Anomalous sound propagation and mode degeneracy in noble gas mixtures

Abstract

In binary gas mixtures, the phase speed of sound usually varies slowly with frequency and gas composition. Huck and Johnson predict, however, that in mixtures of molecules of disparate molecular masses, anomalous dispersion should be possible: a large and rapid change in sound speed over a small composition range in the vicinity of a critical composition, and for frequencies higher than a critical frequency. The effect results from the predicted degeneracy of the sound mode with another possible mode of excitation at critical values of frequency and composition. Here sound adsorption and dispersion data are presented for helium-xenon mixtures, over a range of frequencies and compositions, which include the predicted critical region. Results are compared with improved theoretical predictions based on thirteen-moment equations using reliable intermolecular potentials. Experiment confirms mode degeneracy, and suggest nearly equal mole-fractions, and a frequency of ca . 70 MHz (at 25 °C and 1 bar (1 bar = 10 5 Pa)) as the critical values of composition and frequency.