@article {594, title = {Investigation of mixed ionic/nonionic building blocks for the dual templating of macro-mesoporous silica}, journal = {Journal of Colloid and Interface Science}, volume = {533}, year = {2019}, month = {Jan-01-2019}, pages = {385 - 400}, abstract = {

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.

}, issn = {00219797}, doi = {10.1016/j.jcis.2018.08.068}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0021979718309883}, author = {Roucher, Armand and Emo, Melanie and Vibert, Francois and Stebe, Marie-Jose and Schmitt, Veronique and Jonas, Florian and Backov, Renal and Blin, Jean-Luc} } @article {576, title = {Impact of Nanoporosity on Hydrocarbon Transport in Shales{\textquoteright} Organic Matter}, journal = {Nano Letters}, volume = {18}, year = {2018}, month = {Jan-16-2018}, pages = {832 - 837}, abstract = {

In a context of growing attention for shale gas, the precise impact of organic matter (kerogen) on hydrocarbon recovery from unconventional reservoirs still has to be assessed. Kerogen\&$\#$39;s microstructure is characterized by a very disordered pore network that greatly affects hydrocarbon transport. The specific structure and texture of this organic matter at the nanoscale is highly dependent on its origin. In this study, by the use of statistical physics and molecular dynamics, we shed some new lights on hydrocarbon transport through realistic molecular models of kerogen at different level of maturity [Bousige et al. Nat. Mater. 2016, 15, 576]. Despite the apparent complexity, severe confinement effects controlled by the porosity of the various kerogens allow linear alkanes (from methane to dodecane) transport to be studied only via the self-diffusion coefficients of the species. The decrease of the transport coefficients with the amount of adsorbed fluid can be described by a free volume theory. Ultimately, the transport coefficients of hydrocarbons can be expressed simply as a function of the porosity (volume fraction of void) of the microstructure, thus paving the way for shale gas recovery predictions.

https://pubs-acs-org.libproxy.mit.edu/appl/literatum/publisher/achs/journals/content/nalefd/2018/nalefd.2018.18.issue-2/acs.nanolett.7b04079/20180208/images/medium/nl-2017-04079p_0004.gif

}, keywords = {Kerogen; microporosity; diffusion; hydrocarbons; shales}, issn = {1530-6984}, doi = {10.1021/acs.nanolett.7b04079}, url = {http://pubs.acs.org/doi/10.1021/acs.nanolett.7b04079}, author = {Ama{\"e}l Obliger and Franz-Josef Ulm and Roland Jean-Marc Pellenq} } @article {580, title = {Improving the practicality and safety of artificial corneas: Pre-assembly and gamma-rays sterilization of the Boston Keratoprosthesis}, journal = {The Ocular Surface}, volume = {16}, year = {2018}, month = {Jul-2018}, pages = {322 - 330}, abstract = {

Purpose: To make the Boston keratoprosthesis (B-KPro), together with its carrier corneal graft, more easily procured, transported and stored, as well as less expensive, easier for the surgeon to implant and safer for the patient, it is proposed that the B-KPro-graft combination be pre-assembled by an expert technician, followed by sterilization with gamma ray irradiation (GI) allowing long-term storage at room temperature. For this to be possible, it must be shown that the B-KPro itself (not only the graft) remains unharmed by the irradiation.

Methods: Polymethyl methacrylate (PMMA) discs and B-KPros were submitted to either ethylene oxide sterilization or different doses of GI. Cell biocompatibility, mechanical strength and optical quality were evaluated. The feasibility of assembling the B-KPro to a corneal graft, and gamma-radiate afterwards, was also assessed.

Results: There were no differences in cell biocompatibility between the samples. The optical evaluation showed high levels of transparency for all the groups. The absorbance of ultraviolet was higher for the groups treated with GI. The mechanical evaluation by nanoindentation showed no alterations of the PMMA discs after GI. The flexure test revealed a similar mechanical behavior. Technically, pre-assembly and GI of the B-KPro revealed no problems.

Conclusions: Sterilization of B-KPro using GI has no detrimental influence on the device. The pre-assembly of B-KPro to a donor cornea, followed by gamma sterilization, emerges as an efficient and safe procedure. (C) 2018 Elsevier Inc. All rights reserved.

