Stress-structure coupling and nonlinear rheology of Lennard-Jones liquid

The cross correlation between the two-body density and shear stress of Lennerd-Jones liquids is evaluated by means of equilibrium molecular dynamics (MD) simulation in order to clarify the microscopic structure that determines the shear viscosity. The slowest viscoelastic relaxation is coupled to the shift of the main peak of the static structure factor as is predicted by mode-coupling theory (MCT). The decay of the cross-correlation function in the reciprocal space is explained by the square of the intermediate scattering function, which is also in harmony with MCT. The distortion of the microscopic structure under simple shear is calculated by means of non-equilibrium MD simulation. The linear response relation also holds on the two-body density within the Newtonian regime of shear viscosity, while the structural distortion diminishes in the shear-thinning regime. The transition between the Newtonian and the shear-thinning regimes occurs at the shear rate where the magnitude of the structural distortion becomes as large as that of the equilibrium structure. The breakdown of the Cox-Merz rule is ascribed to the narrow width of the main peak of the static structure factor.