particles. Each particle is discretized as a collection of material points. The information carried by the material points is projected onto a background mesh, where equations of motion are solved. The mesh solution is then used to update the material points. The implicit formulation of MPM allows for unconditional numerical stability and efficient coupling with implicit treatment of unilateral contacts and friction between the particles by the CD method. We use this model to analyse the compaction process of 2D soft-particle packings. The packing can reach high solid fractions by particle shape change and still flow plastically. The compaction is a nonlinear process in which new contacts are formed between particles and the contact areas increase. We find that the evolution of the packing fraction is a slow logarithmic function of the driving stress as a consequence of increasing contact area. We also evidence the effect of friction, which favours strong stress chains and thus the elongation of particles, leading to a larger packing fraction at a given level of compressive stress as compared to a frictionless particle packing.

JF - 3rd International Symposium on Geomechanics from Micro to Macro PB - CRC Press CY - SEP 01-03-2014 Univ Cambridge, Cambridge, ENGLAND VL - Geomechanics from Micro to Macro UR - http://prodinra.inra.fr/record/370208 ER - TY - JOUR T1 - Particle alignment and clustering in sheared granular materials composed of platy particles JF - The European Physical Journal E Y1 - 2014 A1 - Boton, Mauricio A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï AB -By means of molecular dynamics simulations, we investigate the texture and local ordering in sheared packings composed of cohesionless platy particles. The morphology of large packings of platy particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and anisotropic orientations of contacts between particles. We find that particle alignment is strongly enhanced by the degree of platyness and leads to the formation of face-connected clusters of exponentially decaying size. Interestingly, due to dynamics in continuous shearing, this ordering phenomenon emerges even in systems composed of particles of very low platyness differing only slightly from spherical shape. The number of clusters is an increasing function of platyness. However, at high platyness the proportion of face-face interactions is too low to allow for their percolation throughout the system.

VL - 37 IS - 11 JO - Eur. Phys. J. E ER - TY - JOUR T1 - Penetration test in coarse granular material using Contact Dynamics Method JF - Computers and Geotechnics Y1 - 2014 A1 - Quezada, Juan Carlos A1 - Breul, Pierre A1 - Saussine, Gilles A1 - Farhang Radjaï AB -Field tests are widely used for soil characterization in geotechnical applications in spite of implementation difficulties. The light penetrometer is a well-known testing tool for fine soils, but the physical interpretation of the output data in the case of coarse granular materials is far less evident. Indeed, the data are considerably more sensitive in this case to various parameters such as fabric structure, particle shapes or the applied impact energy. In order to achieve a better understanding of the penetration process into a coarse granular material, a numerical study was performed by means of contact dynamics simulations. The penetration of a moving tip in a sample composed of irregular grain shapes was studied and the influence of the driving velocity and input energy on the penetration strength was analyzed. The results show that the latter grows with both the penetration rate and energy, despite the strong fluctuations occur due to a jamming–unjamming process in which the contact network connectivity evolves intermittently in correlation with the penetration strength. This analysis suggests that the time-averaged data provided by a penetrometer is reliable information from which the bulk strength properties of coarse granular materials can be evaluated.

VL - 55 JO - Computers and Geotechnics ER - TY - JOUR T1 - Poroelastic Theory Applied to the Adsorption-Induced Deformation of Vitreous Silica JF - The Journal of Physical Chemistry B Y1 - 2014 A1 - Benoit A. Coasne A1 - Weigel, Coralie A1 - Polian, Alain A1 - Kint, Mathieu A1 - Rouquette, Jérôme A1 - Haines, Julien A1 - Foret, Marie A1 - Vacher, Rene A1 - Ruffle, Benoit AB -When vitreous silica is submitted to high pressures under a helium atmosphere, the change in volume observed is much smaller than expected from its elastic properties. It results from helium penetration into the interstitial free volume of the glass network. We present here the results of concurrent spectroscopic experiments using either helium or neon and molecular simulations relating the amount of gas adsorbed to the strain of the network. We show that a generalized poromechanical approach, describing the elastic properties of microporous materials upon adsorption, can be applied successfully to silica glass in which the free volume exists only at the subnanometer scale. In that picture, the adsorption-induced deformation accounts for the small apparent compressibility of silica observed in experiments.

VL - 118 UR - http://pubs.acs.org/doi/10.1021/jp5094383 IS - 49 JO - J. Phys. Chem. B ER - TY - JOUR T1 - Predicting the settlement of coarse granular materials under vertical loading JF - Scientific Reports Y1 - 2014 A1 - Quezada, Juan Carlos A1 - Saussine, Gilles A1 - Breul, Pierre A1 - Farhang Radjaï AB -Granular materials are widely used in industrial processes despite their complex and poorly understood mechanical behaviour both in static and dynamic regimes. A prototypical example is the settlement and compaction of a granular bed under vibrational loading. The elementary mechanisms of this process are still unclear and there is presently no established theory or methodology to predict the settlement and its statistical variability. By means of a parametric study, carried out on a full-scale track, and a critical analysis of density relaxation laws, we introduce a novel settlement model in coarse granular materials under cyclic loading. Our extensive experimental data indicate that the settlement process is governed by three independent parameters strongly correlated with the vibration intensity and initial packing fraction. We show that the mean settlement is well predicted by the model with its parameter values extracted from experimental data.

Granular materials are both pressure-dependent and density-dependent materials and exhibit a broad range of intricate behaviours due to their discrete nature, dissipative interactions and generic structural disorder^{1}. The packing fraction may vary as a result of particle rearrangements induced by shearing or vibrations and it leads to dramatic changes in the structure and mechanical response of a granular material^{2}^{,3}^{,4}^{,5}^{,6}^{,7}^{,8}. A long-time logarithmic relaxation law of the packing fraction is systematically observed in experiments^{9}^{,10}. In simple compaction models, this behaviour is attributed to the exponentially increasing time for the particles to reach a new configuration of lower packing fraction. The case of settlement under cyclic loading has, however, been much less investigated. The settlement of granular bed occurs due to both compaction and side-wise spreading. An important industrial example is the railway ballast, which undergoes gradual settlement under the static and dynamic overloads induced by train traffics^{11}^{,12}^{,13}^{,14}. The readjustment of differential settlements requires costly operations on fast-train railways. For this reason, an improved understanding of the parameters governing the settlement process is a critical technological challenge for new developments in this field.

