Viscoelastic properties and acoustic relaxation in liquid methane-argon mixtures
Abstract
The shear viscosity ηs and sound attenuation α were measured in liquid methane-argon mixtures in equilibrium with saturated vapors at temperatures of 90 to 180°K and frequencies of 15 to 126 MHz. It is established that the shear viscosity ηs and the “high-frequency” volume viscosity ηVhf are nonmonotonic functions of the composition and substantially different. The rotational degrees of freedom of the CH4 are shown to affect the values of ηs and ηVhf, primarily because of the angular momentum contributions. Dilution of the system with argon has almost no effect on the configurational relaxation times but it does substantially retard the relaxation in momentum space. Acoustic relaxation associated with the excitation and deactivation of intramolecular vibrations occurs at lower temperatures in solutions than in pure CH4 and the relaxation frequency decreases as the argon concentration increases. It is shown that these effects are due to the dependence of the probability of the respective energy transitions on the spectral intensity of the mean force autocorrelator.