Abstract
We discuss the behavior of the extended sound modes of a dense binary hard-sphere mixture. In a dense simple hard-sphere fluid the Enskog theory predicts a gap in the sound propagation at large wave vectors. In a binary mixture the gap is only present for low concentrations of one of the two species. At intermediate concentrations sound modes are always propagating. This behavior is not affected by the mass difference of the two species, but it only depends on the packing fractions. The gap is absent when the packing fractions are comparable and the mixture structurally resembles a metallic glass.
Highlights
Short-wavelength collective modes in dense simple hard-sphere fluids have been studied extensively by both theoretical and experimental researchers in the past few years.[1−4] Two interesting features of the extended hydrodynamic modes in a simple fluid are the softening of the heat mode at the wave vector k where the static structure factor S(k) has its first maximum and the appearance of a gap in the sound propagation in the same large-wave-vector region.l,2 The softening of the heat mode corresponds to the slowing down of structural relaxation in a dense fluid and has been discussed extensively in the literature
This approximation was motivated by the fact that the coefficient of thermal diffusion vanishes in a first-Soninepolynomial approximation, and in the long-wavelength limit it is always much smaller than the diffusion coefficient in mixtures of spheres of not too disparate sizes and masses.1O On the other hand, in the long-wavelength limit the coefficient of thermal diffusion contributes to the damping of the sound modes.[11]
The model used here for the description of the extended sound modes based on the two coupled equations for temperature and longitudinal momentum fluctuations neglects all couplings to the densities and does not contain the coefficient of thermal diffusion
Summary
Follow this and additional works at: https://surface.syr.edu/phy Part of the Physics Commons. Cristina Marchetti Physics Department, Syracuse University, Syracuse, New York 13244. We discuss the behavior of the extended sound modes of a dense binary hard-sphere mixture. In a dense simple hard-sphere fluid the Enskog theory predicts a gap in the sound propagation at large wave vectors. In a binary mixture the gap is only present for low concentrations of one of the two species. At intermediate concentrations sound modes are always propagating. This behavior is not affected by the mass difference of the two species, but it only depends on the packing fractions. The gap is absent when the packing fractions are comparable and the mixture structurally resembles a metallic glass
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