Non-equilibrium molecular dynamics simulation and generalized hydrodynamics of transverse modes in molecular fluids

Abstract

We present a generalized hydrodynamic theory of transverse motion in molecular fluids. The theory considers the coupled motion of the molecular orientation, symmetric and antisymmetric components of the microscopic stress tensor, molecular spin, and transverse momentum density. We have conducted non-equilibrium molecular dynamics simulations, of the Lennard-Jones diatomic fluid, which are designed to test the way in which this theory describes the time dependent responses of orientation, stress and spin to an applied velocity gradient. A primary aim of the work is the evaluation of the dimensionless shear-orientation parameter which appears in this theory and in various theories of depolarized light scattering and flow birefringence. The systems studied correspond to fluorine at 70 K, fluorine at 120 K and carbon dioxide at 273 K, all at approximately atmospheric pressure. We find that may be estimated by measuring the steady state orientation density induced by a uniform applied velocity gradient; we obtain values of =0·46 (F₂/70 K), 0·22 (F₂/120 K), and 0·20 (CO₂/273 K). The generalized hydrodynamic equations are found to give a satisfactory description of the observed time dependent responses. Coupling with the molecular orientation is observed to have a significant effect upon the form of the stress response at long time, and this in turn affects the accuracy with which molecular fluid viscosities may be estimated via the ‘subtraction’ technique of molecular dynamics.