Dynamics of avalanches in sheared and active granular systems

Under shear, granular materials and other amorphous solids fail via localized particle rearrangements that often coalesce into avalanches, but a microscopic explanation of the dynamics of avalanches has remained out of reach. Moreover, dense active matter – including bacterial swarms, biological tissues or even Janus colloidal particles – exhibit many of these same features, with no rigorous explanation.  I will discuss some of our recent work to develop new tools to characterize the dynamics of avalanches and relate them to microscopic structural features. We have used these techniques to quantitatively compare spatiotemporal four-point correlations of the plasticity field during avalanches in both frictionless granular simulations and simulations of coarse-grained elasto-plastic models. We find two length scales: a short “excitable” length scale associated with sites that have finite yield stresses and a large “marginal” length scale associated with sites that have vanishing yield stress. The larger length scale exhibits the same scaling with time in both elasto-plastic and granular simulations, while the short one scales differently in the two models. Finally, I will briefly discuss a direct link between avalanche dynamics in active matter and sheared solids.