Density Mediated Spin Correlations Drive Edge to Bulk Flow Transition in Active Chiral Matter

Abstract: We demonstrate that edge currents develop in active chiral matter -- composed of spinning disk-shaped grains with chirally arranged tilted legs confined in a circular vibrating chamber -- due to boundary shielding over a wide range of densities corresponding to a gas, fluid, and crystal. The edge currents are then shown to increasingly drive circulating bulk flows with area fraction $\phi$ due to increasing spin-coupling between neighbors mediated by frictional contacts, as percolating clusters develop. Edge currents are observed even in the dilute limit. While, at low $\phi$, the average flux vanishes except within a distance of a single particle diameter of the boundary, the penetration depth grows with increasing $\phi$ till a solid body rotation is achieved corresponding to the highest packing, where the particles are fully caged with hexagonal order and spin in phase with the entire packing. A coarse-grained model, based on the increased collisional interlocking of the particles with $\phi$ and the emergence of order, captures the observed flow fields.