Molecular dynamics simulation of unsteady diffusion

Abstract

The behavior of diffusion of monatomic gases is examined by molecular dynamics simulation, using a multi-cell computational method. The gases are modeled as hard spheres, undergoing elastic intermolecular and boundary coollisions in a parallelopiped domain. Accordingly, the process is entirely causal. The computational procedure traces the time evolution of the mixing process, starting with initially unmixed gases. The time history of the diffusion process is described, and diffusion coefficients are determined from the long time behavior. The molecular parameters for binary diffusion correspond to a heliumargon mixture. The effects of Knudsen number are examined. In self diffusion, the early time behavior indicates faster mixing than one would obtain using Fick's law with a constant diffusion coefficient. In binary diffusion, early time behavior of the molecular concentration is found to be non-monatonic. © 2019 Elsevier B.V., All rights reserved.