In this paper, we show that the total settlement *τ _{N}* under vertical cyclic loading is governed by a logarithmic relaxation law as a function of the number N of cycles:

where the three fitting parameters *τ*_{∞}, *B* and *N*_{0} can be evaluated from the loading parameters, namely the frequency *ω* (related to the train speed for ballast) and initial packing fraction of the material. Our experimental correlations between model and loading parameters show consistently that *τ*_{∞} and *B* depend on the dimensionless loading intensity Γ = (*Aω*^{2})/(*pd*^{2}/*m* + *g*), where *A* is the vibration amplitude, *p* is the confining pressure (under the sleeper for ballast), *d* is the average particle diameter, *m* is the average particle mass and *g* is gravitational acceleration. We also find that the parameter *N*_{0} is linked to the initial packing fraction of the material. In fact, this parameter controls the initial settlement rate, and it was systematically determined by means of a light penetrometer in our experiments on ballast material.

This paper discusses the notion of failure in a granular assembly by examining the key microstructural mechanisms which are most likely to trigger the nucleation and propagation of instabilities within a granular material. For this purpose, the key variable to predict the occurrence of failure, known as second-order work, is expressed from variables on the grain scale. The local behaviour incidents (where contacts may open or slide), compared to the global response of the assembly, are analysed by two approaches. First, numerical computations made by a discrete element model confirm the microscopic definition of the second-order work. Secondly, a micromechanical model, based on a homogenization procedure, relating the macroscopic behaviour to microscopic ingredients, namely contact planes, points to a close link between the occurrence of failure on the macroscopic scale as well as on the contact planes.

VL - 342 IS - 3 JO - Comptes Rendus Mécanique ER - TY - JOUR T1 - Stimuli-Responsive Cement-Reinforced Rubber JF - ACS Applied Materials & Interfaces Y1 - 2014 A1 - Musso, Simone A1 - Robisson, Agathe A1 - Sudeep Maheshwar A1 - Franz-Josef Ulm AB -In this work, we report the successful development of a cement–rubber reactive composite with reversible mechanical properties. Initially, the composite behaves like rubber containing inert filler, but when exposed to water, it increases in volume and reaches a stiffness that is intermediate between that of hydrogenated nitrile butadiene rubber (HNBR) and hydrated cement, while maintaining a relatively large ductility characteristic of rubber. After drying, the modulus increases even further up to 400 MPa. Wet/drying cycles prove that the elastic modulus can reversibly change between 150 and 400 MPa. Utilizing attenuated total reflection Fourier transform infrared spectroscopy), we demonstrate that the high pH produced by the hydration of cement triggers the hydrolysis of the rubber nitrile groups into carboxylate anions. Thus, the salt bridges, generated between the carboxylate anions of the elastomer and the cations of the filler, are responsible for the reversible variations in volume and elastic modulus of the composite as a consequence of environmental moisture exposure. These results reveal that cement nanoparticles can successfully be used to accomplish a twofold task: (a) achieve an original postpolymerization modification that allows one to work with carboxylate HNBR (HXNBR) not obtained by direct copolymerization of carboxylate monomers with butadiene, and (b) synthesize a stimuli-responsive polymeric composite. This new type of material, having an ideal behavior for sealing application, could be used as an alternative to cement for oil field zonal isolation applications.

VL - 6 IS - 9 JO - ACS Appl. Mater. Interfaces ER - TY - JOUR T1 - Validity of the t-plot Method to Assess Microporosity in Hierarchical Micro/Mesoporous Materials JF - Langmuir Y1 - 2014 A1 - Anne Galarneau A1 - François Villemot A1 - Rodriguez, Jeremy A1 - Fajula, François A1 - Benoit A. Coasne AB -The *t-plot* method is a well-known technique which allows determining the micro- and/or mesoporous volumes and the specific surface area of a sample by comparison with a reference adsorption isotherm of a nonporous material having the same surface chemistry. In this paper, the validity of the *t-plot* method is discussed in the case of hierarchical porous materials exhibiting both micro- and mesoporosities. Different hierarchical zeolites with MCM-41 type ordered mesoporosity are prepared using pseudomorphic transformation. For comparison, we also consider simple mechanical mixtures of microporous and mesoporous materials. We first show an intrinsic failure of the *t-plot* method; this method does not describe the fact that, for a given surface chemistry and pressure, the thickness of the film adsorbed in micropores or small mesopores (< 10σ, σ being the diameter of the adsorbate) increases with decreasing the pore size (curvature effect). We further show that such an effect, which arises from the fact that the surface area and, hence, the free energy of the curved gas/liquid interface decreases with increasing the film thickness, is captured using the simple thermodynamical model by Derjaguin. The effect of such a drawback on the ability of the *t-plot* method to estimate the micro- and mesoporous volumes of hierarchical samples is then discussed, and an abacus is given to correct the underestimated microporous volume by the *t-plot* method.