}, keywords = {Artificial corneas; Boston keratoprosthesis; Gamma radiation; Gamma rays; Preassembly; Sterilization; Polymethyl methacrylate}, issn = {15420124}, doi = {10.1016/j.jtos.2018.04.002}, url = {https://linkinghub.elsevier.com/retrieve/pii/S1542012418300442}, author = {Gonzalez-Andrades, Miguel and Sharifi, Roholah and Islam, Mohammad Mirazul and Divoux, Thibaut and Haist, Michael and Paschalis, Eleftherios I. and Gelfand, Larisa and Mamodaly, Shamina and Di Cecilia, Luca and Cruzat, Andrea and Franz-Josef Ulm and Chodosh, James and Delori, Francois and Dohlman, Claes H.} } @article {628, title = {Inertial shear flow of assemblies of frictionless polygons: Rheology and microstructure}, journal = {The European Physical Journal E}, volume = {4}, year = {2018}, month = {Jan-05-2018}, pages = { Article Number 2 }, abstract = {

Motivated by the understanding of shape effects in granular materials, we numerically investigate the macroscopic and microstructural properties of anisotropic dense assemblies of frictionless polydisperse rigid pentagons in shear flow, and compare them with similar systems of disks. Once subjected to large cumulative shear strains their rheology and microstructure are investigated in uniform steady states, depending on inertial number I, which ranges from the quasistatic limit (I similar to 10(-5)) to 0.2. In the quasistatic limit both systems are devoid of Reynolds dilatancy, i.e., flow at their random close packing density. Both macroscopic friction angle., an increasing function of I, and solid fraction., a decreasing function of I, are larger with pentagons than with disks at small I, but the differences decline for larger I and, remarkably, nearly vanish for I similar to 0.2. Under growing I, the depletion of contact networks is considerably slower with pentagons, in which increasingly anisotropic, but still well-connected force-transmitting structures are maintained throughout the studied range. Whereas contact anisotropy and force anisotropy contribute nearly equally to the shear strength in disk assemblies, the latter effect dominates with pentagons at small I, while the former takes over for I of the order of 10(-2). The size of clusters of grains in side-to-side contact, typically comprising more than 10 pentagons in the quasistatic limit, very gradually decreases for growing I.

}, keywords = {CONTACT DYNAMICS METHOD; DENSE GRANULAR FLOWS; GEOMETRIC ORIGIN; BEHAVIOR; PACKING; MEDIA; SHAPE}, issn = {1292-8941}, doi = {10.1140/epje/i2018-11608-9}, url = {http://link.springer.com/10.1140/epje/i2018-11608-9}, author = {Emilien Az{\'e}ma and Farhang Radja{\"\i} and Jean-Noel~ Roux} } @article {584, title = {Impact of saccharides on the drying kinetics of agarose gels measured by in-situ interferometry}, journal = {Scientific Reports}, volume = {7}, year = {2017}, month = {Jan-23-2017}, pages = { Article Number: 41185 }, abstract = {

Agarose gels are viscoelastic soft solids that display a porous microstructure filled with water at 90\% w/w or more. Despite an extensive use in food industry and microbiology, little is known about the drying kinetics of such squishy solids, which suffers from a lack of time-resolved local measurements. Moreover, only scattered empirical observations are available on the role of the gel composition on the drying kinetics. Here we study by in-situ interferometry the drying of agarose gels of various compositions cast in Petri dishes. The gel thinning is associated with the displacement of interference fringes that are analyzed using an efficient spatiotemporal filtering method, which allows us to assess local thinning rates as low as 10 nm/s with high accuracy. The gel thinning rate measured at the center of the dish appears as a robust observable to quantify the role of additives on the gel drying kinetics and compare the drying speed of agarose gels loaded with various non-gelling saccharides of increasing molecular weights. Our work shows that saccharides systematically decrease the agarose gel thinning rate up to a factor two, and exemplifies interferometry as a powerful tool to quantify the impact of additives on the drying kinetics of polymer gels.

