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 - Modified Szegö–Widom Asymptotics for Block Toeplitz Matrices with Zero Modes JF - Journal of Statistical Physics Y1 - 2019 A1 - Basor, E. A1 - Dubail, Jerome A1 - Emig, Thorsten A1 - Santachiara, Raoul AB -The Szego-Widom theorem provides an expression for the determinant of block Toeplitz matrices in the asymptotic limit of large matrix dimension n. We show that the presence of zero modes, i.e, eigenvalues that vanish as n, ||<1, when n, requires a modification of the Szego-Widom theorem. A new asymptotic expression for the determinant of a certain class of block Toeplitz matrices with one pair of zero modes is derived. The result is inspired by one-dimensional topological superconductors, and the relation with the latter systems is discussed.

VL - 174 UR - http://link.springer.com/10.1007/s10955-018-2177-8 JO - J Stat Phys ER - TY - JOUR T1 - A minimal power model for human running performance JF - PLOS ONE Y1 - 2018 A1 - Mulligan, Matthew A1 - Adam, Guillaume A1 - Emig, Thorsten ED - Piknova, Barbora KW - ENERGY-COST; OXYGEN-UPTAKE; ATHLETIC RECORDS; ECONOMY; RUNNERS; RUN; ENDURANCE; MARATHON; EXERCISE; EXHAUSTION AB -Models for human running performances of various complexities and underlying principles have been proposed, often combining data from world record performances and bio-energetic facts of human physiology. The purpose of this work is to develop a novel, minimal and universal model for human running performance that employs a relative metabolic power scale. The main component is a self-consistency relation for the time dependent maximal power output. The analytic approach presented here is the first to derive the observed logarithmic scaling between world (and other) record running speeds and times from basic principles of metabolic power supply. Our hypothesis is that various female and male record performances (world, national) and also personal best performances of individual runners for distances from 800m to the marathon are excellently described by this model. Indeed, we confirm this hypothesis with mean errors of (often much) less than 1%. The model defines endurance in a way that demonstrates symmetry between long and short racing events that are separated by a characteristic time scale comparable to the time over which a runner can sustain maximal oxygen uptake. As an application of our model, we derive personalized characteristic race speeds for different durations and distances.

VL - 13 UR - http://dx.plos.org/10.1371/journal.pone.0206645 IS - 11 JO - PLoS ONE ER - TY - JOUR T1 - Role of City Texture in Urban Heat Islands at Nighttime JF - Physical Review Letters Y1 - 2018 A1 - Sobstyl, Jake M. A1 - Emig, Thorsten A1 - Qomi, M. J. Abdolhosseini A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq KW - IDEAL CONDITIONS; CLIMATE-CHANGE; CITIES; SIMULATION; GEOMETRY; ENERGY; HEALTH; IMPACT; MODEL AB -An urban heat island (UHI) is a climate phenomenon that results in an increased air temperature in cities when compared to their rural surroundings. In this Letter, the dependence of an UHI on urban geometry is studied. Multiyear urban-rural temperature differences and building footprints data combined with a heat radiation scaling model are used to demonstrate for more than 50 cities worldwide that city texture-measured by a building distribution function and the sky view factor-explains city-to-city variations in nocturnal UHIs. Our results show a strong correlation between nocturnal UHIs and the city texture.

Despite decades of research seeking to derive the urban energy budget, the dynamics of thermal exchange in the densely constructed environment is not yet well understood. Using New York City as a study site, we present a novel hybrid experimental-computational approach for a better understanding of the radiative heat transfer in complex urban environments. The aim of this work is to contribute to the calculation of the urban energy budget, particularly the stored energy. We will focus our attention on surface thermal radiation. Improved understanding of urban thermodynamics incorporating the interaction of various bodies, particularly in high rise cities, will have implications on energy conservation at the building scale, and for human health and comfort at the urban scale. The platform presented is based on longwave hyperspectral imaging of nearly 100 blocks of Manhattan, in addition to a geospatial radiosity model that describes the collective radiative heat exchange between multiple buildings. Despite assumptions in surface emissivity and thermal conductivity of buildings walls, the close comparison of temperatures derived from measurements and computations is promising. Results imply that the presented geospatial thermodynamic model of urban structures can enable accurate and high resolution analysis of instantaneous urban surface temperatures.

