A solution of the inelastic Enskog kinetic equation that applies to a wide range of values of the coefficients of restitution and densities is used to determine the thermal diffusion factor of an intruder immersed in a granular fluid under gravity. Thermal diffusion gives a criterion for segregation in terms of the parameters of the system (masses, sizes, density and dissipation). In contrast to previous theoretical attempts our model retains the complete nonlinear dependence of the transport coefficients on dissipation, takes into account the influence of both thermal gradients and gravity on segregation and applies for moderate densities. The present analysis extends previous results derived in the dilute limit case and is consistent with the findings of some recent experimental results.
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