NONEQUILIBRIUM MOLECULAR-DYNAMICS SIMULATION OF THE RHEOLOGY OF LINEAR AND BRANCHED ALKANES

Liquid alkanes in the molecular weight range of C-20-C-40 are the main constituents of lubricant basestocks, and their rheological propertie s are therefore of great concern in industrial lubricant applications. Using massively parallel supercomputers and an efficient parallel alg orithm, we have carried out systematic studies of the rheological prop erties of a variety of model liquid alkanes ranging from linear to sin gly branched and multiply branched alkanes. We aim to elucidate the re lationship between the molecular architecture and the viscous behavior . Nonequilibrium molecular dynamics simulations have been carried out for n-decane (C10H22), n-hexadecane (C16H34), n-tetracosane (C24H50), 10-n-hexylnonadecane (C25H52), and squalane (2, 6, 10, 15, 19, 23-hexa methyltetracosane, C-30 H-62). At a high strain rate, the viscosity sh ows a power-law shear thinning behavior over several orders of magnitu de in strain rate, with exponents ranging from - 0.33 to - 0.59. This power-law sheer thinning is shown to be closely related to the orderin g of the molecules. The molecular architecture is shown to have a sign ificant influence on the power-law exponent. At a low strain rate, the viscosity behavior changes to a Newtonian plateau, whose accurate det ermination has been elusive in previous studies. The molecular order i n this regime is essentially that of the equilibrium system, a signatu re of the linear response. The Newtonian plateau is verified by indepe ndent equilibrium molecular dynamics simulations using the Green-Kubo method. The reliable determination of the Newtonian viscosity from non equilibrium molecular simulation permits us to calculate the viscosity index for squalane. The viscosity index is a widely used property to characterize the lubricant's temperature performance, and our studies represent the first approach toward its determination by molecular sim ulation.