@article {669, title = {Fiber plucking by molecular motors yields large emergent contractility in stiff biopolymer networks}, journal = {Soft Matter}, volume = {15}, year = {2019}, month = {Feb-21-2020}, pages = {1481 - 1487}, abstract = {

The mechanical properties of the cell depend crucially on the tension of its cytoskeleton, a biopolymer network that is put under stress by active motor proteins. While the fibrous nature of the network is known to strongly affect the transmission of these forces to the cellular scale, our understanding of this process remains incomplete. Here we investigate the transmission of forces through the network at the individual filament level, and show that active forces can be geometrically amplified as a transverse motor-generated force \"plucks\&$\#$39;\&$\#$39; the fiber and induces a nonlinear tension. In stiff and densely connected networks, this tension results in large network-wide tensile stresses that far exceed the expectation drawn from a linear elastic theory. This amplification mechanism competes with a recently characterized network-level amplification due to fiber buckling, suggesting that that fiber networks provide several distinct pathways for living systems to amplify their molecular forces.

}, issn = {1744-683X}, doi = {10.1039/C8SM00979A}, url = {http://xlink.rsc.org/?DOI=C8SM00979Ahttp://pubs.rsc.org/en/content/articlepdf/2019/SM/C8SM00979A}, author = {Ronceray, Pierre and Broedersz, Chase P. and Lenz, Martin} } @article {588, title = {From cellulose to kerogen: molecular simulation of a geological process}, journal = {Chemical Science}, volume = {8}, year = {2017}, month = {Dec-01-2017}, pages = {8325-8335}, abstract = {

The process by which organic matter decomposes deep underground to form petroleum and its underlying kerogen matrix has so far remained a no man\&$\#$39;s land to theoreticians, largely because of the geological (Myears) timescale associated with the process. Using reactive molecular dynamics and an accelerated simulation framework, the replica exchange molecular dynamics method, we simulate the full transformation of cellulose into kerogen and its associated fluid phase under prevailing geological conditions. We observe in sequence the fragmentation of the cellulose crystal and production of water, the development of an unsaturated aliphatic macromolecular phase and its aromatization. The composition of the solid residue along the maturation pathway strictly follows what is observed for natural type III kerogen and for artificially matured samples under confined conditions. After expulsion of the fluid phase, the obtained microporous kerogen possesses the structure, texture, density, porosity and stiffness observed for mature type III kerogen and a microporous carbon obtained by saccharose pyrolysis at low temperature. As expected for this variety of precursor, the main resulting hydrocarbon is methane. The present work thus demonstrates that molecular simulations can now be used to assess, almost quantitatively, such complex chemical processes as petrogenesis in fossil reservoirs and, more generally, the possible conversion of any natural product into bio-sourced materials and/or fuel.

}, issn = {2041-6520}, doi = {10.1039/C7SC03466K}, url = {http://xlink.rsc.org/?DOI=C7SC03466K}, author = {Atmani, Lea and Christophe Bichara and Roland Jean-Marc Pellenq and Henri Van Damme and Adri CT Van Duin and Raza, Zamaan and Truflandier, Lionel A. and Ama{\"e}l Obliger and Kralert, Paul G. and Franz-Josef Ulm and Jean-Marc Leyssale} } @article {248, title = {Fracture toughness anomalies: Viewpoint of topological constraint theory}, journal = {Acta Materialia}, volume = {121}, year = {2016}, month = {Dec-2016}, pages = {234 - 239}, abstract = {

The relationship between composition, structure, and resistance to fracture remains poorly understood. Here, based on molecular dynamics simulations, we report that sodium silicate glasses and calcium\–silicate\–hydrates feature an anomalous maximum in fracture toughness. In the framework of topological constraint theory, this anomaly is correlated to a flexible-to-rigid transition, driven by pressure or composition for sodium silicate and calcium\–silicate\–hydrates, respectively. This topological transition, observed for an isostatic network, is also shown to correspond to a ductile-to-brittle transition. At this state, the network is rigid but free of eigen-stress and features stress relaxation through crack blunting, resulting in optimal resistance to fracture. Our topological approach could therefore enable the computational design of tough inorganic solids, which has long been a \“holy grail\” within the non-metallic materials chemistry community.

