Size dependence of self-diffusion in a dense hard-disc liquid

For a binary liquid mixture of hard discs well above the freezing density o f the one-component liquid the size dependence of the self-diffusion coeffi cients of small and large particles is calculated by molecular dynamics sim ulation. The observed size effect can be satisfactorily explained by the si ze-dependent cut-off of long-wavelength fluctuations of transverse collecti ve flow and gives no indication of the importance of more 'cooperative' pro cesses at high densities.