Viscosity Predictions using Nonequilibrium Molecular Dynamics Simulations

Having accurate viscosity-pressure data for alkanes is key for a large number of applications that rely on the use of these molecules as lubricants. Molecular simulation methods have emerged over the past 20 years as a means to predict the dependence of viscosity upon pressure and temperature for these systems, when experimental data is difficult to obtain e.g. under high pressure. These methods also allow us to gain insight into the microscopic mechanisms, at the molecular level, occurring in the fluid during viscous flow. Here, we discuss how equilibrium and nonequilibrium molecular dynamics (NEMD) simulations have been developed to determine the viscosity of a wide range of fluids. In particular, we focus on two nonequilibrium approaches, based either on the steady-state response or on the transient response of the fluid to an applied shear rate. We show how these two methods can be applied to determine the viscosity of fluids, with the Transient-Time Correlation Function (TTCF) approach providing a bridge between the equilibrium molecular dynamics simulation results, based on the Green-Kubo formalism, and the steady-state NEMD results. We also discuss several applications of these methods, and compare their performance.