  The intruder segregation dependence on size and density is investigated in
the framework of a hydrodynamic theoretical model for vibrated granular media.
We propose a segregation mechanism based on the difference of densities between
different regions of the granular system, which give origin to a buoyant force
that acts on the intruder. From the analytic solution of the segregation
velocity we can analyze the transition from the upward to downward intruder's
movement.
