Transient ensemble averages in non-Newtonian flow: Symmetry and simulation.

Abstract

Transient (nonequilibrium) cross correlation functions (CCF's) are introduced for the characterization of non-Newtonian flow under conditions of shear startup or cessation. Arguments based on the principles of group-theoretical statistical mechanics imply that these are asymmetric for shearing and symmetric for elongational stress. These expectations are confirmed with nonequilibrium molecular dynamics. Nonequilibrium CCF's are evaluated in the rise transient regime, immediately following the application of shear stress, and in the fall transient regime immediately following its removal. The rise and fall transient CCF's of atomic velocity and pressure tensor components are found to have quite different time dependencies, in line with the rise and fall stress transients themselves. They also correlate well with the non-Newtonian characteristics of the fluid, in that, as the response to the stress becomes increasingly more non-Newtonian, the difference between the equilibrium and rise or fall transient CCF's develops an increasingly sharp peak at short time.