This paper presents conditions of initiation and development of failure in granular materials through a twodimensional discrete element model. General condition for the effective development of failure and its physical characteristics are recalled. Then relation between failure and the second order work expressed in terms of microscopic variables is discussed. Eventually, correspondence between a localized mode of failure marked with shear band patterns and space distribution of negative values of microscopic second-order work is investigated.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - Generic T1 - AIP Conference ProceedingsTumbling sandpiles in a fluid T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Farhang Radjaï A1 - Vincent Topin A1 - Perales, F. A1 - Yann Monerie ED - Yu, A ED - Dong, K ED - Yang, R AB -By means of contact dynamics simulations interfaced with computational fluid dynamics, we analyze the effect of a suspending fluid on the dynamics of collapse and spread of a granular column. We find that the runout distance increases as a power law with the aspect ratio of the column and, for a given aspect ratio, it may be the same in the grain-inertial and fluid-inertial regimes but with considerably longer duration in the latter case. We show that, in both viscous and fluid-inertial regimes, this behavior results from compensation between two effects of the fluid: 1) reduction of the kinetic energy during collapse and 2) enhancement of the flow by lubrication during spread.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - Generic T1 - A benchmark for particle shape dependence T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Gael Combe A1 - Cécile Nouguier-Lehon A1 - Emilien Azéma A1 - Krzysztof Szarf A1 - Baptiste Saint-Cyr A1 - Marie Chaze A1 - Farhang Radjaï A1 - Pascal Villard A1 - Jean-Yves Delenne A1 - Vincent Richefeu A1 - Philippe Sornay A1 - Charles Voivret A1 - CEGEO Group ED - Yu, A ED - Dong, K ED - Yang, R AB -Particle shape is a major parameter for the space-filling and strength properties of granular materials. For a systematic investigation of shape effect, a numerical benchmark test was set up within a collaborative group using different numerical methods and particles of various shape characteristics such as elongation, angularity and nonconvexity. Extensive 2D shear simulations were performed in this framework and the shear strength and packing fraction were compared for different shapes. We show that the results may be analyzed in terms of a low-order shape parameter η describing the degree of distortion from a perfectly circular shape. In particular, the shear strength is an increasing function of η with nearly the same trend for all shapes, the differences being of second order compared to η. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with η from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as η is further increased. Finally, the analysis of contact forces for the same value of η leads to very similar statistics regardless of our specific particle shapes.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - Generic T1 - Capillary states of granular materials in the funicular state T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Jean-Yves Delenne A1 - Vincent Richefeu A1 - Farhang Radjaï ED - Yu, A ED - Dong, K ED - Yang, R AB -Using a multi-phase lattice Boltzmann model, we investigate the capillary states of a 2D granular packing gradually saturated by condensation from a homogeneously injected vapor phase. The internal stresses induced by surface tension and Laplace pressure are directly calculated from the forces acting on the grains with increasing amount of liquid. The evolution of cohesive strength with the amount of liquid reveals four different states reflecting the connectivity of the liquid phase and local grain environments. It increases in the pendular state, characterized by binary liquid bridges holding the grains together, and within the funicular state with an increasing number of liquid clusters connected to several grains. Beyond 40% of saturation, the cohesive strength falls off due to a decreasing Laplace pressure of liquid clusters.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - JOUR T1 - Cohesive granular materials composed of nonconvex particles JF - Physical Review E Y1 - 2013 A1 - Baptiste Saint-Cyr A1 - Farhang Radjaï A1 - Jean-Yves Delenne A1 - Philippe Sornay AB -The macroscopic cohesion of granular materials made up of sticky particles depends on the particle shapes. We address this issue by performing contact dynamics simulations of 2D packings of nonconvex aggregates. We find that the macroscopic cohesion is strongly dependent on the strain and stress inhomogeneities developing inside the material. The largest cohesion is obtained for nearly homogeneous deformation at the beginning of unconfined axial compression and it evolves linearly with nonconvexity. Interestingly, the aggregates in a sheared packing tend to form more contacts with fewer neighboring aggregates as the degree of nonconvexity increases. We also find that shearing leads either to an isotropic distribution of tensile contacts or to the same privileged direction as that of compressive contacts.

VL - 87 IS - 5 JO - Phys. Rev. E ER - TY - Generic T1 - Comparison of the effects of rolling resistance and angularity in sheared granular media T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Taboada, Alfredo ED - Yu, A ED - Dong, K ED - Yang, R KW - angularity KW - anisotropy KW - force distribution KW - Granular material KW - rolling resistance KW - Shear strength KW - solid fraction AB -In this paper, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. For this purpose, we use contact dynamics simulations. By means of a simple shear numerical device, we investigate the mechanical behavior of these materials in the steady state in terms of shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. We find that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 UR - https://hal.archives-ouvertes.fr/hal-00842799 ER - TY - Generic T1 - Granular plastic flow and fabric-based internal variables T2 - 3rd International Symposium on Geomechanics from Micro to Macro Y1 - 2013 A1 - Farhang Radjaï A1 - Stéphane Roux ED - Kenichi Soga ED - Krishna Kumar ED - Giovanna Biscontin ED - Kuo, Matthew AB -A microscopic approach to the quasi-static flow of granular materials requires

fabric parameters pertaining to the contact network. We show that the plastic behavior of

granular materials is correctly predicted with the approximation of perfectly rigid particles

interacting via dissipative contacts as simulated by means of the contact dynamics method.

We investigate the correlations between macroscopic variables (internal friction angle and ...

Soil compaction not only reduces available pore volume in which fluids are stored, but it alters the arrangement of soil constituents and pore geometry, thereby adversely impacting fluid transport and a range of soil ecological functions. Quantitative understanding of stress transmission and deformation processes in arable soils remains limited. Yet such knowledge is essential for better predictions of effects of soil management practices such as agricultural field traffic on soil functioning. Concepts and theory used in agricultural soil mechanics (soil compaction and soil tillage) are often adopted from conventional soil mechanics (e.g. foundation engineering). However, in contrast with standard geotechnical applications, undesired stresses applied by agricultural tyres/tracks are highly dynamic and last for very short times. Moreover, arable soils are typically unsaturated and contain important secondary structures (e.g. aggregates), factors important for affecting their soil mechanical behaviour. Mechanical processes in porous media are not only of concern in soil mechanics, but also in other fields including geophysics and granular material science. Despite similarity of basic mechanical processes, theoretical frameworks often differ and reflect disciplinary focus. We review concepts from different but complementary fields concerned with porous media mechanics and highlight opportunities for synergistic advances in understanding deformation and compaction of arable soils. We highlight the important role of technological advances in non-destructive measurement methods at pore (X-ray tomography) and soil profile (seismic) scales that not only offer new insights into soil architecture and enable visualization of soil deformation, but are becoming instrumental in the development and validation of new soil compaction models. The integration of concepts underlying dynamic processes that modify soil pore spaces and bulk properties will improve the understanding of how soil management affect vital soil mechanical, hydraulic and ecological functions supporting plant growth.

