Self-diffusion in equilibrium and sheared liquid mixtures by molecular dynamics

Abstract

The mass and shear rate, γ, dependence of the self-diffusion tensor, D, of isotopically substituted Lennard-Jones (LJ) and Weeks-Chandler-Anderson liquids are determined here, for the first time, by non-equilibrium molecular dynamics using the Gaussian thermostatted SLLOD algorithm. The mass and composition dependence of the self-diffusion coefficients of solute and solvent are characterized as a function of mass ratio of the two species and composition at zero shear rate. In a LJ mixture containing 10% of impurity atoms, each of the three diagonal elements D _αα of the self-diffusion tensor D, increases initially with shear rate, reaches a maximum (which depends on the mass of the solute particle, m _A), and then decreases with a further increase in shear rate. D(m _A, γ) can be represented well by a polynomial, up to second order in the reduced shear rate γ*. The individual D _αα diverge as the shear rate increases, with diffusion along the flow direction always being the largest. To help interpret the trends in D _αα, directionally resolved velocity autocorrelation functions and cross-correlation functions between the momentum of an impurity particle and its surrounding shell are used.