Translation-orientation coupling and Cox-Merz rule of liquid hexane

Equilibrium and non-equilibrium molecular dynamics simulations are performed on liquid hexane in order to clarify the origin of the Cox-Merz rule of liquids composed of chain-like molecules. The relation between the frequency-dependent complex shear viscosity and the shear-rate dependent nonlinear viscosity follows the Cox-Merz rule as expected. The slowest viscoelastic relaxation mode is explained by the translation-orientation coupling mechanism, and the saturation of the shear-induced orientational order is observed in the non-equilibrium simulation at the onset of the shear thinning. The origin of the Cox-Merz rule is discussed in terms of the translation-orientation coupling.