Nonlinear birefringence in fluids containing nonspherical molecules

The orientation density in nonequilibrium systems is expressed in terms of equilibrium time-correlation functions to quadratic order in the gradients in density, pressure and velocity fields. These are evaluated by mode coupling theory in the reversible bare vertex approximation, at infinite dilution of the nonspherical component and using a two-variable model prediction for the two-point orientation correlations. A very simple nonlinear correction term is found, predicting no nonlinear shear birefringence and a nonlinear acoustic birefringence which scales as ΩI12Г with respect to the linear birefringence (Ω is the driving frequency, I is a dimensionless acoustic intensity and Г is the equilibrium orientation relaxation rate).