}, doi = {10.1038/srep41185}, url = {http://www.nature.com/articles/srep41185}, author = {Mao, Bosi and Divoux, Thibaut and Snabre, Patrick} } @article {245, title = {Inhomogeneity in Cement Hydrates: Linking Local Packing to Local Pressure}, journal = {Journal of Nanomechanics and Micromechanics}, volume = {7}, year = {2017}, month = {Mar-03-2017}, pages = {Article Number: UNSP 04017003}, abstract = {

Nanoscale structural heterogeneities were recently revealed in computational and experimental studies of calcium silicate hydrates in hardened cement pastes. In this work their consequences for the mechanics are analyzed by computing local pressures in model samples at different overall densities, corresponding to different initial water-to-cement ratios. The correlations between pore size distributions, local density, local cohesive energy, and local pressure clearly show how in these materials structural heterogeneities may be the origin of significant mechanical heterogeneities. The results indicate that even at high density pressure, heterogeneities develop during the densification of cement hydrates and result in the coexistence of regions of high positive pressure with regions of negative pressure, in spite of the overall mechanical stability of the samples. Furthermore, the regions of negative pressure, prone to mechanical instabilities and local plastic processes, tend to be localized close to the surface of large mesopores and hence to be more significant at higher initial water content.
Inhomogeneity in Cement Hydrates: Linking Local Packing to Local Pressure. Available from: https://www.researchgate.net/publication/314222493_Inhomogeneity_in_Cement_Hydrates_Linking_Local_Packing_to_Local_Pressure [accessed Oct 2, 2017].

}, issn = {2153-5434}, doi = {10.1061/(ASCE)NM.2153-5477.0000120}, author = {Katerina Ioannidou and Emanuela Del Gado and Franz-Josef Ulm and Roland Jean-Marc Pellenq} } @article {613, title = {Inorganic, Hybridized and Living Macrocellular Foams: {\textquotedblleft}Out of the Box{\textquotedblright} Heterogeneous Catalysis}, journal = {The Chemical Record}, volume = {18}, year = {2017}, month = {Dec-01-2017}, pages = {776-787}, abstract = {

With this personal account we show how the Integrative Chemistry, when combining the sol-gel process and concentrated emulsions, allows to trigger inorganic, hybrid or living materials when dedicated toward heterogeneous catalysis applications. In here we focus on 3D-macrocellular monolithic foams bearing hierarchical porosities and applications thereof toward heterogeneous catalysis where both activities and mass transport are enhanced. We thereby first depict the general background of emulsions, focusing on concentrated ones, acting as soft templates for the design of solid (HIPE) foams, HIPE being the acronym for High Internal Phase Emulsions while encompassing both sol-gel and polymer chemistry. Secondly we extend this approach toward the design of inorganic cellular materials labeled Si(HIPE) and hybrid organic-inorganic foams, labeled Organo-Si(HIPE), where heterogeneous catalysis applications are addressed considering acidic, metallic, enzymatic and bacterial-based modified Si-HIPE. Along, we will show how the fluid hydrodynamic within the macrocellular foams is offering advanced \"out of the box\" heterogeneous catalytic capabilities.

}, keywords = {catalysts; emulsion-templating; integrative chemistry; porous matter; sol-gel process}, doi = {10.1002/tcr.v18.7-810.1002/tcr.201700075}, url = {http://doi.wiley.com/10.1002/tcr.v18.7-8}, author = {Roucher, Armand and Depardieu, Martin and Pekin, Deniz and Morvan, Micka{\"e}l and Backov, Renal} } @article {271, title = {Implicit frictional-contact model for soft particle systems}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {83}, year = {2015}, month = {Oct-2015}, pages = {72 - 87}, abstract = {

We introduce a novel numerical approach for the simulation of soft particles interacting via frictional contacts. This approach is based on an implicit formulation of the Material Point Method, allowing for large particle deformations, combined with the Contact Dynamics method for the treatment of unilateral frictional contacts between particles. This approach is both precise due to the treatment of contacts with no regularization and artificial damping parameters, and robust due to implicit time integration of both bulk degrees of freedom and relative contact velocities at the nodes representing the contact points. By construction, our algorithm is capable of handling arbitrary particle shapes and deformations. We illustrate this approach by two simple 2D examples: a Hertz contact and a rolling particle on an inclined plane. We also investigate the compaction of a packing of circular particles up to a solid fraction well above the jamming limit of hard particles. We find that, for the same level of deformation, the solid fraction in a packing of frictional particles is above that of a packing of frictionless particles as a result of larger particle shape change.