VL - 8 UR - http://www.nature.com/articles/s41598-018-19846-5 JO - Sci Rep ER - TY - JOUR T1 - Conformal field theory of critical Casimir forces between surfaces with alternating boundary conditions in two dimensions JF - Journal of Statistical Mechanics: Theory and Experiment Y1 - 2017 A1 - Dubail, Jerome A1 - Santachiara, Raoul A1 - Emig, Thorsten KW - Casimir effect; conformal field theory; critical exponents and amplitudes AB -Systems as diverse as binary mixtures and inclusions in biological membranes, and many more, can be described effectively by interacting spins. When the critical fluctuations in these systems are constrained by boundary conditions, critical Casimir forces (CCF) emerge. Here we analyze CCF between boundaries with alternating boundary conditions in two dimensions, employing conformal field theory (CFT). After presenting the concept of boundary changing operators, we specifically consider two different boundary configurations for a strip of critical Ising spins: (I) alternating equi-sized domains of up and down spins on both sides of the strip, with a possible lateral shift, and (II) alternating domains of up and down spins of different size on one side and homogeneously fixed spins on the other side of the strip. Asymptotic results for the CCF at small and large distances are derived. We introduce a novel modified Szego formula for determinants of real antisymmetric block Toeplitz matrices to obtain the exact CCF and the corresponding scaling functions at all distances. We demonstrate the existence of a surface renormalization group flow between universal force amplitudes of different magnitude and sign. The Casimir force can vanish at a stable equilibrium position that can be controlled by parameters of the boundary conditions. Lateral Casimir forces assume a universal simple cosine form at large separations.

UR - http://stacks.iop.org/1742-5468/2017/i=3/a=033201?key=crossref.e58e04fb8593248b573c53589bed0f1d JO - J. Stat. Mech. ER - TY - JOUR T1 - Many-body heat radiation and heat transfer in the presence of a nonabsorbing background medium JF - Physical Review B Y1 - 2017 A1 - Müller, Boris A1 - Incardone, Roberta A1 - Antezza, Mauro A1 - Emig, Thorsten A1 - Krüger, Matthias AB -Heat radiation and near-field radiative heat transfer can be strongly manipulated by adjusting geometrical shapes, optical properties, or the relative positions of the objects involved. Typically, these objects are considered as embedded in vacuum. By applying the methods of fluctuational electrodynamics, we derive general closed-form expressions for heat radiation and heat transfer in a system of N arbitrary objects embedded in a passive nonabsorbing background medium. Taking into account the principle of reciprocity, we explicitly prove the symmetry and positivity of transfer in any such system. Regarding applications, we find that the heat radiation of a sphere as well as the heat transfer between two parallel plates is strongly enhanced by the presence of a background medium. Regarding near- and far-field transfer through a gas like air, we show that a microscopic model (based on gas particles) and a macroscopic model (using a dielectric contrast) yield identical results. We also compare the radiative transfer through a medium like air and the energy transfer found from kinetic gas theory.

VL - 95 UR - https://link.aps.org/doi/10.1103/PhysRevB.95.085413 IS - 8 JO - Phys. Rev. B ER - TY - JOUR T1 - Mechanisms of jamming in the Nagel-Schreckenberg model for traffic flow JF - Physical Review E Y1 - 2017 A1 - Bette, Henrik M. A1 - Habel, Lars A1 - Emig, Thorsten A1 - Schreckenberg, Michael AB -We study the Nagel-Schreckenberg cellular automata model for traffic flow by both simulations and analytical techniques. To better understand the nature of the jamming transition, we analyze the fraction of stopped cars P(v=0) as a function of the mean car density. We present a simple argument that yields an estimate for the free density where jamming occurs, and show satisfying agreement with simulation results. We demonstrate that the fraction of jammed cars P(v∈{0,1}) can be decomposed into the three factors (jamming rate, jam lifetime, and jam size) for which we derive, from random walk arguments, exponents that control their scaling close to the critical density.

