The rotation-reptation transition under broken rotational symmetry

In a swirled circular container, granular particles can change their sense of rotation when the packing density is increased, exhibiting a transition from rotational to reptational motion. In addition, here we report a ‘snake mode’ that arises at a lower packing density, where particles form a chain like cluster that rotates with the same frequency as the container. We investigate experimentally transitions between these three modes under the influence of geometrical distortions which break the rotational symmetry of the container. The driving mechanism for the rotational motion of the clusters is also discussed.