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}, issn = {13596454}, doi = {10.1016/j.actamat.2016.09.004}, author = {Mathieu Bauchy and Wang, Bu and Wang, Mengyi and Yu, Yingtian and Mohammad Javad Abdolhosseini Qomi and Smedskjaer, Morten M. and Christophe Bichara and Franz-Josef Ulm and Roland Jean-Marc Pellenq} } @article {249, title = {Free Volume Theory of Hydrocarbon Mixture Transport in Nanoporous Materials}, journal = {The Journal of Physical Chemistry Letters}, volume = {7}, year = {2016}, month = {Jun-10-2016}, pages = {3712 - 3717}, abstract = {
Despite recent focus on shale gas, hydrocarbon recovery from the ultraconfining and disordered porosity of organic matter in shales (kerogen) remains poorly understood. Key aspects such as the breakdown of hydrodynamics at the nanoscale and strong adsorption effects lead to unexplained non-Darcy behaviors. Here, molecular dynamics and statistical mechanics are used to elucidate hydrocarbon mixture transport through a realistic molecular model of kerogen [Bousige, C.; et al. Nat. Mater. 2016, 15, 576]. Owing to strong adsorption effects, velocity cross-correlations between the mixture components and between molecules of the same species are shown to be negligible. This allows estimation of each component permeance from its self-diffusivity, which can be obtained from single-component data. These permeances are found to scale with the reciprocal of the alkane length and decrease with the number of adsorbed molecules following a simple free volume theory, therefore allowing mixture transport prediction as a function of the amount of trapped fluid.
Abstract Image
}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.6b01684}, author = {Ama{\"e}l Obliger and Roland Jean-Marc Pellenq and Franz-Josef Ulm and Benoit A. Coasne} } @article {644, title = {First-principles prediction of kink-pair activation enthalpy on screw dislocations in bcc transition metals: V, Nb, Ta, Mo, W, and Fe}, journal = {Physical Review B}, volume = {91}, year = {2015}, month = {Mar-09-2015}, pages = { Article Number 094105 }, abstract = {

The atomistic study of kink pairs on screw dislocations in body-centered cubic (bcc) metals is challenging because interatomic potentials in bcc metals still lack accuracy and kink pairs require too many atoms to be modeled by first principles. Here, we circumvent this difficulty using a one-dimensional line tension model whose parameters, namely the line tension and Peierls barrier, are reachable to density functional theory calculations. The model parameterized in V, Nb, Ta, Mo, W, and Fe, is used to study the kink-pair activation enthalpy and spatial extension. Interestingly, we find that the atomistic line tension is more than twice the usual elastic estimates. The calculations also show interesting group tendencies with the line tension and kink-pair width larger in group V than in group VI elements. Finally, the present kink-pair activation energies are shown to compare qualitatively with experimental data and potential origins of quantitative discrepancies are discussed.

}, issn = {1098-0121}, doi = {10.1103/PhysRevB.91.094105}, url = {https://link.aps.org/doi/10.1103/PhysRevB.91.094105}, author = {Dezerald, Lucile and Proville, L. and Ventelon, Lisa and Willaime, F. and Rodney, D.} } @article {162, title = {Fractal analysis of surface roughness of montmorillonite clay self-supported films: Effects of exchanged cations and of mechanical tensile stress}, journal = {Colloids and Surfaces A: Physicochemical and Engineering Aspects}, volume = {486}, year = {2015}, month = {Dec-05-2015}, pages = {38 - 44}, abstract = {

In this paper, surface roughness of self-supported montmorillonite clay films was studied by the use of fractal analysis to study its links with various inter-foliar cations, mechanical properties, and permeability. The Hurst exponent of fractional Brownian motion was estimated by maximum likelihood from optical images in reflection mode. Images corresponding to films with different inter-foliar exchanged cations visually show significant texture variations. It is confirmed by a continuous decrease of H parameter toward less correlated textures as one goes from the samples exchanged with monovalent ions to those exchanged with divalent and finally trivalent ions. The same films were analyzed while they were submitted to mechanical tensile stress. The films exchanged with the most charged cations or less hydrated ones do not show any evidence of texture change and the H parameter is almost constant. Concerning the permeability study, less permeable parent cakes \– with hydrated monovalent cations \– lead to relatively smooth and less disturbed surface films with high H parameter. In the opposite, very permeable cakes give birth to anti-persistent relief.

