Molecular simulation of the viscosity of asymmetric dense mixtures

Abstract

The shear viscosity of asymmetric, binary mixtures, consisting of small and long-chain molecules, was computed by means of reversed non-equilibrium molecular dynamics simulations. The molecules were modelled as flexible chains of tangent spheres that interact through a combination of site–site Lennard-Jones (LJ) 12–6 intermolecular forces. The calculations were performed in order to elucidate the mechanisms responsible for the experimentally observed marked decrease in the viscosity of a fluid consisting of large molecules, on addition of lighter species.The simulations of pure species indicate that the chains exhibit a range of configurational shapes, but are in general quite folded, even in a dilute state, when there are no other chains or monomers present; the average radius of gyration of a particular chain is nearly temperature independent and only weakly dependent on density. Analysis of the behaviour of the chains with the different number of segments indicates that the resulting viscosity is proportional to the square root of their moment of inertia. The simulations carried out on the binary mixture, consisting of a monomer and a 16-segment chain species, indicate that the viscosity decrease can be broadly attributed to density, mixing and structural effects. The decrease in density and mixing effects led to a large decrease in viscosity, primarily dominated by the effect of mixing the two species. The structural changes resulted in an increase in viscosity, as the presence of monomers led to configurational relaxation of hexadecamer and to a localization of monomers in the vicinity of the hexadecamers.