Nonequilibrium molecular dynamics study of shear and shear-free flows in simple fluids

Abstract

Nonequilibrium molecular dynamics simulations have been performed in order to compare the characteristics of planar Couette, planar elongation, uniaxial stretching, and biaxial stretching flows in simple fluids at different strain rates. After deriving the periodic boundary conditions for general flow fields and introducing some methodological improvements for elongation flow calculations we simulated the combination of shear and shear-free flows as well. We found that even at high strain rates where simple fluids exhibit strong non-Newtonian behavior (shear-thinning) it is a reasonable approximation to consider the two planar flows to be rotationally equivalent. This is because in planar Couette flow the in-plane normal stress difference of simple fluids is approximately zero even far from equilibrium. Similarly to planar Couette flow, the trace of the pressure tensor and the internal energy vary approximately as function of the 3/2 power of the strain rate in shear free flows. However, the individual diagonal elements of elongation flow pressure tensors deviate considerably from this approximation. In the extension direction the pressure seems to have a minimum in terms of the strain rate in every shear-free flow. We have discussed the implications of these results.