Secondary flow in ensembles of nonconvex granular particles under shear

Studies of granular materials, both theoretical and experimental, are often restricted to convex grain shapes. We demonstrate that a nonconvex grain shape can lead to a qualitatively novel macroscopic dynamics. Spatial crosses (hexapods) are continuously sheared in a split-bottom container. Thereby, they develop a secondary flow profile that is completely opposite to that of rod-shaped or lentil-shaped convex grains in the same geometry. The crosses at the surface migrate towards the rotation center and sink there mimicking a reverse Weissenberg effect. The observed surface flow field suggests the existence of a radial outward flow in the depth of the granular bed, thus, forming a convection cell. This flow field is connected with a dimple formed in the rotation center. The effect is strongly dependent on the particle geometry and the height of the granular bed.

Automated summary

This study demonstrates that nonconvex grain shapes can lead to qualitatively novel macroscopic dynamics, different from convex grains. Hexapod grains at the surface migrate towards the rotation center and sink, mimicking a reverse Weissenberg effect. The observed surface flow field suggests a radial outward flow in the depth of the granular bed, forming a convection cell.