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

Marbles in a swirled teacup exhibit a curious dynamical transition: at small densities, the marbles circulate in the same direction as the container, but at larger densities, they transition to circulating in the opposite direction. This phenomenon occurs in a range of swirled and vibrated granular systems, from industrial vibration mills to baby rattles, each observation challenging our intuitive understanding of angular momentum conservation. Using an accelerating coordinate system, we use experiments and simulations to identify the cause for the transition to counterrotation. At low densities, the grains roll freely in the container, but as density increases, antiferromagnetic-like frictional interactions between beads lead to geometrical frustration, resolved only via global circulation. I will furthermore show that in the new frame of reference this system resembles a rotating drum.