The astonishing behavior of non-convex grains in granular shear experiments*

Granular material composed of regular tetrapods and hexapods is exposed to continuous shear in a split-bottom shear cell. We observe the generation of a secondary convective flow which is astonishingly opposite to that previously observed with cylinders and ellipsoids. The convective flow drags the particles at the surface towards the rotation center, where it is pulled downward, leaving a dimple in the center. The details of this phenomenon are extremely sensitive to the particle geometry and the height of the granular bed. We propose a hypothesis that explains this behavior qualitatively.