  We present a hydrodynamic theoretical model for "Brazil nut" size segregation
in granular materials. We give analytical solutions for the rise velocity of a
large intruder particle immersed in a medium of monodisperse fluidized small
particles. We propose a new mechanism for this particle size-segregation due to
buoyant forces caused by density variations which come from differences in the
local "granular temperature". The mobility of the particles is modified by the
energy dissipation due to inelastic collisions and this leads to a different
behavior from what one would expect for an elastic system. Using our model we
can explain the size ratio dependence of the upward velocity.
