Granular stick-slip dynamics across scales: How friction and grain shape influence collective grain-scale motion, force transmission, and bulk stability

Granular materials are ubiquitous in industry and nature, from pills and produce to boulders and sandy shores. Characterizing the stability and flow of these complex materials is of great importance for prediction and mitigation of destructive geophysical phenomena like earthquakes in which an elastically driven granular material (in a relatively long, stable "stick") yields (a rapid energy-releasing "slip") until it reaches another stable sticking configuration. The stick-slip dynamics of bulk granular media has been well-modeled for a variety of macroscopic control parameters like driving rate and load elasticity. However, the role that particle-scale interactions play in these bulk dynamics – via changes to collective grain motion during slips and the mesoscale force networks that stabilize the material during sticks – is not fully understood. In this talk, I present experiments using photoelastic, quasi-2D grains that elucidate granular stick-slip dynamics across scales. I focus in particular on the influences of friction between grains and a substrate, grain angularity, and confining system geometry on the dynamics of a grain-scale intruder pushed through the medium. By examining intruder motion, average flow fields of grains during slips, and spatial extent of force networks during sticks, I show that substrate friction controls whether the material exhibits robust stick-slip or intermittent flow akin to clogging; that grain angularity constrains collective grain circulation, changes how forces propagate within the medium, and enhances energy release in slip events; and that the system and intruder sizes control a transition from continual yielding to stick-slip. I lastly pose open questions about collective mesoscale structures formed by angular grains and the potential for slip prediction based on signatures in the force network during sticking periods.