Soft-sphere Liquid Research Articles

Pressure tensor and viscosity coefficients of a soft sphere liquid under shear

General properties and consequences of the distortion of the structure of a simple liquid subjected to a planar shear flow are reported. In particular, the orientational distribution of particles in the first coordination shell around a given particle is analyzed, and the effect of this distribution on the pressure tensor is discussed. The distorted distribution gives rise to a set of non-Newtonian viscosity coefficients reflecting the occurrence of normal pressure differences in the liquid. Numerical values of these viscosities are given for a soft sphere fluid at 7/8 of the freezing density using the technique of nonequilibrium molecular dynamics. A wide range of shear rates is considered and all viscosity coefficients are found to be functions of the shear rate.

Discrete hydrodynamics: Numerical results for the soft-sphere viscosity

Numerical calculations have been done of the viscosity of the soft-sphere liquid, using a new molecular dynamics technique. It is based on a formulation of hydrodynamics which is discrete in space and time, and exactly renormalizable. The present data turn out to be sufficient to estimate the viscosity, but determination of the full equations of motion (and therefore renormalization) requires further calculations using a smaller discrete time interval; these are presently under way. The present results indicate that this method is more than 100 times more efficient than previous (Green-Kubo or nonequilibrium molecular dynamics) methods. This suggests that the discrete formulation is the most natural way to approach hydrodynamics.

Discrete hydrodynamics: Determination of equations of motion by simulation

A detailed procedure is described for computing discrete equations of motion for a fluid, to implement a new method which turns out to be substantially more efficient than previous methods for calculating transport coefficients. This paper describes the calculations of discrete averages from molecular dynamics data and the numerical extraction of the equation-of-motion coefficients in a way which makes maximum use of the geometric symmetry of the problem. Extrapolation to the infinite-system limit and eventual computation of transport coefficients by renormalization are discussed. The method described in detail here is briefly sketched and applied numerically to computing the viscosity of the soft-sphere liquid in a subsequent paper.