VL - 95 UR - https://link.aps.org/doi/10.1103/PhysRevE.95.012311 IS - 1 JO - Phys. Rev. E ER - TY - JOUR T1 - Methodology for Estimation of Nanoscale Hardness via Atomistic Simulations JF - Journal of Nanomechanics and Micromechanics Y1 - 2017 A1 - Qomi, M. J. Abdolhosseini A1 - Ebrahimi, Davoud A1 - Mathieu Bauchy A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm KW - Atomistic simulation; Friction; Cohesion; Hardness; Tobermorite; Montmorillonite AB -Statistical mechanics has provided powerful techniques to measure mechanical properties of materials at the nanoscale and paved the way for bottom-up computational materials design. The introduction of such techniques in civil engineering applications, namely construction and geotechnical materials, remains limited to the elastic and fracture properties. This paper presents an atomistic approach to calculate the nanoscale cohesion, friction angle, and hardness. This method is based on the application of biaxial external deformation, or stress, in the weakest crystallographic direction in the material. The onset of the failure is characterized by investigating the unloading paths from several points on the stress-strain curve. Such calculations of the failure stress along different deformation paths provide multiple failure Mohr circles in the normal-shear stress space, which is found to provide a failure envelope akin to the Mohr-Coulomb failure criterion that is widely used for the plastic analysis of granular geomaterials. The failure envelope characterizes the nanoscale cohesion and friction angle, which in conjunction with continuum mechanics can be utilized to estimate the nanoscale hardness of layered materials. Application of this method to tobermorite and Na-montmorillonite crystals yields values that are close to the experimental measurements obtained using nanoindentation and atomic force microscopy techniques. (C) 2017 American Society of Civil Engineers.

VL - 7 UR - http://ascelibrary.org/doi/10.1061/%28ASCE%29NM.2153-5477.0000127 IS - 4 JO - J. Nanomech. Micromech. ER - TY - JOUR T1 - Nonequilibrium Fluctuational Quantum Electrodynamics: Heat Radiation, Heat Transfer, and Force JF - Annual Review of Condensed Matter Physics Y1 - 2017 A1 - Bimonte, Giuseppe A1 - Emig, Thorsten A1 - Kardar, Mehran A1 - Krüger, Matthias AB -Quantum-thermal fluctuations of electromagnetic waves are the cornerstone of quantum statistics and inherent to phenomena such as thermal radiation and van der Waals forces. Although the principles are found in elementary texts, recent experimental and technological advances make it necessary to come to terms with counterintuitive consequences at short scales—the so-called near-field regime. We focus on three manifestations: (*a*) The Stefan–Boltzmann law describes radiation from macroscopic bodies but fails for small objects. (*b*) The heat transfer between two bodies at close proximity is dominated by evanescent waves and can be orders of magnitude larger than the classical (propagating) contribution. (*c*) Casimir forces, dominant at submicron separation, are not sufficiently explored for objects at different temperatures (at least experimentally). We explore these phenomena using fluctuational quantum electrodynamics (QED), introduced by Rytov in the 1950s, combined with scattering formalisms. This enables investigation of different material properties, shapes, separations, and arrangements.

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 - Temperature distribution and heat radiation of patterned surfaces at short wavelengths JF - Physical Review E Y1 - 2017 A1 - Emig, Thorsten AB -We analyze the equilibrium spatial distribution of surface temperatures of patterned surfaces. The surface is exposed to a constant external heat flux and has a fixed internal temperature that is coupled to the outside heat fluxes by finite heat conductivity across the surface. It is assumed that the temperatures are sufficiently high so that the thermal wavelength (a few microns at room temperature) is short compared to all geometric length scales of the surface patterns. Hence the radiosity method can be employed. A recursive multiple scattering method is developed that enables rapid convergence to equilibrium temperatures. While the temperature distributions show distinct dependence on the detailed surface shapes (cuboids and cylinder are studied), we demonstrate robust universal relations between the mean and the standard deviation of the temperature distributions and quantities that characterize overall geometric features of the surface shape.

