The nature of extreme shear thinning in simple liquids

Abstract

The structural and dynamical changes that take place in simple liquids during extreme shear thinning are illuminated by Non-Equilibrium Molecular Dynamics. Long range order develops, which is characterized by linear lines of molecules spontaneously forming along the shear flow lines. These ‘strings’ of molecules pack into a triangular lattice, when viewed in a plane orthogonal to their length, and are very slow to drift en masse and appear stationary when viewed in cross-section over many Maxwell relaxation times. Computations were performed on cubic MD cells containing Lennard-Jones molecules at reduced densities close to the triple point for this pair potential. The transition from amorphous liquid to the ‘string phase’ takes place sharply at reduced shear rates of ˙γ = 2·3 ∓ 0·1. Increasing the shear rate leads to a decrease in recoverable shear strain, making a limiting shear stress a useful modelling parameter. Symmetry breaking structural distortions appear also at these shear rates. Calculations of force and pressure tensor component time autocorrelation functions, the component shear rigidity moduli and the component self-diffusion coefficients have revealed that the string phase has characteristics of a solid in directions orthogonal to the streaming direction but it is liquid-like along the flow lines. The establishment of this ‘two time-scale’ material is a consequence of the string phase. Oscillations appear in the orthogonal direction autocorrelation functions of a frequency of order ˙γ. The Reynolds Number is not a useful parameter for indicating the onset of turbulence in these microscopic periodic systems.