}, issn = {09277757}, doi = {10.1016/j.colsurfa.2015.09.001}, author = {Zabat, M. and Harba, R. and Henri Van Damme} } @article {137, title = {Fracture toughness of calcium{\textendash}silicate{\textendash}hydrate from molecular dynamics simulations}, journal = {Journal of Non-Crystalline Solids}, volume = {419}, year = {2015}, month = {Jul-01-2015}, pages = {58 - 64}, abstract = {

Concrete is the most widely manufactured material in the world. Its binding phase, calcium\–silicate\–hydrate (C\–S\–H), is responsible for its mechanical properties and has an atomic structure fairly similar to that of usual calcium silicate glasses, which makes it appealing to study this material with tools and theories traditionally used for non-crystalline solids. Here, following this idea, we use molecular dynamics simulations to evaluate the fracture toughness of C\–S\–H, inaccessible experimentally. This allows us to discuss the brittleness of the material at the atomic scale. We show that, at this scale, C\–S\–H breaks in a ductile way, which prevents one from using methods based on linear elastic fracture mechanics. Knowledge of the fracture properties of C\–S\–H at the atomic scale opens the way for an upscaling approach to the design of tougher cement paste, which would allow for the design of slender environment-friendly infrastructures, requiring less material.

}, issn = {00223093}, doi = {10.1016/j.jnoncrysol.2015.03.031}, author = {Mathieu Bauchy and Hadrien Laubie and Mohammad Javad Abdolhosseini Qomi and Christian G. Hoover and Franz-Josef Ulm and Roland Jean-Marc Pellenq} } @article {313, title = {Fl{\"u}gge{\textquoteright}s Conjecture: Dissipation- versus Deflection-Induced Pavement{\textendash}Vehicle Interactions}, journal = {Journal of Engineering Mechanics}, volume = {140}, year = {2014}, month = {Aug-2014}, pages = {Article Number: 04014053 }, abstract = {

The dissipation occurring below a moving tire in steady-state conditions in contact with a viscoelastic pavement is expressed using two different reference frames: a fixed observer attached to the pavement and a moving observer attached to the pavement\–tire contact surface. The first approach is commonly referred to as dissipation-induced pavement\–vehicle interaction (PVI), the second as deflection-induced PVI. Based on the principle of frame independence, it is shown that both approaches are strictly equal, from a thermodynamic point of view, and thus predict the same amount of dissipated energy. This equivalence is illustrated through application to two pavement systems: a viscoelastic beam and a viscoelastic plate both resting on an elastic foundation. The amount of dissipated energy in the pavement structure needs to be supplied by the vehicle to maintain constant speed, thus contributing to the rolling resistance, associated excess fuel consumption, and greenhouse gas emissions. The model here proposed can be used to quantify the dissipated energy and contribute to the development of engineering methods for the sustainable design of pavements.

}, issn = {0733-9399}, doi = {10.1061/(ASCE)EM.1943-7889.0000754}, author = {Arghavan Louhghalam and Mehdi Akbarian and Franz-Josef Ulm} } @article {77, title = {First-principles molecular dynamics study of glassy GeS2: Atomic structure and bonding properties}, journal = {Physical Review B}, volume = {88}, year = {2013}, month = {Nov-04-2013}, type = {Article}, abstract = {

The structure of glassy GeS2 is studied in the framework of density functional theory, by using a fully self-consistent first-principles molecular dynamics (FPMD) scheme. A comparative analysis is performed with previous molecular dynamics data obtained within the Harris functional (HFMD) total energy approach. The calculated total neutron structure factor exhibits an unprecedented agreement with the experimental counterpart. In particular, the height of the first sharp diffraction peak (FSDP) improves considerably upon the HFMD results. Both the Ge and the S subnetworks are affected by a consistent number of miscoordinations, coexisting with the main tetrahedral structural motif. Glassy GeS2 features a short-range order quite similar to the one found in glassy GeSe2, a notable exception being the larger number of edge-sharing connections. An electronic structure localization analysis, based on the Wannier functions formalism, provides evidence of a more enhanced ionic character in glassy GeS2 when compared to glassy GeS2.

}, issn = {1098-0121}, doi = {10.1103/PhysRevB.88.174201}, author = {Celino, M. and Le Roux, S. and Guido Ori and Benoit A. Coasne and Bouzid, A. and Boero, M. and Carlo Massobrio} } @article {308, title = {Fabric evolution and accessible geometrical states in granular materials}, journal = {Granular Matter}, volume = {14}, year = {2012}, month = {Mar-02-2012}, pages = {259 - 264}, abstract = {

We analyze the geometrical states of granular materials by means of a fabric tensor involving the coordination number and fabric anisotropy as the lowest-order descriptors of the contact network. In particular, we show that the fabric states in this representation are constrained by steric exclusions and the condition of mechanical equilibrium required in the quasi-static limit. A simple model, supported by numerical data, allows us to characterize the range of accessible fabric states and the joint evolution of fabric parameters. The critical state in this framework appears as a jammed state in the sense of a saturation of contact gain and loss along the principal strain-rate directions.

