Coupling of velocity gradients to orientation densities

Velocity gradients and orientation densities are coupled in liquids composed of anisometric molecules, as is evidenced by flow birefringence and by the Rytov dip observed in VH depolarized light scattering. The coupling strength can be related to a dimensionless parameter R which can be expressed in terms of correlations over molecular quantities or, when evaluated by a hydrodynamic model, in terms of geometric factors. Both cases are discussed. Although R depends on the temporal behaviour of molecular quantities, i.e. it is a dynamic factor, it is actually a ratio of dynamic factors and can be approximated by static or equilibrium quantities. In the hydrodynamic limit, i.e. for brownian particles, this approximation is rigorously valid.