The molecular dynamics study of shear thinning of the Lennard-Jones fluid

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An extensive series of non-equilibrium molecular dynamics, NEMD, simulations have been performed of sheat thinning of the Lennard-Jones fluid. Calculations on over 100 state points have been used to characterise and parameterise the thermodynamic and dynamical behaviour of the fluid at high densities in the region in which non-newtonian phenomena become pronounced. This is typically at a shear rate of order unity in Lennard-Jones reduced units. The formalism derived by Hanley and Evans was used to characterise the shear rate dependence of the internal energy, pressure and viscosity. It was also shown to apply equally well to the shear rigidity modulus and self-diffusion coefficient. For the thermodynamic properties, they increase in proportion to the shear rate to the power of 1.5, whereas for the viscosity and self-diffusion coefficient the power is 0.5 but with coefficients of opposite sign. This is consistent with a demonstrated applicability of the Stokes-Einstein relationship in the shear thinning regime. The nature of the shear thinning is presented in terms of structure, through projected molecular coordinates on the MD cell sides, and dynamics via the force autocorrelation functions. A number of alternative empirical correlations involving the shear stress and other state variables are also given.