VL - 128 JO - Soil and Tillage Research ER - TY - JOUR T1 - Micromechanical analysis of second order work in granular media JF - Granular Matter Y1 - 2013 A1 - Hadda, Nejib A1 - François Nicot A1 - Bourrier, Franck A1 - Sibille, Luc A1 - Farhang Radjaï A1 - Darve, Félix AB -This paper examines instabilities in granular materials from a microscopic point of view through numerical simulations conducted using a discrete element method on two three-dimensional specimens. The detection and the tracking of grain scale deformation mechanisms constitute the key point for a better understanding the failure process and puzzling out what lies behind the vanishing of the macroscopic second order work. For this purpose, the second order work from microscopic variables, involving contact force and branch vector, was introduced and tracked numerically. Then, all contacts depicting negative values of the second order work were deeply investigated, especially their spatial distribution (homogeneity, agglomeration, dispersion

An original method based on atomic force microscopy (AFM) in contact mode was developed to abrade progressively the surface of tablets made of starch or gluten polymers isolated from wheat. The volume of the material removed by the tip was estimated from the analysis of successive topographic images of the surface, and the shear force was measured by keeping a constant normal force. Our data together with a simple tribological model provide clear evidence for a higher hardness and shear strength of starch compared to gluten. Gluten appears to have mechanical properties close to soft materials, such as talc, whereas starch displays higher hardness close to calcite. Our results are in a better agreement with structural properties of gluten (complex protein network) and starch (granular and semi-cristalline structure) than earlier studies by micro-indentation. This work shows that the AFM scratching method is relevant for the characterization of any polymer surface, in particular in application to materials made of different polymers at the nano-scale.

VL - 49 IS - 12 JO - European Polymer Journal ER - TY - JOUR T1 - Packings of irregular polyhedral particles: Strength, structure, and effects of angularity JF - Physical Review E Y1 - 2013 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Dubois, Frédéric AB -

We present a systematic numerical investigation of the shear strength and structure of granular packings composed of irregular polyhedral particles. The angularity of the particles is varied by increasing the number of faces from 8 (octahedronlike shape) to 596. We find that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below 46 faces). At the same time, the packing fraction increases to a peak value but declines for more angular particles. We analyze the connectivity and anisotropy of the microstructure by considering both the contacts and branch vectors joining particle centers. The increase of the shear strength with angularity is shown to be due to a net increase of the fabric and force anisotropies but at higher particle angularity a rapid falloff of the fabric anisotropy is compensated by an increase of force anisotropy, leading thus to the saturation of shear strength.

VL - 8713 IS - 6 JO - Phys. Rev. E ER - TY - Generic T1 - Penetration strength of coarse granular materials from DEM simulations T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Quezada, Juan Carlos A1 - Saussine, Gilles A1 - Breul, Pierre A1 - Farhang Radjaï ED - Yu, A ED - Dong, K ED - Yang, R AB -Field tests are widely used for soil characterization in geotechnical applications in spite of implementation difficulties. The light penetrometer test is a well-known testing tool for fine soils, but the physical interpretation of the output data in the case of coarse granular materials is far less evident. In fact, the data are considerably more sensitive to various parameters such as fabric structure, particles shape or the applied impact energy. In order to achieve a better understanding of the underlying phenomena, we performed a numerical study by means contact dynamics DEM simulations. We consider the penetration of a moving tip into a sample composed of irregular grain shapes and we analyze the influence of the driving velocity and applied energy on the penetration strength. We find that the latter grows with both the penetration rate and energy. Force fluctuations on the tip involve a jamming-unjamming process. The typology of contact network and inter-granular friction play a major role in the fluctuations and measured values of the cone penetration strength.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - JOUR T1 - Quasistatic rheology and microstructural description of sheared granular materials composed of platy particles JF - Physical Review E Y1 - 2013 A1 - Boton, Mauricio A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï A1 - Lizcano, Arcesio AB -This is the first paper of a series devoted to the micro-mechanical modeling of clayey soils, by means of discrete element simulations. We specifically focus here on the effect of the platy shape of particles by reducing the interactions between particles to mechanical contact forces (i.e., neither electrostatic repulsion nor van der Waals forces are taken into account). The particles are three-dimensional square plates, approximated as spheropolyhedra. Several samples composed of particles of different levels of platyness (related to the ratio of length to thickness) were numerically prepared and sheared up to large deformations. We analyzed the shear strength, packing fraction, orientation of the particles, connectivity, fabric of the interactions network, and interaction forces as functions of the platyness. We find that both the mechanical behavior and microstructure are strongly dependent on the degree of platyness. The principal underlying phenomenon is the alignment of particle faces along a particular direction. This ordering phenomenon, which emerges even for shapes that deviate only slightly from that of a sphere, enhances the ability of the packing to develop an anisotropic structure leading to large shear strength, especially as a consequence of the fabric and mobilization of friction forces. Moreover, the connectivity of the packings and their packing fraction also evolve with the platyness. In particular, the packing fraction evolves in a nonmonotonic fashion, as observed in other granular materials composed of elongated or angular particles.

VL - 87 IS - 3 JO - Phys. Rev. E ER - TY - JOUR T1 - Reactive elastomeric composites: When rubber meets cement JF - Composites Science and Technology Y1 - 2013 A1 - Robisson, Agathe A1 - Sudeep Maheshwar A1 - Musso, Simone A1 - Jeffrey J. Thomas A1 - Auzerais, Francois M. A1 - Han, Dingzhi A1 - Qu, Meng A1 - Franz-Josef Ulm AB -This paper describes a novel reactive composite material comprised of hydrogenated nitrile butadiene rubber (HNBR) compounded with slag cement. The composite initially looks and behaves like rubber, but when exposed to water it simultaneously swells and stiffens due to hydration of the cement component. The material eventually reaches a stiffness that is intermediate between that of HNBR and hydrated cement, while maintaining a relatively large ductility that is more characteristic of rubber. This behavior, which is ideal for sealing applications, differentiates this material from conventional swellable materials that become less stiff upon swelling. The development of this new type of material was motivated by the requirements of oilfield zonal isolation, where alternatives to cement are needed for some challenging sealing applications. A mechanism for the swelling and stiffening of the reactive composite is proposed: water diffuses into the HNBR matrix and is converted to bound water through hydration reactions with the cement, causing the effective solid filler content of the composite to increase. A model is proposed that treats the composite as a cellular solid with a continuous filler phase (hydrated cement). This model is able to reproduce the observed increase in the elastic modulus with time during exposure to water.