VL - 95 UR - http://link.aps.org/doi/10.1103/PhysRevE.95.052104 IS - 5 JO - Phys. Rev. E 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 - Effect of Polydispersity of Clay Platelets on the Aggregation And Mechanical Properties of Clay at the Mesoscale JF - Clays and Clay Minerals Y1 - 2016 A1 - Ebrahimi, Davoud A1 - Andrew J. Whittle A1 - Roland Jean-Marc Pellenq AB -The results from mesoscale simulations of the formation and evolution of microstructure for assemblies of Na-smectite particles based on assumed size distributions of individual clay platelets are presented here. The analyses predicted particle arrangements and aggregation (i.e. platelets linked in face-face configurations) and are used to link geometric properties of the microstructure and mechanical properties of the particle assemblies. Interactions between individual ellipsoidal clay platelets are represented using the Gay-Berne potential based on atomistic simulations of the free energy between two Na-smectite clay-platelets in liquid water, following a novel coarse-graining method developed previously. The current study describes the geometric (aggregate thickness, orientation, and porosity) and elastic properties in the ‘jammed states’ from the mesoscale simulations for selected ranges of clay particle sizes and confining pressures. The thickness of clay aggregates for monodisperse assemblies increases (with average stack thickness consisting of n = 3-8 platelets) with the diameter of the individual clay platelets and with the level of confining pressure. Aggregates break down at high confining pressures (50-300 atm) due to slippage between the platelets. Polydisperse simulations generate smaller aggregates (n = 2) and show much smaller effects of confining pressure. All assemblies show increased order with confining pressure, implying more anisotropic microstructure. The mesoscale simulations are also in good agreement with macroscopic compression behavior measured in conventional 1-D laboratory compression tests. The mesoscale assemblies exhibit cubic symmetry in elastic properties. The results for larger platelets (D = 1000 Å) are in good agreement with Nano-indentation measurements on natural clays and shale samples.

VL - 64 IS - 4 Special Issue: SI JO - clays clay miner ER - TY - JOUR T1 - Electromagnetic Casimir energy of a disk opposite a plane JF - Physical Review A Y1 - 2016 A1 - Emig, Thorsten A1 - Graham, Noah AB -Building on work by J. Meixner [Z. Naturforschung **3a**, 506 (1948)], we show how to compute the exact scattering amplitude (or T-matrix) for electromagnetic scattering from a perfectly conducting disk. This calculation is a rare example of a nondiagonal T-matrix that can nonetheless be obtained in a semianalytic form. We then use this result to compute the electromagnetic Casimir interaction energy for a disk opposite a plane, for arbitrary orientation angle of the disk, for separations greater than the disk radius. We find that the proximity force approximation (PFA) significantly overestimates the Casimir energy, in the case of both the ordinary PFA, which applies when the disk is parallel to the plane, and the “edge PFA”, which applies when the disk is perpendicular to the plane.

We study the shift of rotational levels of a diatomic polar molecule due to its van der Waals interaction with a gently curved dielectric surface at temperature T, and submicron separations. The molecule is assumed to be in its electronic and vibrational ground state, and the rotational degrees are described by a rigid rotor model. We show that under these conditions retardation effects and surface dispersion can be neglected. The level shifts are found to be independent of T, and given by the quantum state averaged classical electrostatic interaction of the dipole with its image on the surface. We use a derivative expansion for the static Green's function to express the shifts in terms of surface curvature. We argue that the curvature induced line splitting is experimentally observable, and not obscured by natural linewidths and thermal broadening.

VL - 94 UR - https://link.aps.org/doi/10.1103/PhysRevA.94.022509 IS - 2 JO - Phys. Rev. A ER - TY - JOUR T1 - Velocity statistics of the Nagel-Schreckenberg model JF - Physical Review E Y1 - 2016 A1 - Bain, Nicolas A1 - Emig, Thorsten A1 - Franz-Josef Ulm A1 - Schreckenberg, Michael AB -The statistics of velocities in the cellular automaton model of Nagel and Schreckenberg for traffic are studied. From numerical simulations, we obtain the probability distribution function (PDF) for vehicle velocities and the velocity-velocity (vv) correlation function. We identify the probability to find a standing vehicle as a potential order parameter that signals nicely the transition between free congested flow for sufficiently large number of velocity states. Our results for the vv correlation function resemble features of a second order phase transition. We develop a 3-body approximation that allows us to relate the PDFs for velocities and headways. Using this relation, an approximation to the velocity PDF is obtained from the headway PDF observed in simulations. We find a remarkable agreement between this approximation and the velocity PDF obtained from simulations.

