Highly-correlated, Spontaneous Gear-like Motion in an Active Granular System

We study a system of vibrated self-propelled granular particles on a horizontal plate within a circular boundary. The particles are square and designed to have polar motion along one body diagonal. When they hit the boundary they align along the boundary but also "walk" along the boundary. Given a large enough initial density, particles spontaneously migrate to the boundary, form a ring and perform a stable 1D rotational gear-like motion with a direction chosen by their net polarization. For a fully polarized single ring we find that the collective velocity surpasses the free single-particle velocity. This collective velocity increases as the density of particles in the ring increases, which is counterintuitive for a normal traffic problem. The spatial correlations of particle velocity in the fully polarized ring shows anti-correlations between nearest neighbors, which indicate they form pairs in the ring. The temporal correlation shows that velocity fluctuations are also anticorrelated in time. We also use two or more rings of particles to explore the effects of pressure and drag between layers.