Sound-propagation gap in fluid mixtures

TL;DR

This paper investigates how the propagation of sound waves in dense binary fluid mixtures depends on concentration and packing fractions, revealing a gap at certain conditions and its relation to structural properties.

Contribution

It demonstrates that the sound-propagation gap in binary mixtures depends on packing fractions and concentration, not on mass difference, providing new insights into fluid mixture behavior.

Findings

Sound-propagation gap exists only at low concentrations of one species.

The gap disappears when packing fractions are similar, resembling metallic glass structures.

Mass difference does not influence the presence of the gap.

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.