VL - 75 JO - Composites Science and Technology ER - TY - JOUR T1 - Rheology of three-dimensional packings of aggregates: Microstructure and effects of nonconvexity JF - Physical Review E Y1 - 2013 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Baptiste Saint-Cyr A1 - Jean-Yves Delenne A1 - Philippe Sornay AB -We use 3D contact dynamics simulations to analyze the rheological properties of granular materials composed of rigid aggregates. The aggregates are made from four overlapping spheres and described by a nonconvexity parameter depending on the relative positions of the spheres. The macroscopic and microstructural properties of several sheared packings are analyzed as a function of the degree of nonconvexity of the aggregates. We find that the internal angle of friction increases with nonconvexity. In contrast, the packing fraction increases first to a maximum value but declines as nonconvexity further increases. At high level of nonconvexity, the packings are looser but show a higher shear strength. At the microscopic scale, the fabric and force anisotropy, as well as friction mobilization are enhanced by multiple contacts between aggregates and interlocking, revealing thus the mechanical and geometrical origins of shear strength.

VL - 87 IS - 5 JO - Phys. Rev. E ER - TY - Generic T1 - Settlement statistics of a granular layer composed of polyhedral particles T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Quezada, Juan Carlos A1 - Saussine, Gilles A1 - Breul, Pierre A1 - Farhang Radjaï ED - Yu, A ED - Dong, K ED - Yang, R AB -We use 3D contact dynamics simulations to investigate the mechanical equilibrium and settlement of a granular material composed of irregular polyhedral particles confined between two horizontal frictional planes. We show that, as a consequence of mobilized wall-particle friction force at the top and bottom boundaries, the transient deformation induced by a constant vertical load increment is controlled by the aspect ratio (thickness over width) of the packing as well as the stress ratio. The transient deformation declines considerably for increasingly smaller aspect ratios and grows with the stress ratio. From the simulation data for a large number of independent configurations, we find that sample-to-sample fluctuations of the deformation have a broad distribution and they scale with the average deformation.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - Generic T1 - Shear strength and microstructure of 3D assemblies of platy particles T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Boton, Mauricio A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï A1 - Lizcano, Arcesio ED - Yu, A ED - Dong, K ED - Yang, R KW - Clays KW - DEM KW - Microstructure KW - Shear strength AB -As a first step towards particle-scale modeling of clayey soils, we investigate the mechanical behavior and microstructure of assemblies of three-dimensional rectangular platy particles by means of the discrete element method. Several samples composed of particles of different levels of platyness (ratio of width to thickness) were numerically prepared and sheared to large deformations. We analyze the shear strength, packing fraction, connectivity, contact and force anisotropies, and mobilization of friction forces as functions of platyness.We find that both the mechanical behavior and microstructure are strongly dependent on the degree of platyness. This happens, in particular, because of the alignment of particle faces along a particular direction. Additionally, as observed for other granular materials with complex shapes, the packing fraction passes by a peak value before decreasing for larger values of platyness.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 UR - https://hal.archives-ouvertes.fr/hal-00842787 ER - TY - Generic T1 - Shear strength, force distributions and friction mobilization in sheared packings composed of angular particles T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï ED - Yu, A ED - Dong, K ED - Yang, R AB -In this paper, we explore the effect of particle shape angularity on the mechanical behavior of sheared granular packings. A first series of contact dynamics simulations is performed in 2D with regular polygons with an increasing number of sides ranging from 3 (triangles) to 60. Then, in order to approach “idealized” angular particles, a second series of simulations is performed in 3D with irregular polyhedra with the number of faces ranging from 8 (octahedron-like) to 596. A counterintuitive finding is that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below 6 sides in 2D and 46 faces in 3D). A micromechanical analysis of force and contact orientations, all enhanced by face-face and face-side contacts, reveals that this increase is due to an increase of both contact and force anisotropies, and the saturation for higher angularities is a consequence of a rapid fall-off of the contact and normal force anisotropies compensated by an increase of the tangential force anisotropy.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - JOUR T1 - Collapse Dynamics and Runout of Dense Granular Materials in a Fluid JF - Physical Review Letters Y1 - 2012 A1 - Vincent Topin A1 - Yann Monerie A1 - Perales, F. A1 - Farhang Radjaï VL - 109 IS - 18 JO - Phys. Rev. Lett. ER - TY - Generic T1 - Effect of Particle Shape non-Convexity on the Rheology of Granular Media : 3D Contact Dynamics Simulations T2 - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) Y1 - 2012 A1 - Baptiste Saint-Cyr A1 - Emilien Azéma A1 - Jean-Yves Delenne A1 - Farhang Radjaï A1 - Philippe Sornay ED - Onate, E ED - Owen, DRJ KW - force transmission KW - Granular Materials KW - non-convexity KW - particle shape KW - texture AB -We analyze the effect of particle shape non-convexity on the quasi-static behavior of granular materials by means of contact dynamics simulations. The particles are regular aggregates of four overlapping spheres described by a nonconvexity parameter depending on the relative positions of the particles. Several packings are first submitted to isotropic compression without friction. We find that, as in 2D, the solid fraction of isotropic packings increases with non-convexity up to a maximum value and then declines to be nearly equal to that of a packing composed of only spheres. It is also remarkable that the coordination number increases quickly and saturates so that the packings composed of grains with a high level of nonconvexity are looser but more strongly connected. Then, the quasi-static behavior, structural and force anisotropies are analyzed by subjecting each packing to a triaxial compression. We find that the shear strength increases with non-convexity. We show that this increase results from the presence of multiple contacts between trimers leading to enhanced frictional interlocking.

JF - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) CY - OCT 26-28 2011 Barcelona, SPAIN VL - PARTICLE-BASED METHODS II: FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-00686453 ER - TY - JOUR T1 - Experimental determination of the fracture toughness via microscratch tests: Application to polymers, ceramics, and metals JF - Journal of Materials Research Y1 - 2012 A1 - Ange-Therese Akono A1 - Randall, Nicholas X. A1 - Franz-Josef Ulm AB -We analyze the geometrical states of granular materials by means of a fabric tensor involving the coordination number and fabric anisotropy as the lowest-order descriptors of the contact network. In particular, we show that the fabric states in this representation are constrained by steric exclusions and the condition of mechanical equilibrium required in the quasi-static limit. A simple model, supported by numerical data, allows us to characterize the range of accessible fabric states and the joint evolution of fabric parameters. The critical state in this framework appears as a jammed state in the sense of a saturation of contact gain and loss along the principal strain-rate directions.

