Relations between transport coefficients in Lennard-Jones fluids and in liquid metals

Several simple approximate hard-sphere relations for transport coefficients are compared with the results of molecular dynamics (MD) simulations performed on Lennard Jones (LJ) fluids. Typically the individual transport coefficients: self-diffusion coefficients, D, shear viscosity, eta(s), bulk viscosity, eta(B), and thermal conductivity, lambda, agree within a factor of two of the exact results over the fluid and liquid parts of the phase diagram, which seems reasonable in view of the approximations involved in the models. We have also considered the ratio, lambda/eta(s), and the product, D eta(s), for which simple analytic expressions exist in the hardsphere models. These two quantities also agree within a factor of two of the simulation values and hard sphere analytic expressions. Using time correlation functions, Tankeshwar has recently related the ratio lambda/D to thermodynamic quantities, in particular, to the differences in specific heats, C-p-C-v, and to the isothermal compressibility, kappa(T). Using D and thermodynamic values taken solely from LJ MD simulations, his relation was tested and found to give typically better than similar to 20% agreement at liquid densities, deteriorating somewhat as density decreases into the gas phase. Finally liquid metals are considered. In this case, lambda is dominated by its electronic contribution, which is related approximately to the electrical conductivity by the Wiedemann-Franz Law. Some theoretical results for the electrical conductivity of Na are referenced, which allow a semiquantitative understanding of the measured thermal conductivity of the liquid metal. Shear viscosity is also discussed and, following the work of Tosi, is found to be dominated by ionic contributions; Nevertheless, at the melting temperature of Na, a relation emerges between thermal conductivity, electrical resistivity and shear viscosity.