Elastic Signature of Flow Events in Supercooled Liquids Under Shear

International audienceUsing numerical simulation of a 2D Lennard-Jones system, we study the crossover from shear thinning to Newtonian flow. We find that the short-time elastic response of our system essentially does not change through this crossover, and show that, in the Newtonian regime, thermal activation triggers shear transformations, i.e., local irreversible shear events that produce Eshelby (long-ranged, anisotropic) deformation fields as previously seen in low-T glasses. Quite surprisingly, these Eshelby fields are found to persist much beyond the α-relaxation time, and shear thinning to coincide with the emergence of correlations between shear relaxation centers

Robustness of avalanche dynamics in sheared amorphous solids as probed by transverse diffusion

Using numerical simulations, we perform an extensive finite-size analysis of the transverse diffusion coefficient in a sheared 2D amorphous solid, over a broad range of strain rates, at temperatures up to the supercooled liquid regime. We thus obtain direct qualitative evidence for the persistence of correlations between elementary plastic events up to the vicinity of the glass transition temperature $T_g$. A quantitative analysis of the data, combined with a previous study of the $T$- and $\dot\gamma$-dependence of the macroscopic stress \cite{ChattorajCaroliLemaitre2010}, leads us to conclude that the average avalanche size remains essentially unaffected by temperature up to $T\sim0.75 T_g$

Noise Amplification in Frictional Systems: Oscillatory Instabilities

It was discovered recently that frictional granular materials can exhibit an important mechanism for instabilities, i.e the appearance of pairs of complex eigenvalues in their stability matrix. The consequence is an oscillatory exponential growth of small perturbations which are tamed by dynamical nonlinearities. The amplification can be giant, many orders of magnitude, and it ends with a catastrophic system-spanning plastic event. Here we follow up on this discovery, explore the scaling laws characterizing the onset of the instability, the scenarios of the development of the instability with and without damping, and the nature of the eventual system spanning events. The possible relevance to earthquake physics and to the transition from static to dynamic friction is discussed.Comment: arXiv admin note: text overlap with arXiv:1901.0237

Frictional Active Brownian Particles

Frictional forces affect the rheology of hard-sphere colloids, at high shear rate. Here we demonstrate, via numerical simulations, that they also affect the dynamics of active Brownian particles, and their motility induced phase separation. Frictional forces increase the angular diffusivity of the particles, in the dilute phase, and prevent colliding particles from resolving their collision by sliding one past to the other. This leads to qualitatively changes of motility-induced phase diagram in the volume-fraction motility plane. While frictionless systems become unstable towards phase separation as the motility increases only if their volume fraction overcomes a threshold, frictional system become unstable regardless of their volume fraction. These results suggest the possibility of controlling the motility induced phase diagram by tuning the roughness of the particles

Transition from Static to Dynamic Friction in an Array of Frictional Disks

The nature of an instability that controls the transition from static to dynamical friction is studied in the the context of an array of frictional disks that are pressed from above on a substrate. In this case the forces are all explicit and Newtonian dynamics can be employed without any phenomenological assumptions. We show that an oscillatory instability that had been discovered recently is responsible for the transition, allowing individual disks to spontaneously reach the Coulomb limit and slide with dynamic friction. The transparency of the model allows a full understanding of the phenomenon, including the speeds of the waves that travel from the trailing to the leading edge and vice versa

Designing Phononic Band Gaps with Sticky Potentials

Spectral gaps in the vibrational modes of disordered solids are key design elements in the synthesis and control of phononic metamaterials that exhibit a plethora of novel elastic and mechanical properties. However, reliably producing these gaps often require a high degree of network specificity through complex control optimization procedures. In this work, we present as an additional tool to the existing repertoire, a numerical scheme that rapidly generates sizeable spectral gaps in absence of any fine tuning of the network structure or elastic parameters. These gaps occur even in disordered polydisperse systems consisting of relatively few particles ($N \sim 10^2-10^3$). Our proposed procedure exploits sticky potentials that have recently been shown to suppress the formation of soft modes, thus effectively recovering the linear elastic regime where band structures appear, at much shorter length scales than in conventional models of disordered solids. Our approach is relevant to design and realization of gapped spectra in a variety of physical setups ranging from colloidal suspensions to 3D-printed elastic networks.Comment: 12 pages, 11 figures, Frontiers in Physics (in press

