Geometric frustration induces the transition between rotation and counterrotation in swirled granular media.

2018 
Granular material in a swirled container exhibits a curious transition as the number of particles is increased: at low density the particle cluster rotates in the same direction as the swirling motion of the container, while at high density it rotates in the opposite direction. We investigate this phenomenon experimentally and numerically using a co-rotating reference frame in which the system reaches a statistical steady-state. In this steady-state the particles form a cluster whose translational degrees of freedom are stationary, while particles constantly circulate around the cluster's center of mass. We show that the transition to counterrotation is friction-dependent. At high particle densities, frictional effects result in geometrical frustration which prevents particles from cooperatively rolling and spinning. Consequently, the particle cluster rolls like a rigid body on the container wall, leading to counterrotation. Numerical simulations verify that both wall-disc friction and disc-disc friction are critical control parameters leading to the observed counterrotation in the lab frame.
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