}, issn = {00225096}, doi = {10.1016/j.jmps.2015.06.007}, author = {Saeid Nezamabadi and Farhang Radja{\"\i} and Julien Averseng and Jean-Yves Delenne} } @article {151, title = {Inference of the phase-to-mechanical property link via coupled X-ray spectrometry and indentation analysis: Application to cement-based materials}, journal = {Cement and Concrete Research}, volume = {67}, year = {2015}, month = {Jan-2015}, pages = {271 - 285}, abstract = {

A novel approach for the chemo-mechanical characterization of cement-based materials is presented, which combines the classical grid indentation technique with elemental mapping by scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS). It is illustrated through application to an oil-well cement system with siliceous filler. The characteristic X-rays of major elements (silicon, calcium and aluminum) are measured over the indentation region and mapped back on the indentation points. Measured intensities together with indentation hardness and modulus are considered in a clustering analysis within the framework of Finite Mixture Models with Gaussian component density function. The method is able to successfully isolate the calcium-silica-hydrate gel at the indentation scale from its mixtures with other products of cement hydration and anhydrous phases; thus providing a convenient means to link mechanical response to the calcium-to-silicon ratio quantified independently via X-ray wavelength dispersive spectroscopy. A discussion of uncertainty quantification of the estimated chemo-mechanical properties and phase volume fractions, as well as the effect of chemical observables on phase assessment is also included.

}, issn = {00088846}, doi = {10.1016/j.cemconres.2014.09.001}, author = {Konrad J. Krakowiak and Wilson, William and James, Simon and Musso, Simone and Franz-Josef Ulm} } @article {154, title = {Internal friction and absence of dilatancy of packings of frictionless polygons}, journal = {Physical Review E}, volume = {91}, year = {2015}, month = {Jan-30-2015}, pages = {Article Number: 010202}, abstract = {

By means of numerical simulations, we show that assemblies of frictionless rigid pentagons in slow shear flow possess an internal friction coefficient (equalto0.183\±0.008 with our choice of moderately polydisperse grains) but no macroscopic dilatancy. In other words, despite side-side contacts tending to hinder relative particle rotations, the solidfraction under quasistatic shear coincides with that of isotropic random close packings of pentagonal particles. Properties of polygonal grains are thus similar to those of disks in that respect. We argue that continuous reshuffling of the force-bearing network leads to frequent collapsing events at the microscale, thereby causing the macroscopic dilatancy to vanish. Despite such rearrangements, the shear flow favors an anisotropic structure that is at the origin of the ability of the system to sustain shear stress.

}, issn = {1539-3755}, doi = {10.1103/PhysRevE.91.010202}, author = {Emilien Az{\'e}ma and Farhang Radja{\"\i} and Jean-Noel~ Roux} } @article {159, title = {Interplay of curvature and temperature in the Casimir{\textendash}Polder interaction}, journal = {Journal of Physics: Condensed Matter}, volume = {27}, year = {2015}, month = {Jun-03-2015}, pages = {214018}, abstract = {

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.

}, issn = {0953-8984}, doi = {10.1088/0953-8984/27/21/214018}, author = {Bimonte, Giuseppe and Emig, Thorsten} } @article {146, title = {Investigating carbon materials nanostructure using image orientation statistics}, journal = {Carbon}, volume = {84}, year = {2015}, month = {Apr-2015}, pages = {160 - 173}, abstract = {

A new characterization method of the lattice fringe images of turbostratic carbons is proposed. This method is based on the computation of their orientation field without explicit detection of fringes. It allows meaningful insights into the material nanostructure and nanotexture at several scales, either qualitatively or quantitatively. The calculation of pairwise spatial statistics of the orientation field at short distance provides measurements of the coherence lengths along any direction, in particular along and orthogonally to the layers. These statistics also allow representing orientation coherence patterns typical of the observed nanostructure. At larger distances, the mean disorientation of the fringes is computed and information about the homogeneity of the sample is obtained. An experimental validation is carried out on various artificial images and an application to the characterization of four bulk turbostratic carbons is provided.

