The mutual diffusion coefficient in a dilute hard sphere gas mixture

The mutual diffusion coefficient is calculated in the ’’successive uncorrelated binary collision approximation’’ (SUBC approximation). It is shown that the ’’single collision approximation’’ agrees with the first Chapman–Enskog approximation and how higher approximations can be obtained by including more information on the collision processes. One of the long standing puzzles of mean free path theory—the fact that only collisions between unlike molecules contribute to mutual diffusion—is solved quite naturally within the SUBC approximation.