  Granular material in a swirled container exhibits a curious transition as the
number of particles is increased: at low densities the particle cluster rotates
in the same direction as the swirling motion of the container, while at high
densities 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 the
individual particles constantly circulate around the cluster's center of mass,
similar to a ball rolling along the wall within a rotating drum. We show that
the transition to counterrotation is friction-dependent. At high particle
densities, frictional effects result in geometric frustration which prevents
particles from cooperatively rolling and spinning. Consequently, the particle
cluster rolls like a rigid body with no-slip conditions on the container wall,
which necessarily counterrotates around its own axis. Numerical simulations
verify that both wall-disc friction and disc-disc friction are critical for
inducing counterrotation.