}, issn = {00086223}, doi = {10.1016/j.carbon.2014.11.048}, author = {J. Philippe Da-Costa and Weisbecker, P. and Baptiste Farbos and Jean-Marc Leyssale and G{\'e}rard L. Vignoles and Germain, C.} } @article {184, title = {Ion-specific adsorption and electroosmosis in charged amorphous porous silica}, journal = {Phys. Chem. Chem. Phys.}, volume = {17}, year = {2015}, month = {Oct-14-2015}, pages = {24683 - 24695}, abstract = {

Monovalent and divalent aqueous electrolytes confined in negatively charged porous silica are studied by means of molecular simulations including free energy calculations. Owing to the strong cation adsorption at the surface, surface charge overcompensation (overscreening) occurs which leads to an effective positive surface next to the Stern layer, followed by a negatively charged diffuse layer. A simple Poisson-Boltzmann model in which the single-ion potential of mean force is introduced is shown to capture the most prominent features of ion density profiles near an amorphous silica surface. Nevertheless, due to its mean-field nature, which fails to account for correlations, this simple model does not predict overscreening corresponding to charge inversion at the surface. Such an overscreening drastically affects the transport of confined electrolytes as it leads to flow reversal when subjected to an electric field. A simple continuum theory is shown to capture how the electro-osmotic flow is affected by overscreening and by the apparent enhanced viscosity of the confined electrolytes. Comparison with available experimental data is discussed, as well as the implications of these phenomena for \ζ-potential measurements.

}, issn = {1463-9076}, doi = {10.1039/C5CP03818A}, author = {Hartkamp, Remco and Bertrand Siboulet and Jean-Francois Dufreche and Benoit A. Coasne} } @article {311, title = {An improved technique for characterizing the fracture toughness via scratch test experiments}, journal = {Wear}, volume = {313}, year = {2014}, month = {May-15-2014}, pages = {117 - 124}, abstract = {

The scratch test consists in pushing a tool across the surface of a weaker material at a given penetration depth; and it has several applications in Science and Engineering including strength testing of rocks and ceramics, damage of polymers and metals and quality control of thin films and coatings. Despite numerous attempts in the scientific literature, the application of scratch tests to the characterization of fracture properties remains a challenge and a heavily controversial topic. Therefore, this investigation aims at articulating a rigorous theoretical and experimental framework in order to assess the fracture toughness at both the macroscopic and the microscopic length scales, using scratch tests. First, we apply optical microscopy and scanning electron microscopy to investigate the physical evidence of crack initiation, crack propagation and material removal mechanisms during scratch tests. Then, we employ Finite Element simulations of crack growth during macroscopic scratch tests to assess the influence of the blade back-rake angle, the friction coefficient between the blade and the material and the wear flat of the blade on the scratching forces, thus testing the robustness of our Linear Elastic Fracture Mechanics scratch model. Finally, at the microscopic scale, a meticulous scratch probe calibration procedure is described to improve the accuracy of the fracture properties determination by addressing important issues such as moisture content, specimen surface cleanliness and choice of reference material. In summary, we bring forward a robust, convenient and accurate method that is applied to polymers, ceramics and metals and can be further applied to the multi-scale study of fracture processes in complex and challenging materials such as gas shale, cement paste and cortical bone.

Fig.1. (color online) (a) Macroscopic scratch tests experiments on paraffin wax with a{\textellipsis}

Fig.2. (color online) Fracture processes in scratch tests at both the macroscopic and{\textellipsis}

Fig.3. (color online) (a) Numerical Scratch Fracture model: 0<=θ<=20{\textdegree} is the blade{\textellipsis}

Fig.4. (color online) (a) Ratio of the stress intensity factor in mode II (in-plane{\textellipsis}

Fig.5. (color online) Wear flat configuration: the interface between the blade and the{\textellipsis}

Fig.6. (color online) Example of damage to the scratch probe as a result of misuse or{\textellipsis}

}, issn = {00431648}, doi = {10.1016/j.wear.2014.02.015}, author = {Ange-Therese Akono and Franz-Josef Ulm} } @article {275, title = {Initiation of immersed granular avalanches}, journal = {Physical Review E}, volume = {89}, year = {2014}, month = {May-09-2014}, pages = {Article Number: 052203}, abstract = {