VL - 93 IS - 2 JO - Phys. Rev. E ER - TY - JOUR T1 - Advances in design and modeling of porous materials JF - The European Physical Journal Special Topics Y1 - 2015 A1 - Ayral, Andre A1 - Calas-Etienne, Sylvie A1 - Benoit A. Coasne A1 - Deratani, André A1 - Evstratov, Alexis A1 - Anne Galarneau A1 - Grande, Daniel A1 - Hureau, Matthieu A1 - Jobic, Hervé A1 - Morlay, Catherine A1 - Parmentier, Julien A1 - Prelot, Bénédicte A1 - Rossignol, Sylvie A1 - Simon-Masseron, Angélique A1 - Thibault-Starzyk, Frédéric AB -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 - Casimir-Polder force between anisotropic nanoparticles and gently curved surfaces JF - Physical Review D Y1 - 2015 A1 - Bimonte, Giuseppe A1 - Emig, Thorsten A1 - Kardar, Mehran AB -The Casimir--Polder interaction between an anisotropic particle and a surface is orientation dependent. We study novel orientational effects that arise due to curvature of the surface for distances much smaller than the radii of curvature by employing a derivative expansion. For nanoparticles we derive a general short distance expansion of the interaction potential in terms of their dipolar polarizabilities. Explicit results are presented for nano-spheroids made of SiO2 and gold, both at zero and at finite temperatures. The preferred orientation of the particle is strongly dependent on curvature, temperature, as well as material properties.

VL - 92 IS - 2 JO - Phys. Rev. D 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.

To study the critical Casimir force between chemically structured boundaries immersed in a binary mixture at its demixing transition, we consider a strip of Ising spins subject to alternating fixed spin boundary conditions. The system exhibits a boundary induced phase transition as function of the relative amount of up and down boundary spins. This transition is associated with a sign change of the asymptotic force and a diverging correlation length that sets the scale for the crossover between different universal force amplitudes. Using conformal field theory and a mapping to Majorana fermions, we obtain the universal scaling function of this crossover, and the force at short distances.

VL - 112 IS - 6 JO - EPL ER - TY - Generic T1 - C-S-H across Length Scales: From Nano to Micron T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 Y1 - 2015 A1 - Mohammad Javad Abdolhosseini Qomi A1 - Enrico Masoero A1 - Mathieu Bauchy A1 - Franz-Josef Ulm A1 - Emanuela Del Gado A1 - Roland Jean-Marc Pellenq ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -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 - 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 - Interplay of curvature and temperature in the Casimir–Polder interaction JF - Journal of Physics: Condensed Matter Y1 - 2015 A1 - Bimonte, Giuseppe A1 - Emig, Thorsten AB -We study the Casimir-Polder interaction at finite temperatures between a polarizable small, anisotropic particle and a non-planar surface using a derivative expansion. We obtain the leading and the next-to-leading curvature corrections to the interaction for low and high temperatures. Explicit results are provided for the retarded limit in the presence of a perfectly conducting surface.

VL - 27 IS - 21 JO - J. Phys.: Condens. Matter 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 - 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 - 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 - Reversing the critical Casimir force by shape deformation JF - Physics Letters B Y1 - 2015 A1 - Bimonte, Giuseppe A1 - Emig, Thorsten A1 - Kardar, Mehran AB -The exact critical Casimir force between periodically deformed boundaries of a 2D semi-infinite strip is obtained for conformally invariant classical systems. Only two parameters (conformal charge and scaling dimension of a boundary changing operator), along withthe solution of an electrostatic problem, determine the Casimir force, rendering the theory practically applicable to any shape and arrangement. The attraction between any two mirror symmetric objects follows directly from our general result. The possibility of purely shape induced reversal of the force, as well as occurrence of stable equilibrium points, is demonstrated for certain conformally invariant models, including the tricritical Ising model.

