RAYLEIGH-BRILLOUIN SCATTERING FROM FLUID MIXTURES

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Abstract
We apply the memory-function formalism to a binary fluid mixture with a view to analyse the Rayleigh-Brillouin spectrum. By limiting the choice of dynamical variables in the generalized Langevin equation to the set of conserved orthogonal variables, A {ζ1(k, t), ζ(k, t), θ(k, t), J(k, t)}, and making the Markov approximation in the memory function matrix, we recover the macroscopic hydrodynamic results previously obtained by Mountain and Deutch. In the above set, ζ1 is related to concentration fluctuations, ζ to orthogonal density fluctuations, and θ to orthogonal energy density fluctuations. J is the longitudinal momentum density. We then extend the above set to include the first order time derivatives in a manner analogous to the previous treatment of Tong and Desai for one component fluids. In addition to the coupling between the heat flux, q and the diffusion flux, Jd which is present in the hydrodynamic limit, we also find possible couplings of the viscosity stress tensor σ with q and Jd for finite values of k. We discuss the results of this extended analysis, especially the relevance of these couplings to the light and neutron scattering experiments

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