Developments in the hard sphere model for self-diffusion and shear viscosity

Abstract

Abstract Examination of shear viscosity (η) and self-diffusion coefficient ( D ) data for liquid methane, together with equivalent hard sphere diameters (σ ϱ ) derived from liquid densities along the solid-liquid coexistence curve, is used to show that the Stokes-Einstein equation in the hydrodynamic slip limit is obeyed only at very high densities. The product D ησ ϱ / kT differs significantly both at low and at very high densities from values obtained from molecular dynamics (MD) simulations of a hard sphere fluid. Values of η/η E (η E = Enskog dense fluid viscosity) for methane, used in place of those from MD simulations, gave the expected values of A η (translational-rotational coupling factor) for nitrogen ( A η > 1), and (within the uncertainty of the experimental data on which the calculation is based) for the liquefied rare gases ( A η = 1) at temperatures within 20–25% of the critical temperatures. Apparent departures from smooth hard sphere behaviour, of the liquefied rare gases at temperatures approaching the triple points, are accounted for by the assumption of less dense packing in the liquids at low solid-liquid transition temperatures than for a hard sphere fluid.