Self-Diffusion in Granular Flows

The mixing of identical grains in a granular flow was experimentally studied and found to be a self-diffusive process. The flow, produced by vertically vibrating a horizontal layer of grains, is two-dimensional allowing us to study self-diffusion in the perpendicular direction to the flow plane. The self-diffusion coefficient depends only on the flow velocity and not on the independent values of frequency and amplitude of vibration. Its growth with the flow velocity is consistent with a linear dependence. Our findings support the picture of the velocity field as consisting of convective and fluctuating parts and agree qualitatively with recent hydrodynamical models of granular flows.