Oscillatory Instabilities in Frictional Granular Matter

Frictional granular matter is shown to be fundamentally different in its plastic responses to external strains from generic glasses and amorphous solids without friction. While regular glasses exhibit plastic instabilities due to a vanishing of a real eigenvalue of the Hessian matrix, frictional granular materials can exhibit a previously unnoticed additional mechanism for instabilities, i.e. the appearance of a pair of complex eigenvalues leading to oscillatory exponential growth of perturbations which are tamed by dynamical nonlinearities. This fundamental difference appears crucial for the understanding of plasticity and failure in frictional granular materials. The possible relevance to earthquake physics is discussed.Comment: Supplementary materials follows the main text Animation of the instability: https://vimeo.com/31013227

Oscillatory Instabilities in 3-Dimensional Frictional Granular Matter

The dynamics of amorphous granular matter with frictional interactions cannot be derived in general from a Hamiltonian and therefore displays oscillatory instabilities stemming from the onset of complex eigenvalues in the stability matrix. These instabilities were discovered in the context of one and two dimensional systems, while the three dimensional case was never studied in detail. Here we fill this gap by deriving and demonstrating the presence of oscillatory instabilities in a three dimensional granular packing. We study binary assemblies of spheres of two sizes interacting via classical Hertz and Mindlin force laws for the longitudinal and tangent interactions, respectively. We formulate analytically the stability matrix in 3D and observe that a couple of complex eigenvalues emerges at the onset of the instability as in the case of frictional disks in two-dimensions. The dynamics then shows oscillatory exponential growth in the Mean-Square-Displacement, followed by a catastrophic event. The generality of these results for any choice of forces that break the symplectic Hamiltonian symmetry is discussed.Comment: arXiv admin note: substantial text overlap with arXiv:1903.10887, arXiv:1901.0237

A Surrogate-Assisted Extended Generative Adversarial Network for Parameter Optimization in Free-Form Metasurface Design

Metasurfaces have widespread applications in fifth-generation (5G) microwave communication. Among the metasurface family, free-form metasurfaces excel in achieving intricate spectral responses compared to regular-shape counterparts. However, conventional numerical methods for free-form metasurfaces are time-consuming and demand specialized expertise. Alternatively, recent studies demonstrate that deep learning has great potential to accelerate and refine metasurface designs. Here, we present XGAN, an extended generative adversarial network (GAN) with a surrogate for high-quality free-form metasurface designs. The proposed surrogate provides a physical constraint to XGAN so that XGAN can accurately generate metasurfaces monolithically from input spectral responses. In comparative experiments involving 20000 free-form metasurface designs, XGAN achieves 0.9734 average accuracy and is 500 times faster than the conventional methodology. This method facilitates the metasurface library building for specific spectral responses and can be extended to various inverse design problems, including optical metamaterials, nanophotonic devices, and drug discovery

Effet d'une faible température sur les mécanismes élémentaires de la déformation plastique dans les matériaux amorphes

Using numerical simulations of a model two-dimensional Lennard-Jones glass, we study the effect of small temperatures on the elementary mechanisms of deformation in amorphous materials. A very extensive data set covering several decades of shear rate at various temperatures below and up to the glass transition was compiled. Measurements, which include transverse diffusion, macroscopic stress, and coarse-grained fields (strain, stress) and their spatial correlations, lead us to propose that the avalanche dynamics previously identified in athermal simulations continues to be at work -- and nearly unchanged -- up to the glass transition. It is then argued that in this range, thermal fluctuation essentially shift the strains at which dissipative events take place, which results in a sharp drop of the macroscopic stress level at the lowest temperaturesPar la mise en œuvre de simulations numériques d'un modèle bidimensionnel de verre de Lennard-Jones, nous étudions l'effet de la température sur les mécanismes élémentaires de la déformation dans les matériaux amorphes. Nous présentons un ensemble très complet de données couvrant plusieurs décades de taux de cisaillement à différentes températures en dessous et jusqu'à la transition vitreuse. Les mesures, qui portent sur la diffusion transverse, la contrainte macroscopique ainsi que sur des champs mésoscopiques (déformation, contrainte) et leurs corrélations spatiales, conduisent à proposer que la dynamique des avalanches identifiée précédemment dans les simulations athermiques continue d'être à l'œuvre - en restant presque inchangée - jusqu'à la transition vitreuse. Nous arguons que dans la gamme de paramètres utilisée l'effet des fluctuations thermiques revient à déplacer les seuils auxquels les événements dissipatifs se produisent, ce qui se traduit par une forte baisse du niveau de contrainte macroscopique aux températures les plus basse