Kinetic energy relaxation of a test particle in a dense fluid

Abstract

The rate constant kε for the relaxation of the kinetic energy of a solute test hard sphere in a dense hard sphere solvent is studied. Microscopic boundary layer and ring kinetic theory methods are used to construct a simple expression for kε. It is found that kε−1 has three additive contributions arising from (a) uncorrelated binary collisions, (b) coupling to the solvent energy density, and (c) coupling to the solvent shear modes. Thus kε−1 is determined by the Enskog collisional relaxation rate and the rates of the hydrodynamic flows in the solvent of heat and momentum. The competition between these effects is studied as a function of particle size and mass ratios and solvent density. The regimes examined at higher density vary from the identical solute–solvent case, where kε is mainly collisional with a small hydrodynamic heat flow correction, to the large, massive solute case, where kε is governed by solvent momentum flow according to a Stokes law type relation.