By means of coupled molecular dynamics-computational fluid dynamics simulations, we analyze the initiation of avalanches in a granular bed of spherical particles immersed in a viscous fluid and inclined above its angle of repose. In quantitative agreement with experiments, we find that the bed is unstable for a packing fraction below 0.59 but is stabilized above this packing fraction by negative excess pore pressure induced by the effect of dilatancy. From detailed numerical data, we explore the time evolution of shear strain, packing fraction, excess pore pressures, and granular microstructure in this creeplike pressure redistribution regime, and we show that they scale excellently with a characteristic time extracted from a model based on the balance of granular stresses in the presence of a negative excess pressure and its interplay with dilatancy. The cumulative shear strain at failure is found to be ≃ 0.2, in close agreement with the experiments, irrespective of the initial packing fraction and inclination angle. Remarkably, the avalanche is triggered when dilatancy vanishes instantly as a result of fluctuations while the average dilatancy is still positive (expanding bed) with a packing fraction that declines with the initial packing fraction. Another nontrivial feature of this creeplike regime is that, in contrast to dry granular materials, the internal friction angle of the bed at failure is independent of dilatancy but depends on the inclination angle, leading therefore to a nonlinear dependence of the excess pore pressure on the inclination angle. We show that this behavior may be described in terms of the contact network anisotropy, which increases with a nearly constant connectivity and levels off at a value (critical state) that increases with the inclination angle. These features suggest that the behavior of immersed granular materials is controlled not only directly by hydrodynamic forces acting on the particles but also by the influence of the fluid on the granular microstructure.

}, issn = {1539-3755}, doi = {10.1103/PhysRevE.89.052203}, author = {Patrick Mutabaruka and Jean-Yves Delenne and Kenichi Soga and Farhang Radja{\"\i}} } @article {277, title = {Internal Structure of Inertial Granular Flows}, journal = {Physical Review Letters}, volume = {112}, year = {2014}, month = {Feb-21-2014}, pages = {Article Number: 078001}, abstract = {

We analyze inertial granular flows and show that, for all values of the inertial number I, the effective friction coefficient arises from three different parameters pertaining to the contact network and force transmission: (1)\ contact anisotropy, (2)\ force chain anisotropy, and (3)\ friction mobilization. Our extensive 3D numerical simulations reveal that increases with I mainly due to an increasing contact anisotropy and partially by friction mobilization whereas the anisotropy of force chains declines as a result of the destabilizing effect of particle inertia. The contact network undergoes topological transitions, and beyond I0.1 the force chains break into clusters immersed in a background \“soup\” of floating particles. We show that this transition coincides with the divergence of the size of fluidized zones characterized from the local environments of floating particles and a slower increase of with I.

Figure

Figure

Figure

Figure

Figure

Figure

}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.112.078001}, author = {Emilien Az{\'e}ma and Farhang Radja{\"\i}} } @article {83, title = {Ionic liquid confined in silica nanopores: molecular dynamics in the isobaric-isothermal ensemble}, journal = {Molecular Physics}, volume = {112}, year = {2014}, month = {Avr-03-2014}, pages = {1350-1361}, type = {Article}, abstract = {

Molecular dynamics simulations in the isobaric-isothermal ensemble are used to investigate the structure and dynamics of an ionic liquid confined at ambient temperature and pressure in hydroxylated amorphous silica nanopores. The use of the isobaric-isothermal ensemble allows estimating the effect of confinement and surface chemistry on the density of the confined ionic liquid. The structure of the confined ionic liquid is investigated using density profiles and structural order parameters while its dynamics is assessed by determining the mobility and ionic conductivity of the confined phase. Despite the important screening of the electrostatic interactions (owing to the small Debye length in ionic liquids), the local structure of the confined ionic liquid is found to be mostly driven by electrostatic interactions. We show that both the structure and dynamics of the confined ionic liquid can be described as the sum of a surface contribution arising from the ions in contact with the surface and a bulk-like contribution arising from the ions located in the pore centre; as a result, most properties of the confined ionic liquid are a simple function of the surface-to-volume ratio of the host porous material. In contrast, the ionic conductivity of the confined ionic liquid, which is a collective dynamical property, is found to be similar to the bulk. This study sheds light on the complex behaviour of hybrid materials made up of ionic liquid confined in inorganic porous materials.

