Combined shear and elongational flow by non-equilibrium molecular dynamics

Abstract

We continue our development of group theory statistical mechanics applied to non-newtonian shear and elongational flow. We discuss some new aspects of shear flow discovered by non-equilibrium molecular dynamics, NEMD. We investigate the origins of time reversal asymmetry in newly discovered cross-correlation functions. Using the profile unbiased thermostat, PUT, shear flow algorithm we discover that the strong phase is stable in 3D, for typical system sizes considered by molecular dynamics. In addition, for the first time we apply NEMD, to simultaneous shear and elongational non-newtonian flow. New equations of motion are constructed to enforce the elongational flow. We apply transient flows to a model Lennard-Jones liquid and monitor the thermodynamics and mechanical response, directly, and from transient time correlation functions. The value of the shear viscosity can be increased or lowered by the presence of simultaneous shear and elongational flow, in which the main velocity flow directions are perpendicular or parallel, respectively. A combination of shear and elongational flow can produce a heat flux, the thermal equivalent of the Weissenberg effect.