Interfacial viscosities via stress autocorrelation functions

Abstract

Transport properties of simple liquids (the Lennard-Jones model) have been determined, from stress correlation functions, for both the homogeneous liquid and in narrow pores of molecular dimensions. Viscosity profiles are defined by means of a resolution of the total stress tensor and its autocorrelation function into auto- and cross-components of single molecules. This enables a precise definition of viscosity profiles and provides a method of determination from molecular dynamics (MD) simulations of the interfacial regions. Preliminary shear viscosity profiles for liquids in periodic rectangular cavities (between two semi-infinite walls) have been obtained for both the transverse and longitudinal components. Profiles of these transport coefficients have no direct experimentally accessible counterparts, but could be useful for bulk fluid mechanics predictions. Results are presented for (i) a hard, elastic wall–molecule interaction and (ii) a Lennard-Jones (10–4) cohesive wall. Elongational viscosities, which have not previously been investigated via MD correlation functions have also been computed for the homogeneous fluid and the interfacial system. The results are discussed in relation to current experimentation on interfacial viscosities (including recent non-equilibrium molecular dynamics studies).