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.