Computer simulation of shear flow in simple fluids

Abstract

Computer simulation of shear flow in fluids has become an important technique in our understanding of the non-linear behaviour of fluids subjected to an external field. An important method for studying fluids from the atomic or molecular level is Molecular Dynamics. Conventional molecular dynamics simulations of shear flow have been performed at constant shear rate and constant pressure. In this thesis we describe how to perform simulations at constant shear stress or constant pressure, which are the usual experimental conditions. The theory used in deriving the constant stress equations is quite general and can be applied for fields other than shear flow, allowing one to simulate at constant thermodynamic force or flux. The constant pressure simulations show clearly the difference between shear thinning and shear dilatancy, a point that has caused confusion in the rheological literature. Efficient computational methods are important as some simulations can take hundreds of hours of computer time. The arrival of a vector processor at the A.N.U. has necessitated the experimentation with algorithms, with substantial performance gains over a standard code. These increases in performance have allowed us to complete a thorough study of shear flow at constant pressure for the soft sphere system, which complements a large body of data at constant density from other workers. Also a thorough study of shear flow in two dimensions has been completed. The two dimensional system shows a large dependence on system size, and we have been able to show the transition from small system behaviour to large system behaviour.