A three-variable theory on the translation–reorientation coupling of molecular liquids

Abstract

The two-variable theory on the translation–reorientation coupling observed in the light-scattering spectra of molecular liquids is extended to include the time derivative of the orientation as one of the explicit variables. The novel “three-variable theory” has the mathematical form more proper than the two-variable one in that the shear viscosity is directly proportional to the time integration of the time-correlation function of the random shear stress and that the translation–reorientation coupling constant is given by the covariance of the random shear stress and random acceleration of the orientation. Molecular dynamics simulations on the isotropic phase of Gay–Berne fluid are performed to test the performance of the three-variable theory. The decay times of all the memory functions are much shorter than the corresponding ones of the two-variable theory, and they are rather insensitive to the density of fluid even near the isotropic–nematic transition where reorientational correlation time is long. These characteristics demonstrate that the three-variable theory provides a better starting point to discuss the translation–reorientation coupling based on the molecular interaction and liquids structure.