VL - 743 JO - Physics Letters B ER - TY - Generic T1 - Rheology and Micromechanical Analysis of Granular Media Composed of Platy Particles: A Step Toward the DEM Simulation of Clayey Soils T2 - 15th Pan-American Conference on Soil Mechanics and Geotechnical Engineering (PCSMGE) / 8th South American Congress on Rock Mechanics (SCRM) Y1 - 2015 A1 - Boton, Mauricio A1 - Estrada, Nicolas A1 - Emilien Azéma ED - Manzanal, D ED - Sfriso, AO AB -This work was carried out in the framework of a larger project devoted

to the micro-mechanical modeling of clayey soils by means of discrete element

simulations. Here, we study the rheology and microstructure of granular media

composed of platy particles. 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. Specifically, we

revisit and analyze in detail an ordering phenomenon described briefly in a

previous paper [1] and investigate its consequences in terms of the alignment of

particles and cluster formation. We find that particle alignment is strongly

enhanced by the degree of platyness and leads to the formation of face-connected

clusters. Due to dynamics, this spontaneous clustering is a robust characteristic of

granular systems composed of platy particles even in the absence of attraction

forces.

JF - 15th Pan-American Conference on Soil Mechanics and Geotechnical Engineering (PCSMGE) / 8th South American Congress on Rock Mechanics (SCRM) CY - NOV 15-18 2015 Buenos Aires, ARGENTINA VL - FROM FUNDAMENTALS TO APPLICATIONS IN GEOTECHNICS ER - TY - Generic T1 - Shrinkage Due to Colloidal Force Interactions T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete Structures CONCREEP 10 Y1 - 2015 A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -

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 - Generic T1 - Transient regime to fluidized chimney within a granular bed by means of a 2D DEM/LBM modeling T2 - IV International Conference on Particle-based Methods (PARTICLES 2015) Y1 - 2015 A1 - Jeff Ngoma A1 - Pierre Philippe A1 - Stéphane Bonelli A1 - Cuellar, Pablo A1 - Jean-Yves Delenne A1 - Farhang Radjaï ED - Onate, E ED - Bischoff, M ED - Owen, DRJ KW - DEM KW - Fluidization KW - Granular Materials KW - LBM AB -

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 - Atomic-scale modelling of elastic and failure properties of clays JF - Molecular Physics Y1 - 2014 A1 - György Hantal A1 - Brochard, Laurent A1 - Hadrien Laubie A1 - Ebrahimi, Davoud A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Benoit A. Coasne KW - clay KW - elastic properties KW - fracture KW - reactive molecular simulation AB -The elastic and failure properties of a typical clay, illite, are investigated using molecular simulation. We employ a reactive (ReaxFF) and a non-reactive (ClayFF) force field to assess the elastic properties of the clay. As far as failure is concerned, ReaxFF was used throughout the study; however, some calculations were also performed with ClayFF. A crack parallel to the clay layers is found to have low fracture resistance when submitted to a tensile loading perpendicular to the crack. The mechanism of both yield and fracture failures is decohesion in the interlayer space. In contrast, under shear loading, the nanoscale failure mechanism is a stick-slip between clay layers. No fracture propagation is observed as the clay layers slide on top of each other. The low fracture resistance in mode I and the stick-slip failure in mode II are both the consequence of the lack of chemical bonds between clay layers where the cohesion is provided by non-covalent interactions. This work, which provides a description of the failure of clays at the microscopic scale, is the first step towards describing the failure of clays at a larger scale where the polycrystalline distribution of clay grains must be taken into account.

VL - 112 IS - 9-10 ER - TY - JOUR T1 - Casimir-Polder interaction for gently curved surfaces JF - Physical Review D Y1 - 2014 A1 - Bimonte, Giuseppe A1 - Emig, Thorsten A1 - Kardar, Mehran AB -We use a derivative expansion for gently curved surfaces to compute the leading and the next-to-leading curvature corrections to the Casimir-Polder interaction between a polarizable small particle and a nonplanar surface. While our methods apply to any homogeneous and isotropic surface, explicit results are presented here for perfect conductors. We show that the derivative expansion of the Casimir-Polder potential follows from a resummation of its perturbative series, for small in-plane momenta. We consider the retarded, nonretarded and classical high-temperature limits.

