Transport coefficients of soft sphere fluids

Abstract

This study explores the effects of interaction softness on the transport properties of simple fluids in which the particles interact via the potential, phi(r) = epsilon(r/sigma)(-n), with n in the range 6-1152. Molecular dynamics simulation has been used to compute the self-diffusion coefficient, D, the shear viscosity eta(s), bulk viscosity eta(b), and the thermal conductivity, lambda, over a wide packing fraction range. A number of semi-empirical formulae for these transport coefficients have been tested against this data. It was found that the transport coefficients were proportional to the exponential of the two-body component of the entropy. The quality of agreement was best for D and worst for the thermal conductivity. A cell model for the fluid based on the simulated mean square force was not successful in correlating the diffusion coefficient data over the wide range of n. We also consider several variants on the Stokes-Einstein relationship, which provide an informative insight into the global behaviour of (mainly) the self-diffusion coefficient and shear viscosity.