VL - 14 IS - 2 SI JO - Granular Matter ER - TY - JOUR T1 - Force chains and contact network topology in sheared packings of elongated particles JF - Physical Review E Y1 - 2012 A1 - Emilien Azéma A1 - Farhang Radjaï AB -By means of contact dynamic simulations, we investigate the contact network topology and force chains in two-dimensional packings of elongated particles modeled by rounded-cap rectangles. The morphology of large packings of elongated particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and orientations of contacts between particles. We show that particle elongation affects force distributions and force/fabric anisotropy via various local structures allowed by steric exclusions and the requirement of force balance. As a result, the force distributions become increasingly broader as particles become more elongated. Interestingly, the weak force network transforms from a passive stabilizing agent with respect to strong force chains to an active force-transmitting network for the whole system. The strongest force chains are carried by side/side contacts oriented along the principal stress direction.

VL - 85 IS - 3 JO - Phys. Rev. E ER - TY - Generic T1 - Microscopic Origins of Shear Strength in Packings Composed of Elongated Particles T2 - 9th International Workshop on Bifurcation and Degradation in Geomaterials (IWBDG 2011) Y1 - 2012 A1 - Emilien Azéma A1 - Farhang Radjaï ED - Stéphane Bonelli ED - Dascalu, Cristian ED - François Nicot KW - Elongated particles KW - Fabric properties KW - force transmission KW - Harmonic decomposition AB -We investigate the rheology, force transmission and texture of granular materials composed of elongated particles by means of contact dynamics simulations. The particles have a rounded-cap rectangular (RCR) shape described by a single elongation parameter varying from 0 for a circular particle to 1 for an infinitely thin or long particle. We study the quasi-static behavior, structural and force anisotropies as a function of the elongation parameter for packings submitted to biaxial compression. The shear strength is found to increase linearly with this parameter whereas the solid fraction both at the initial isotropic state and in the critical state is nonmonotonous. We show that for these elongated particles a harmonic decomposition of the stress tensor provides a fairly good approximation of the internal state. Our data suggest that the increase of shear strength with reflects both enhanced friction mobilization and anisotropic particle orientation as the elongation of the particles increases.

JF - 9th International Workshop on Bifurcation and Degradation in Geomaterials (IWBDG 2011) CY - MAY 23-26 2011 Porquerolles, FRANCE VL - Springer Series in Geomechanics and Geoengineering - ADVANCES IN BIFURCATION AND DEGRADATION IN GEOMATERIALS UR - https://hal.archives-ouvertes.fr/hal-00686728 ER - TY - JOUR T1 - Nonlinear effects of particle shape angularity in sheared granular media JF - Physical Review E Y1 - 2012 A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï AB -We analyze the effects of particle shape angularity on the macroscopic shear behavior and texture of granular packings simulated by means of the contact dynamics method. The particles are regular polygons with an increasing number of sides ranging from 3 (triangles) to 60. The packings are analyzed in the steady shear state in terms of their shear strength, packing fraction, connectivity, and fabric and force anisotropies, as functions of the angularity. An interesting finding is that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below six sides). In contrast, the packing fraction declines towards a constant value, so that the packings of more angular particles are looser but have higher shear strength. We show that the increase of the shear strength at low angularity is due to an increase of both contact and force anisotropies, and the saturation of the shear strength for higher angularities is a consequence of a rapid fall-off of the contact and normal force anisotropies compensated by an increase of the tangential force anisotropy. This transition reflects clearly the rather special geometrical properties of these highly angular shapes, implying that the stability of the packing relies strongly on the side-side contacts and the mobilization of friction forces.

VL - 86 IS - 4 JO - Phys. Rev. E ER - TY - JOUR T1 - Particle shape dependence in 2D granular media JF - EPL (Europhysics Letters) Y1 - 2012 A1 - Baptiste Saint-Cyr A1 - Krzysztof Szarf A1 - Charles Voivret A1 - Emilien Azéma A1 - Vincent Richefeu A1 - Jean-Yves Delenne A1 - Gael Combe A1 - Cécile Nouguier-Lehon A1 - Pascal Villard A1 - Philippe Sornay A1 - Marie Chaze A1 - Farhang Radjaï AB -Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter

By means of extensive contact dynamics simulations, we investigate the mechanical equilibrium and deformation of a granular material composed of irregular polyhedral particles confined between two horizontal frictional planes. We show that, as a consequence of mobilized wall-particle friction forces at the top and bottom boundaries, the transient deformation induced by a constant vertical load is controlled by the aspect ratio (thickness over width) of the packing as well as the stress ratio. The transient deformation declines considerably for increasingly smaller aspect ratios and grows with the stress ratio. From the simulation data for a large number of independent configurations, we find that sample-to-sample fluctuations of the deformation have a broad distribution and they scale with the average deformation. We also analyze the evolution of particle connectivity during settlement and with the applied force. The face-face and edge-face contacts between polyhedral particles concentrate strong force chains with a growing proportion as a function of the applied force.

VL - 86 IS - 3 JO - Phys. Rev. E ER - TY - JOUR T1 - Structure and properties of nanoscale materials: theory and atomistic computer simulation JF - International Journal of Nanotechnology Y1 - 2012 A1 - Christophe Bichara A1 - Marsal, P A1 - Mottet, C A1 - Roland Jean-Marc Pellenq A1 - F. Ribeiro A1 - Andres Saùl A1 - G. Tréglia A1 - Weissker, H–Ch AB -We present a review of a few research topics developed within the "Theory and Atomistic Computer Simulation" Department at CINaM. The bottom line of the scientific activity is to use up–to–date theoretical and computer simulation techniques to address physics and materials science problems, often at the nanometric scale, in close contact with experimental groups. It ranges from the study of the structure and properties of molecular systems for organic electronics to metallic clusters and alloys, magnetic oxides, nuclear fuels and carbon–based nanostructures. These studies are motivated by fundamental research questions as well as more applied goals including environmental and energy issues, or information technologies. This broad spectrum of activities requires a large range of techniques, from theory and ab initio calculations to semi–empirical models incorporated in Monte Carlo or molecular dynamics simulations.