}, keywords = {amorphous silica, ionic liquids, molecular dynamics, surface}, issn = {0026-8976}, doi = {10.1080/00268976.2014.902138}, author = {Guido Ori and Fran{\c c}ois Villemot and Lydie Viau and Andre Vioux and Benoit A. Coasne} } @article {312, title = {Irwin׳s conjecture: Crack shape adaptability in transversely isotropic solids}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {68}, year = {2014}, month = {Aug-2014}, pages = {1 - 13}, abstract = {

The planar crack propagation problem of a flat elliptical crack embedded in a brittle elastic anisotropic solid is investigated. We introduce the concept of crack shape adaptability: the ability of three-dimensional planar cracks to shape with the mechanical properties of a cracked body. A criterion based on the principle of maximum dissipation is suggested in order to determine the most stable elliptical shape. This criterion is applied to the specific case of vertical cracks in transversely isotropic solids. It is shown that contrary to the isotropic case, the circular shape (i.e. penny-shaped cracks) is not the most stable one. Upon propagation, the crack first grows non-self-similarly before it reaches a stable shape. This stable shape can be approximated by an ellipse of an aspect ratio that varies with the degree of elastic anisotropy. By way of example, we apply the so-derived crack shape adaptability criterion to shale materials. For this class of materials it is shown that once the stable shape is reached, the crack propagates at a higher rate in the horizontal direction than in the vertical direction. We also comment on the possible implications of these findings for hydraulic fracturing operations.

Fig.1. Top view of an elliptical crack (en, et and eb are the unit vectors normal to{\textellipsis}

Fig.2. Definition of the θ angle for a penny-shaped crack

Fig.3. Definition of the α-angle for an elliptical vertical crack (θ=π/2)

Fig.4. Zoom at the crack tip (vertical crack)

Fig.5. Comparison of the H(ϕ)/H2 obtained numerically {\textendash} solid line {\textendash} and its{\textellipsis}

Fig.6. Comparison of the H(ϕ)/H2 obtained numerically {\textendash} solid line {\textendash} and its{\textellipsis}

}, issn = {00225096}, doi = {10.1016/j.jmps.2014.03.004}, author = {Hadrien Laubie and Franz-Josef Ulm} } @article {284, title = {An interdisciplinary approach towards improved understanding of soil deformation during compaction}, journal = {Soil and Tillage Research}, volume = {128}, year = {2013}, month = {Apr-2013}, pages = {61 - 80}, abstract = {

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

}, issn = {01671987}, doi = {10.1016/j.still.2012.10.004}, author = {Keller, T. and Lamand, M. and Peth, S. and Berli, M. and Jean-Yves Delenne and Baumgarten, W. and Rabbel, W. and Farhang Radja{\"\i} and Rajchenbach, J. and Selvadurai, A.P.S. and Or, D.} } @article {317, title = {Identification of rolling resistance as a shape parameter in sheared granular media}, journal = {Physical Review E}, volume = {84}, year = {2011}, month = {Jul-28-2011}, pages = {Article Number: 011306 Part: 1}, abstract = {

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.

}, issn = {1539-3755}, doi = {10.1103/PhysRevE.84.011306}, author = {Estrada, Nicolas and Emilien Az{\'e}ma and Farhang Radja{\"\i} and Taboada, Alfredo} } @article {291, title = {Impact of Chemical Impurities on the Crystalline Cement Clinker Phases Determined by Atomistic Simulations}, journal = {Crystal Growth \& Design}, volume = {11}, year = {2011}, month = {Jul-2011}, pages = {2964 - 2972}, abstract = {
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The presence of chemical substitutions is believed to play a crucial role in the hydration reactions, structure, and elastic properties of cement clinker phases. Hence, substitutions are of great technological interest, as more efficient production of cement clinkers would result in a reduction of CO2 emissions, as well as possible economic benefits. Here we use a combination of classical and quantum mechanical simulation methods to study the detailed physicochemical changes of the clinker phases alite (Ca3SiO5) and belite (Ca2SiO4) when Mg2+, Al3+ and Fe3+ guest ions are incorporated into their structure. Using classical force field methods, we considered random substitutions among possible sites and different compositions in order to identify the preferential substitution sites on the crystalline structures. Then, the resulting structural changes that take place to accommodate the guest ions are investigated and discussed in detail. Using quantum mechanical density functional theory calculations the electronic structure of representative configurations has been computed to determine the potential impact of impurities on the reactivity.

}, issn = {1528-7483}, doi = {10.1021/cg200212c}, author = {Hegoi Manzano and Engin Durgun and Mohammad Javad Abdolhosseini Qomi and Franz-Josef Ulm and Roland Jean-Marc Pellenq and Jeffrey C Grossman} }