Heaping and secondary flows in sheared granular materials

Split-bottom cylindrical containers are well established devices in experiments where granular materials are continuously sheared. They are characterized by localized broad shear bands. In such shear experiments, shape-anisotropic grains develop a secondary flow profile in the radial direction. A macroscopically observable consequence is the formation of a heap in the center of the container. This effect is found for all investigated types of prolate and oblate grains, but it is completely absent for spherical particles. There are qualitative differences in the behavior of short (moderate aspect ratio < 8) and long grains. The fill height of the granulate in the container considerably affects the time scale for heap formation, while the height of the heap is less dramatically influenced. Under reversal of the shear direction, the heap collapses within a few rotations, before the secondary flow in the reverse direction establishes a new heap.