Viscoelastic shear thinning of liquids. A molecular-dynamics study

Abstract

A molecular-dynamics investigation into the microscopic response of simple liquids subjected to severe shearing conditions is discussed. The quantitative behaviour of a model Lennard-Jones liquid and experimental data on more complicated molecular fluids at high levels of stress are strikingly similar, provided appropriate adjustments of characteristic relaxation time-scales are made.The simulations give insights into the structural origin of material failure at high shear stresses and reveal, at high pressures, a restructuring of the medium into layers, which permits easier flow and hence reduces its viscosity. This change is also manifest in an increase of internal pressure (at constant volume) and a decrease in low-frequency shear rigidity modulus and thermal conductivity.Associated temperature distributions in the non-linear region of viscosity are consistent with macroscopic continuum theory, suggesting that the thermal effects due to shear are essentially decoupled from any parallel structural changes.