VL - 90 ER - TY - JOUR T1 - Mesoscale properties of clay aggregates from potential of mean force representation of interactions between nanoplatelets JF - Journal of Chemical Physics Y1 - 2014 A1 - Ebrahimi, Davoud A1 - Andrew J. Whittle A1 - Roland Jean-Marc Pellenq AB -Face-to-face and edge-to-edge free energy interactions of Wyoming Na-montmorillonite platelets were studied by calculating potential of mean force along their center to center reaction coordinate using explicit solvent (i.e., water) molecular dynamics and free energy perturbation methods. Using a series of configurations, the Gay-Berne potential was parametrized and used to examine the mesoscale aggregation and properties of platelets that are initially random oriented under isothermal-isobaric conditions. Aggregates of clay were defined by geometrical analysis of face-to-face proximity of platelets with size distribution described by a log-normal function. The isotropy of the microstructure was assessed by computing a scalar order parameter. The number of platelets per aggregate and anisotropy of the microstructure both increases with platelet plan area. The system becomes more ordered and aggregate size increases with increasing pressure until maximum ordered state at confining pressure of 50 atm. Further increase of pressure slides platelets relative to each other leading to smaller aggregate size. The results show aggregate size of (3-8) platelets for sodium-smectite in agreement with experiments (3-10). The geometrical arrangement of aggregates affects mechanical properties of the system. The elastic properties of the meso-scale aggregate assembly are reported and compared with nanoindentation experiments. It is found that the elastic properties at this scale are close to the cubic systems. The elastic stiffness and anisotropy of the assembly increases with the size of the platelets and the level of external pressure. (C) 2014 AIP Publishing LLC.

VL - 140 IS - 15 ER - TY - Generic T1 - Mesoscale simulation of clay aggregate formation and mechanical properties T2 - 3rd International Symposium on Geomechanics from Micro to Macro Y1 - 2014 A1 - Ebrahimi, Davoud A1 - Roland Jean-Marc Pellenq A1 - Andrew J. Whittle ED - Kenichi Soga ED - Krishna Kumar ED - Giovanna Biscontin AB -This paper proposes a novel methodology for understanding the meso-scale aggregation of clay platelets in water. We use Molecular Dynamics simulations using the CLAYFF force fields to represent the interactions between two layers of Wyoming montmorillonite (Na-smectite) in bulk water. The analyses are used to establish the potential of mean force at different spacings between the layers for edge-to-edge and face-to-face interactions. This is accomplished by finding the change in free energy as a function of the separation distance between the platelets using thermodynamic perturbation theory with a simple overlap sampling method. These nanoscale results are then used to calibrate the Gay–Berne (GB) potential that represents each platelet as a single-site ellipsoidal body. A coarse-graining upscaling approach then uses the GB potentials and molecular dynamics to represent the meso-scale aggregation of clay platelets (at submicron length scale). Results from meso-scale simulations obtain the equilibrium/jamming configurations for mono-disperse clay platelets. The results show aggregation for a range of clay platelets dimensions and pressures with mean stack size ranging from 3 to 8 platelets. The particle assemblies become more ordered and exhibit more pronounced elastic anisotropy at higher confining pressures. The results are in good agreement with previously measured nano-indentation moduli over a wide range of clay packing densities.

JF - 3rd International Symposium on Geomechanics from Micro to Macro CY - SEP 01-03 2014 Univ Cambridge, Cambridge, ENGLAND VL - Geomechanics from Micro to Macro: Granular Matter IS - 3 JO - Granular Matter 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 - Generic T1 - AIP Conference Proceedings A multiscale description of failure in granular materials T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Hadda, Nejib A1 - François Nicot A1 - Sibille, Luc A1 - Farhang Radjaï A1 - Tordesillas, Antoinette A1 - Darve, Félix ED - Yu, A ED - Dong, K ED - Yang, R AB -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 - 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 - JOUR T1 - Environmental scanning electron microscopy (ESEM) and nanoindentation investigation of the crack tip process zone in marble JF - Acta Geotechnica Y1 - 2013 A1 - Brooks, Z. A1 - Franz-Josef Ulm A1 - Einstein, H. H. AB -This study explores the interaction between crack initiation and nanomechanical properties in the crack-tip fracture process zone of Carrara marble. Specimens with preexisting cracks were loaded in a uniaxial testing machine until the process zone appeared at the tips of the preexisting cracks. ESEM analysis reveals an increase in microcrack density in the process zone with increased loading of the specimen. Nanoindentation testing comprised of lines and grids of single nanoindentations located both near and far from the process zone shows a decrease in both indentation modulus and indentation hardness near grain boundaries in intact material, and with closeness to the process zone. Ultimately, the study confirms that the crack-tip process zone manifests itself as an area of reduced indentation hardness and indentation modulus in marble.

