Motion of a Rigid Body in a Special Lorentz Gas: Loss of Memory Effect

Linear motion of a rigid body in a special kind of Lorentz gas is mathematically analyzed. The rigid body moves against gas drag according to Newton’s equation. The gas model is a special Lorentz gas consisting of gas molecules and background obstacles, which was introduced in~(Tsuji and Aoki: J. Stat. Phys. $\mathbf{\text{146}}$, 620–645, 2012). The specular boundary condition is imposed on the resulting kinetic equation. This study complements the numerical study by Tsuji and Aoki cited above — although the setting in this paper is slightly different from theirs, qualitatively the same asymptotic behavior is proved: The velocity $V(t)$ of the rigid body decays exponentially if the obstacles undergo thermal motion; if the obstacles are motionless, then the velocity $V(t)$ decays algebraically with a rate $t^{-5}$ independent of the spatial dimension. This demonstrates the idea that interaction of the molecules with the background obstacles destroy the memory effect due to recollision.