On the existence of sound propagation gaps in disparate mass gas mixtures

Several scattering experiments indicate that the hydrodynamic theory is valid beyond the wavevector region in which the Brillouin-shift is linear with the wavevector. In this article extended hydrodynamic calculations are presented concerning the disparate mass mixture He+Xe, using Navier-Stokes equations. The calculations of the eigenvalues of the hydrodynamic matrix reveal the existence of sound propagation gaps for a low density gas mixture with x Xe < 0·16 at kσ ⋍ 0·006, similar to those which are obtained in a one component fluid at a triple point density for kσ = 2π. Results concerning the wavevector dependence of the amplitudes of the Rayleigh-Brillouin triplet are also given, as well as a discussion concerning ultrasound experiments. The predictions presented here can be tested by light scattering experiments.