ABSTRACT
We investigate the effects produced on the light scattering spectrum by the anisotropic diffusion of impurities in an incompressible nematic solvent. The spectrum is calculated by using a fluctuating hydrodynamic description when the system is both in a fully thermodynamic equilibrium state and in a nonequilibrium steady state induced by a dye-concentration gradient. In the former state, the isotropic pretransitional phase as well as the nematic phase of the solvent are considered. This spectrum is symmetric (Lorentzian) with respect to the frequency shifts, but anisotropic through its explicit dependence on the ratio of the diffusion coefficients of the dye parallel and normal to the mean molecular axis of the nematic. The values of these coefficients were taken from experimental measurements of diffusion of methylred and nitrosodimethylaniline in a N -( p -methoxybenzylidene)- p -butylaniline (MBBA) solvent. This anisotropy changes the height and the width at mid height with respect to the isotropic case in amounts which for MBBA vary up to 36% and 26%. We also calculate the spectrum in the presence of a concentration gradient of the dye and find that its presence gives rise to an asymmetry of the spectrum in its dependence on the frequency shift; its maximum increases and is displaced with respect to its equilibrium position. The size and direction of this shift are proportional to the magnitude of the dye-concentration gradient and depend on its relative orientation with respect to the scattering vector. For small dimensionless concentration gradients ( approximately 10(-2) ), this effect is maximum when these vectors are parallel and the scattering angle is low ( approximately 1 degrees ). The maximum degree of departure from equilibrium is significant and turns out to be approximately 55%. In view of the significant changes in the spectrum, our theoretical analysis suggests that these effects might be observable.
