  We perform a two-dimensional molecular-dynamics study of a model for sheared
bidisperse granular systems under conditions of simple shear and Poiseuille
flow. We propose a mechanism for particle-size segregation based on the
observation that segregation occurs if the viscous length scale introduced by a
liquid in the system is smaller than of the order of the particle size. We show
that the ratio of shear rate to viscosity must be small if one wants to find
size segregation. In this case the particles in the system arrange themselves
in bands of big and small particles oriented along the direction of the flow.
Similarly, in Poiseuille flow we find the formation of particle bands. Here, in
addition, the variety of time scales in the flow leads to an aggregation of
particles in the zones of low shear rate and can suppress size segregation in
these regions. The results have been verified against simulations using a full
Navier-Stokes description for the liquid.
