Segregation due to surface flows of granular mixtures

We present a theoretical model of surface flows for a mixture of two grain species, differing by their size and their surface properties. The rolling stream is assumed to be homogeneous. Exchanges between the grains at rest and the rolling stream are modelled via binary collisions. The model predicts that during the filling of a 2D silo, continuous segregation appears inside the static phase. This fits the observations when the size difference between the species is small. We argue that when the size difference is large, segregation occurs directly inside the rolling stream.