Stick-slip dynamics generated by granular materials with varying grain angularity*

Experiments on two-dimensional granular materials and corresponding simulations and theoretical treatments typically feature circular or elliptical grains. However, grains found in natural systems often have flat faces that introduce local rotational constraints; these have been shown to affect, for example, the jamming transition, discontinuous shear thickening, and ordered states in colloids and thermalized grains. In this work, we experimentally investigate the effects of grain angularity on stick-slip dynamics in a slowly driven granular medium consisting of regular polygonal grains; the gravity-packed granular bed is sheared via a weighted slider pulled by a spring. We find that packings of triangular or square grains have slightly higher shear strengths than packings of pentagons, hexagons, heptagons, or disks. Moreover, as the number of sides increases, sticking periods become shorter on average with the material yielding at smaller applied stresses. We report on measurements of the pulling force on the slider, particle dynamics during slip events, and properties of force-bearing contact networks identified via photoelasticity. The data are consistent with a continuous change in dynamics as the circular grain limit is approached.