Thermal conductivity and bulk viscosity of simple fluids. A molecular-dynamics study

Abstract

New non-equilibrium molecular-dynamics (MD) simulation methods are used to obtain the specific heats, thermal conductivities, bulk moduli and bulk viscosities of Lennard-Jones (LJ) and soft-sphere (SS) fluids in the range 0.7 < T^* < 4.5 and 0.4 < ρ^* < 1.1. The thermal conductivity is obtained by monitoring the cooling curve of one molecule which has been given an excess of kinetic energy above its surroundings in the MD cell. A simple Newtonian-cooling decay fit yields thermal conductivities with a weak temperature dependence, in agreement with experimental data on argon. Thermal relaxation of an MD cell of molecules subjected to velocity scaling is also demonstrated as a promising route to C_v and (∂p/∂T)_v. A small volume change in the MD cell produces a pressure relaxation which, if conducted isothermally, gives the bulk viscosity. Values are compatible with other work at normal liquid densities but do not exhibit the large increase near the critical point observed experimentally, which is perhaps a reflection of the isochoric constraints on the MD. The ratio of bulk to shear viscosity is 0.49 near the LJ triple point and decreases in the supercooled region. The ratio of thermal conductivity to shear viscosity increases with volume from 1 to 6 in the range studied. The moduli agree excellently with fluctuation theory and equation-of-state estimates.