We develop and extend a method to compute the local yield stresses at the atomic scale in model two-dimensional Lennard-Jones glasses produced via differing quench protocols [1-2]. This technique allows us to sample the plastic rearrangements in a non-perturbative manner for different loading directions on a well controlled length scale. Plastic activity upon shearing correlates strongly with the locations of low yield stresses in the quenched states. The distribution of local yield stresses is also shown to strongly depend on the quench protocol: the more relaxed the glass, the higher the local plastic thresholds. The local yield stress varies with the shear orientation tested and strongly correlates with the plastic rearrangement locations when the system is sheared correspondingly. Plastic rearrangements appear therefore as the consequence of shear transformation zones encoded in the glass structure that possess weak slip planes along different orientations. We then justify the length scale employed in this work and extract the yield threshold statistics as a function of the size of the probing zones. Finally, the evolution of local yield stress distributions upon shear deformation is studied [3]. We show how the plasticity-induced anisotropy in amorphous solids can be explained by the depletion of the soft zones in the direction of deformation with respect to the reverse direction. In the most relaxed system, systematically presenting a localization of the plastic deformation through the formation of a shear band, the persistence of plastic activity is explained by local softening. It is thus argued that this method makes it possible to derive physically grounded models of plasticity for amorphous materials by directly revealing the relevant details of the shear transformation zones that mediate this process.

ci-joint

The dynamics of an individual flexible filaments in viscous flows is key to deciphering the rheological behavior of many complex fluids and soft materials. It also underlies a wealth of biophysical processes from flagellar propulsion to intracellular streaming. The interaction between an elongated object and a given flow depends strongly on the properties of the object - flexibility, aspect ratio and dimensions - and the flow geometry. This interaction is governed by the elasto-viscous number comparing elastic and viscous forces. During the last years, we have studied different configurations of this problem by combining microfluidics, microfabrication and microscopy. In this seminar, I will focus more on the dynamics of actin filament, used here as a model filament, in shear and pure straining flows and will describe the different morphologies the filament adopts when the elasto-viscous number increases. In shear flow, simulations matching the experimental conditions are also been performed and allow the complete phase diagram to be understood.

Active systems are made of interacting particles which extract energy from their environment to perform a directed motion. For instance, bacteria consume some nutrients to exert forces with their flagella, which allows them to move persistently by alternating running and tumbling periods. In contrast with equilibrium, active systems constantly dissipate energy: this offers the opportunity to build some autonomous engines extracting work at constant temperature. I will discuss how to design efficient active engines which operate by either exploiting the current of some asymmetric obstacles or driving some confining walls with cyclic protocols. Moreover, active particles can undergo spontaneous transitions driven by collective effects. I will explore the role of dissipation in the emergence of such collective states, to shed light on transitions between a phase separation with purely repulsive interactions and a collective motion despite the lack of aligning interactions.

Can one infer the rheological law of a deformable system? Example of an
epithelial tissue.

We will present preliminary investigations on how to efficiently collect
information on the mechanical behaviour of a deformable material, in
other words on the differential equation that relates stress and
deformation histories. We apply this method to an epithelial tissue
stimulated from a rheological law at the intra-cellular level.