Diffusion of intruders in a granular gas thermostatted by a bath of elastic hard spheres

Abstract: The Boltzmann kinetic equation is considered to compute the transport coefficients associated with the mass flux of intruders in a granular gas. Intruders and granular gas are immersed in a gas of elastic hard spheres (molecular gas). We assume that the granular particles are sufficiently rarefied so that the state of the molecular gas is not affected by the presence of the granular gas. Thus, the gas of elastic hard spheres can be considered as a thermostat (or bath) at a fixed temperature $T_g$. In the absence of spatial gradients, the system achieves a steady state where the temperature of the granular gas $T$ differs from that of the intruders $T_0$ (energy nonequipartition). Approximate theoretical predictions for the temperature ratios $T/T_g$ and $T_0/T_g$ and the kurtosis $c$ and $c_0$ associated with the granular gas and the intruders compare very well with Monte Carlo simulations for conditions of practical interest. For states close to the steady homogeneous state, the Boltzmann equation for the intruders is solved by means of the Chapman--Enskog method to first order in the spatial gradients. As expected, the diffusion transport coefficients are given in terms of the solutions of a set of coupled linear integral equations which are approximately solved by considering the first-Sonine approximation. In dimensionless form, the transport coefficients are nonlinear functions of the mass and diameter ratios, the coefficients of restitution, and the (reduced) bath temperature. Interestingly, previous results derived from a suspension model based on an effective fluid-solid interaction force are recovered when $m/m_g\to \infty$ and $m_0/m_g\to \infty$, where $m$, $m_0$, and $m_g$ are the masses of the granular, intruder, and molecular gas particle, respectively. Finally, as an application of our results, thermal diffusion segregation is exhaustively analysed.