Surface flows of granular mixtures: II. Segregation with grains of different size

We present the generalization of a theoretical model for segregation of gra nular mixtures due to surface flows, published in J. Phys. I France 6, 1295 (1996). Our generalized model is valid for grains differing ba their size and/or their surface properties; in the present paper, we describe the case of two species with the same surface properties but two different sizes. T he rolling stream is assumed to be homogeneous. Exchanges between the grain s at rest and the rolling stream are modelized via binary collisions. The m odel predicts that during the filling of a two-dimensional silo, continuous segregation appears inside the static phase: small (respectively large) gr ains tend to stop uphill (respectively downhill), although both species rem ain present everywhere. This fits the observations when the size difference between the species is small. When the size difference is large, a differe nt regime is observed. We argue that in this case, segregation occurs direc tly inside the rolling stream.