VL - 9 IS - 3-7 ER - TY - JOUR T1 - Tensile strength and fracture of cemented granular aggregates JF - The European Physical Journal E Y1 - 2012 A1 - Rafik Affès A1 - Jean-Yves Delenne A1 - Yann Monerie A1 - Farhang Radjaï A1 - Vincent Topin AB -Cemented granular aggregates include a broad class of geomaterials such as sedimentary rocks and some biomaterials such as the wheat endosperm. We present a 3D lattice element method for the simulation of such materials, modeled as a jammed assembly of particles bound together by a matrix partially filling the interstitial space. From extensive simulation data, we analyze the mechanical properties of aggregates subjected to tensile loading as a function of matrix volume fraction and particle-matrix adhesion. We observe a linear elastic behavior followed by a brutal failure along a fracture surface. The effective stiffness before failure increases almost linearly with the matrix volume fraction. We show that the tensile strength of the aggregates increases with both the increasing tensile strength at the particle-matrix interface and decreasing stress concentration as a function of matrix volume fraction. The proportion of broken bonds in the particle phase reveals a range of values of the particle-matrix adhesion and matrix volume fraction for which the cracks bypass the particles and hence no particle damage occurs. This limit is shown to depend on the relative toughness of the particle-matrix interface with respect to the particles.

VL - 35 IS - 11 JO - Eur. Phys. J. E ER - TY - JOUR T1 - An atomistic modelling of the porosity impact on UO2 matrix macroscopic properties JF - Journal of Nuclear Materials Y1 - 2011 A1 - Andrei Jelea A1 - Colbert, M. A1 - F. Ribeiro A1 - G. Tréglia A1 - Roland Jean-Marc Pellenq AB -The porosity impact on the UO_{2} matrix thermomechanical properties was investigated using atomistic simulation techniques. The porosity modifies the thermal expansion coefficient and this is attributed to pore surface effects. The elastic moduli at 0 K and at finite temperature decrease with porosity, this variation being well approximated using affine functions. These results agree with other mesoscale model predictions and experimental data, showing the ability of the semiempirical potential atomistic simulations to give an overall good description of the porous UO_{2}. However, the surface effects are incompletely described.

VL - 415 IS - 2 JO - Journal of Nuclear Materials ER - TY - Generic T1 - Compressive strength of an unsaturated granular material during cementation T2 - Symposium on Science and Technology of Powders and Sintered Materials (STPMF 2009) Y1 - 2011 A1 - Jean-Yves Delenne A1 - Soulié, Fabien A1 - Moulay Saïd El Youssoufi A1 - Farhang Radjaï AB -

The cohesive behaviour of unsaturated granular materials is due to the presence of cohesive bonds between grains. These bonds can have various physico-chemical characteristics and may evolve with environmental conditions. We study the case of a granular material partially saturated by an aqueous solution. The bonds are thus initially of capillary type and the mechanical strength is weak. At low relative humidity, the phase change of water involves crystallization of the solute at the contact points between grains, generating thus solid bonds. The mechanical strength of the material is then enhanced. An experimental study of the evolution of the mechanical strength during crystallization of the solute shows clearly the transition from capillary regime to cemented regime. This transition is not correlated with the mass of the crystallized solute, but rather with the residual degree of saturation. This behavior is analyzed here in the light of discrete element simulations. We introduce a local cohesion law that accounts for transition from capillary to cemented bonding. This law is formulated in terms of the degree of crystallization as a result of the evaporation of water at the boundary of the sample. The cohesion of the packing is initially of capillary type. A crystallization front then spreads from the sample boundaries to the center of the sample, and the strength increases as a result. Uniaxial compression allows us to determine the strength at different times. The numerical strength agrees well with the experimental data, and reveals strength enhancement as the solute crystallizes, as well as the transition from capillary to cementation regime.

JF - Symposium on Science and Technology of Powders and Sintered Materials (STPMF 2009) CY - MAY 25-27 2009 Montpellier, FRANCE VL - POWDER TECHNOLOGY JO - Powder Technology ER - TY - Generic T1 - Creep behaviour of confined layers of polyhedral grains T2 - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) Y1 - 2011 A1 - Quezada, Juan Carlos A1 - Farhang Radjaï A1 - Breul, Pierre A1 - Saussine, Gilles ED - Onate, E ED - Owen, DRJ AB -By means of contact dynamics simulations, we investigate the creep deformation of a thin granular layer composed of irregular polyhedral particles under the action of a constant vertical overload applied on a horizontal wall on top of the layer. We show that the total deformation induced by the overload increases with the ratio between the vertical and confining horizontal stresses and the aspect ratio of the sample. The effect of the aspect ratio is a consequence of the mobilized wall-grain friction forces at the top and bottom boundaries, that lead to enhanced strength by stabilizing strong force chains at the center of the sample. We also evidence the influence of loading history due to strain-induced fabric change or inertial effects resulting from the instant application of the overload. The topology of the contact network evolves in close correlation with creep. In particular, the face/face contacts between polyhedral particles concentrate largest force chains and their number is an increasing function of creep. A crucial feature of a confined granular system is the statistical variability of the mechanical response that we analyzed for creep deformations by performing a large number of simulations for independent initial configurations. Our data indicate that the distribution of fluctuations with respect to the mean creep falls off exponentially.

JF - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) CY - OCT 26-28 2011 Barcelona, SPAIN VL - PARTICLE-BASED METHODS II: FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-00761094 ER - TY - JOUR T1 - From liquid to solid bonding in cohesive granular media JF - Mechanics of Materials Y1 - 2011 A1 - Jean-Yves Delenne A1 - Soulié, Fabien A1 - Moulay Saïd El Youssoufi A1 - Farhang Radjaï AB -We study the transition of a granular packing from liquid to solid bonding in the course of drying. The particles are initially wetted by a liquid brine and the cohesion of the packing is ensured by capillary forces, but the crystallization of the solute transforms the liquid bonds into partially cemented bonds. This transition is evidenced experimentally by measuring the compressive strength of the samples at regular intervals of times. Our experimental data reveal three regimes: (1) Up to a critical degree of saturation, no solid bonds are formed and the cohesion remains practically constant; (2) The onset of cementation occurs at the surface and a front spreads towards the center of the sample with a nonlinear increase of the cohesion; (3) All bonds are partially cemented when the cementation front reaches the center of the sample, but the cohesion increases rapidly due to the strengthening of cemented bonds. We introduce a model based on a parametric cohesion law at the bonds and a bond crystallization parameter. This model predicts correctly the phase transition and the relation between microscopic and macroscopic cohesion.

