Microscopic Simulation of Rheology: Molecular Dynamics Computations and Percolation theory

Abstract

The rheology of any liquid can now, in principle, be predicted from first principles using the computer simulation technique of molecular dynamics. Computations on simple molecular fluids close to the solid phase co-existence line have revealed that shear thinning and shear thickening, as well as other non-Newtonian effects are manifest by all liquids at large enough shear rates. This discovery leads to simplifications in predicting non-Newtonian behaviour. The shear viscosity as a function of shear rate for a wide range of disparate experimental and simulated liquids fall on a “universal” curve, when normalised by internally derived parameters.

The rheology of intermediate density fluids at volume fractions ca. 25% has not been studied by simulation with the same degree of interpretive success. We reveal that there is a close link between the rheology of this part of the phase diagram and microscopic parameters, using ideas borrowed from Percolation Theory. We establish this relationship directly by molecular dynamics simulation. This is a first step towards modelling the rheology of weakly and strongly aggregating colloidal dispersions.

Key Words:

• Rheology
• molecular dynamics
• computer simulation
• percolation theory
• Lennard-Jones potential