Light scattering by a fluid in a nonequilibrium steady state. I. Small gradients

Abstract

The equations derived in the previous paper for the unequal- and equal-time correlation functions of the microscopic densities of mass, momentum, and energy are solved and applied to light scattering for the case of a fluid subject to a small temperature gradient. The deviations of the dynamical structure factor $S(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},\ensuremath{\omega})$ and the intensities of the Rayleigh and Brillouin lines from their equilibrium behavior are computed to first order in the temperature gradient. The shape and intensity of the Rayleigh line remains the same as in equilibrium. The shapes and the intensities of the Brillouin lines deviate from their equilibrium values by terms proportional to the temperature gradient, leading to an asymmetry in the heights and intensities of the two lines. This asymmetry in the intensities is caused by a mode-coupling effect: the coupling of two sound modes to the heat flux. Owing to restrictions on the theory, the predicted change in the shape of the Brillouin lines is too small to be detected, but the change in their integrated intensities might be observable.