VL - 43 IS - 10 JO - Mechanics of Materials ER - TY - JOUR T1 - Identification of rolling resistance as a shape parameter in sheared granular media JF - Physical Review E Y1 - 2011 A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Taboada, Alfredo AB -Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

VL - 84 IS - 1 JO - Phys. Rev. E ER - TY - Generic T1 - Modeling Porous Granular Aggregates T2 - 9th International Workshop on Buifurcation and Degradation in Geomaterials (IWBDG 2011) Y1 - 2011 A1 - Rafik Affès A1 - Vincent Topin A1 - Jean-Yves Delenne A1 - Yann Monerie ED - Stéphane Bonelli ED - Dascalu, Cristian ED - François Nicot AB -We rely on 3D simulations based on the Lattice Element Method (LEM) to analyze the failure of porous granular aggregates under tensile loading. We investigate crack growth by considering the number of broken bonds in the particle phase as a function of the matrix volume fraction and particle-matrix adhesion. Three regimes are evidenced, corresponding to no particle damage, particle abrasion and particle fragmentation, respectively. We also show that the probability density of strong stresses falls off exponentially at high particle volume fractions where a percolating network of jammed particles occurs. Decreasing the matrix volume fraction leads to increasingly broader stress distribution and hence a higher stress concentration. Our findings are in agreement with 2D results previously reported in the literature.

JF - 9th International Workshop on Buifurcation and Degradation in Geomaterials (IWBDG 2011) PB - Springer Netherlands CY - MAY 23-26 2011 Porquerolles, FRANCE VL - Springer Series in Geomechanics and Geoengineering - ADVANCES IN BIFURCATION AND DEGRADATION IN GEOMATERIALS SN - 978-94-007-1420-5 ER - TY - Generic T1 - Onset of Immersed Granular Avalanches by DEM-LBM Approach T2 - 9th International Workshop on Buifurcation and Degradation in Geomaterials (IWBDG 2011) Y1 - 2011 A1 - Jean-Yves Delenne A1 - Mansouri, M. A1 - Farhang Radjaï A1 - Moulay Saïd El Youssoufi A1 - Seridi, A. ED - Stéphane Bonelli ED - Dascalu, Cristian ED - François Nicot AB -We present 3D grain-fluid simulations based on the discrete element method interfaced with the lattice Boltzmann method and applied to investigate the initiation of underwater granular flows. We prepare granular beds of 800 spherical grains with different values of the initial solid fraction in a biperiodic rectangular box. In order to trigger an avalanche, the bed is instantaneously tilted to a finite slope angle above the maximum angle of stability. We simulate the dynamics of the transient flow for different solid fractions. In agreement with the experimental work of Iverson (Water Resour Res 36(7):1897–1910, 2000) and Pailha et al. (Phys Fluids 20(11):111701, 2008), we find that the flow onset is controlled by the initial solid fraction.

JF - 9th International Workshop on Buifurcation and Degradation in Geomaterials (IWBDG 2011) PB - Springer Netherlands CY - MAY 23-26 2011 Porquerolles, FRANCE VL - Springer Series in Geomechanics and Geoengineering - ADVANCES IN BIFURCATION AND DEGRADATION IN GEOMATERIALS SN - 978-94-007-1420-5 ER - TY - JOUR T1 - Rheology of granular materials composed of nonconvex particles JF - Physical Review E Y1 - 2011 A1 - Baptiste Saint-Cyr A1 - Jean-Yves Delenne A1 - Charles Voivret A1 - Farhang Radjaï A1 - Philippe Sornay AB -By means of contact dynamics simulations, we investigate the shear strength and internal structure of granular materials composed of two-dimensional nonconvex aggregates. We find that the packing fraction first grows as the nonconvexity is increased but declines at higher nonconvexity. This unmonotonic dependence reflects the competing effects of pore size reduction between convex borders of aggregates and gain in porosity at the nonconvex borders that are captured in a simple model fitting nicely the simulation data both in the isotropic and sheared packings. On the other hand, the internal angle of friction increases linearly with nonconvexity and saturates to a value independent of nonconvexity. We show that fabric anisotropy, force anisotropy, and friction mobilization, all enhanced by multiple contacts between aggregates, govern the observed increase of shear strength and its saturation with increasing nonconvexity. The main effect of interlocking is to dislocate frictional dissipation from the locked double and triple contacts between aggregates to the simple contacts between clusters of aggregates. This self-organization of particle motions allows the packing to keep a constant shear strength at high nonconvexity.

VL - 84 IS - 4 JO - Phys. Rev. E ER - TY - JOUR T1 - Scratching as a Fracture Process: From Butter to Steel JF - Physical Review Letters Y1 - 2011 A1 - Ange-Therese Akono A1 - Reis, P. M. A1 - Franz-Josef Ulm AB -We present results of a hybrid experimental and theoretical investigation of the fracture scaling in scratch tests and show that scratching is a fracture dominated process. Validated for paraffin wax, cement paste, Jurassic limestone and steel, we derive a model that provides a quantitative means to relate quantities measured in scratch tests to fracture properties of materials at multiple scales. The scalability of scratching for different probes and depths opens new venues towards miniaturization of our technique, to extract fracture properties of materials at even smaller length scales.

VL - 106 IS - 20 ER - TY - Generic T1 - Stress fields in granular solids: Effect of composition T2 - Symposium on Science and Technology of Powders and Sintered Materials (STPMF 2009) Y1 - 2011 A1 - Vincent Topin A1 - Farhang Radjaï A1 - Jean-Yves Delenne AB -We use the lattice element method to investigate stress fields at the sub-particle scale in granular solids composed of particles embedded in a cementing matrix. The stress distributions are found to be similar in 2D and 3D samples subjected to vertical loading with free lateral boundaries. We find that the number of strong forces falls off exponentially at high particle volume fractions where a percolating network of jammed particles occurs. The influence of the matrix volume fraction and particle/matrix stiffness ratio with respect to stress distribution is analyzed in 2D and 3D. We show that both decreasing the matrix volume fraction and increasing the stiffness ratio lead to increasingly broader distributions within a limit beyond which the distribution is independent of one or both of these parameters.

JF - Symposium on Science and Technology of Powders and Sintered Materials (STPMF 2009) CY - MAY 25-27 2009 Montpellier, FRANCE VL - POWDER TECHNOLOGY JO - Powder Technology ER -