VL - 8 IS - 3 JO - Acta Geotech. ER - TY - JOUR T1 - Mesoscopic Monte Carlo simulations of microstructure and conductivity of zirconia-based ceramics JF - J. Eur. Ceram. Soc. Y1 - 2013 A1 - P. Epicoco A1 - Benoit A. Coasne A1 - A. Gioia A1 - P. Papet A1 - I. Cabodi A1 - M. Gaubil AB -Zirconia-based refractories are ceramics that serve as furnaces for the production of high-quality speciality glasses. These materials are obtained by cooling a melt composed of different oxides and oxide precursors. The microstructure of these refractories consists of monoclinic zirconia embedded in a three-dimensional interconnected amorphous phase. Using mesoscopic Monte Carlo simulations, the present paper addresses the effect of the amount of amorphous phase and temperature on the microstructure of the material. The simulated microstructures resemble those obtained using tomography experiments. A theoretical model describing the electrical behavior of the material as a function of temperature and composition is also reported. In this model, the conductivity of the ZrO2 particles is assumed to be constant, but the overall conductivity of the sample depends on its tortuosity s. Comparison with experimental data suggests that the present model provides a realistic picture of the electrical behavior of zirconia-based ceramics and leads to quantitative predictions.

VL - 61 IS - 13 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 - 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 - 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 - 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 - Nanoscale Elastic Properties of Montmorillonite upon Water Adsorption JF - Langmuir Y1 - 2012 A1 - Ebrahimi, Davoud A1 - Roland Jean-Marc Pellenq A1 - Andrew J. Whittle AB -Smectites are an important group of clay minerals that experience swelling upon water adsorption. This paper uses molecular dynamics with the CLAYFF force field to simulate isothermal isobaric water adsorption of interlayer Wyoming Na-montmorillonite, a member of the smectite group. Nanoscale elastic properties of the clay–interlayer water system are calculated from the potential energy of the model system. The transverse isotropic symmetry of the elastic constant matrix was assessed by calculating Euclidean and Riemannian distance metrics. Simulated elastic constants of the clay mineral are compared with available results from acoustic and nanoindentation measurements.

VL - 28 IS - 49 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 - 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 - Generic T1 - Springer Series in Geomechanics and Geoengineering Multiscale and Multiphysics Processes in Geomechanics SIMULATION OF HYDRATION AND ELASTIC PROPERTIES OF MONTMORILLONITE USING MOLECULAR DYNAMICS T2 - International Workshop on Multiscale and Multiphysics Processes in Geomechanics Y1 - 2011 A1 - Roland Jean-Marc Pellenq A1 - Andrew J. Whittle A1 - Ebrahimi, Davoud ED - Borja, Ronaldo I. ED - Wu, Wei AB -This article describes molecular-level simulations of the adsorption isotherm and elastic properties for a Wyoming Na-montmorillonite using the General Utility Lattice Program. The selected clay mineral includes isomorphous substitutions in both the silica tetrahedral and octahedral gibbsite sheets. The resulting negative charges are balanced by Na+ cations. The authors have investigated the importance of different interatomic potentials in understanding the adsorption of water molecules (at ambient temperature, 300K). A unique combination of NPT and Grand Canonical ensemble simulations using Core–Shell potential shows intra-layer adsorption, while the same approach based on empirical CLAYFF potential produces only interlayer water adsorption. There is a correspondingly large difference in the theoretical elastic modulus normal to the clay sheets using the two different sets of force potentials. This article discusses the underlying reasons for these differences and reviews the experimental evidence supporting intra-layer water absorption in montmorillonite.

JF - International Workshop on Multiscale and Multiphysics Processes in Geomechanics PB - Springer Berlin Heidelberg CY - JUN 23-25 2010 Stanford Univ, Stanford, CA VL - Springer Series in Geomechanics and Geoengineering SN - 978-3-642-19629-4 ER -