}, issn = {1434-5021}, doi = {10.1007/s10035-012-0321-8}, author = {Farhang Radja{\"\i} and Jean-Yves Delenne and Emilien Az{\'e}ma and St{\'e}phane Roux} } @article {314, title = {Force chains and contact network topology in sheared packings of elongated particles}, journal = {Physical Review E}, volume = {85}, year = {2012}, month = {Mar-19-2012}, pages = {Article Number: 031303 Part: 1}, abstract = {

By means of contact dynamic simulations, we investigate the contact network topology and force chains in two-dimensional packings of elongated particles modeled by rounded-cap rectangles. The morphology of large packings of elongated particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and orientations of contacts between particles. We show that particle elongation affects force distributions and force/fabric anisotropy via various local structures allowed by steric exclusions and the requirement of force balance. As a result, the force distributions become increasingly broader as particles become more elongated. Interestingly, the weak force network transforms from a passive stabilizing agent with respect to strong force chains to an active force-transmitting network for the whole system. The strongest force chains are carried by side/side contacts oriented along the principal stress direction.

}, issn = {1539-3755}, doi = {10.1103/PhysRevE.85.031303}, author = {Emilien Az{\'e}ma and Farhang Radja{\"\i}} } @article {301, title = {Fracture scaling relations for scratch tests of axisymmetric shape}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {60}, year = {2012}, month = {Mar-2012}, pages = {379 - 390}, abstract = {

Scratch testing and scratch test analysis continues to gain momentum in Applied Mechanics, due to the possibility offered by this method to assess fracture properties at very fine scales. In this paper, we derive general scratch force scaling relations for axisymmetric scratch probes defined by single variable monomial functions. These relations are used to define fracture criteria with and without consideration of the development of shear stresses at the probe\–material interface. The approach is illustrated for common scratch probe geometries: conical probe, flat punch, and hemi-spherical probe. Application of the proposed method to micro-scratch tests on two materials (an aluminum alloy and a thermoplastic polymer) using a Rockwell probe (a conical probe ending in a hemi-spherical shape) illustrates the versatility of the approach: First, the scratch force-depth scaling relations provide a means to determine the degree of the homogeneous function characterizing the scratch probe. Second, the fracture criteria enable an experimental assessment of the fracture toughness. The good agreement between the fracture toughness determined by scratching and values reported in the open literature show the potential of the proposed method for determining fracture properties of materials at even smaller scales.

Fig.1. Axisymmetric scratch probe geometry

Fig.2. Scaling of the normalized fracture force and the perimeter function, β, of the{\textellipsis}

Fig.3. Forces in three scratch tests on aluminum alloy 2024 (AA 2024) obtained with a{\textellipsis}

Fig.4. Fracture toughness determination from the scratch response of AA 2024 with a{\textellipsis}

Fig.5. Response and analysis of three scratch tests on a thermoplastic polymer (Delrin{\textellipsis}

}, issn = {00225096}, doi = {10.1016/j.jmps.2011.12.009}, author = {Ange-Therese Akono and Franz-Josef Ulm} } @article {316, title = {From liquid to solid bonding in cohesive granular media}, journal = {Mechanics of Materials}, volume = {43}, year = {2011}, month = {Oct-2011}, pages = {529 - 537}, abstract = {

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.

Fig. 1. (a) Photograph of the sample in the process of drying; (b) The press used for{\textellipsis}

Fig. 2. Compressive yield stress σY, normalized by the yield stress σLY due only to{\textellipsis}

Fig. 3. Photograph of a partially dried sample in the second regime

Fig. 4. Schematic representation of the evolution of bridge: from a liquid to a solid{\textellipsis}

Fig. 5. Evolution of the global crystallization index with time; solid line:{\textellipsis}

Fig. 6. The parametric adhesion law

}, issn = {01676636}, doi = {10.1016/j.mechmat.2011.06.008}, author = {Jean-Yves Delenne and Souli{\'e}, Fabien and Moulay Sa{\"\i}d El Youssoufi and Farhang Radja{\"\i}} }