Rheology, dynamics, and structure of hydrocarbon blends: a molecular dynamics study of n-hexane/n-hexadecane mixtures

Equilibrium molecular dynamics (EMD) and non-equilibrium molecular dynamics (NEMD) simulations are carried out on n-hexane/n-hexadecane binary mixture s. Using EMD, dynamic properties such as the viscosity, self-diffusivity, a nd rotational relaxation, as well as static structural properties are compu ted at different compositions. Upon mixing, a slowing down of the dynamics of the smaller species is observed, while the dynamics of the long species increases. The fluids exhibit non-ideal mixing due to a non-random distribu tion of the methylene units of the chain molecules. By examining the short range structure of the fluid, a local clustering of the smaller chains is o bserved. NEMD is used to simulate shear Couette flow and compute the shear viscosity of the fluid, particularly in the non-linear viscoelastic regime, Phenomena such as shear thinning, normal stress effects and molecular alig nment are observed. Two shear thinning transitions are observed for the cas e of mixtures. It is shown that this is a result of the two species having differing relaxation times and varying tendencies to align with the shear f low. The longer chains shear thin first and align more readily than the sho rter ones. The effect this has on the overall properties and shear-thinning spectrum, and its relation to the dynamics of each species, is examined. ( C) 1999 Elsevier Science S.A. All rights reserved.