Segregation time-scale in bi-disperse granular flows

Using a discrete simulation method, we investigate numerically two-dimensional bi-disperse chute flows formed of a layer of larger grains overlaid by a layer of smaller grains, and analyze their evolution for different slopes and different volume fraction of large beads. As size segregation occurs, the vertical position of the centre of mass of the large beads is shown to increase exponentially with time with a typical time scale decreasing with their volume fraction. A simple model balancing lift and drag forces acting on large particles recovers this dynamics, successfully predicts the typical time scale of segregation, and permits to relate this typical time scale to the flow dynamics.