KINETIC-THEORY OF TRANSPORT-COEFFICIENTS FOR SIMPLE LIQUIDS

The temperature behaviors of self-diffusion coefficient D, viscosity coefficient η, and thermal conductivity λ for a simple liquid are quite different from each other while they are similar for a gas. The differences in liquid arise because the carriers of the mass, momentum and energy transfers are different. This is discussed by using simple kinetic theory in terms of the mean free path. In particular, the exponential decrease of the liquid viscosity with the rise in temperature is shown to come from the decrease in the mean free path for vacancies. The thermal conductivity which exhibits a temperature behavior similar to that of the viscosity coefficient and dissimilar to that of the diffusion coefficient is discussed in terms of the energy transfer by vacancies of small sizes (phonons) and sizes comparable to the molecular size. The microscopic origin for the diverse temperature behaviors can be traced to the potential contributions to those relevant currents, which appear in the Kubo-Green formulas for η and λ. There is no such potential contribution for D, and the diffusion coefficient must be calculated by examining the molecular motion just as we do so for the case of gas. © 1990.