Shear-induced angular dependence of the liquid pair correlation function

A formal expansion in spherical harmonics or Cartesian tensors of the pair correlation function of a liquid subjected to a shear rate is discussed. Expressions for the coefficients to tensor rank 4 are evaluated via a nonequilibrium molecular-dynamics simulation of an inverse twelve soft-sphere liquid undergoing Couette flow. It is shown that the expansion converges slowly if the product {τ}{γ}>0.05, where {τ} is the Maxwell relaxation time and {γ} is the shear rate. Further, the fourth-rank coefficient that represents cubic symmetry is significant for our model system. The microstructure of a shear liquid is demonstrated by intensity plots of particles around a given central particle. We have derived expressions for the expansion coefficients using a relaxation-time model and the comparison between them and the simulations is generally very good.