Ultrasonic Relaxation and Its Relation to Structure in Viscous Liquids

Abstract

Measurements of ultrasonic propagation of longitudinal waves in pentachlorobiphenyl were made over a large range of viscosity at frequencies in the range 7.5 to 52.5 Mc. The data at low viscosities demonstrated the presence of both shear and compressional viscous processes. Measurements in the relaxation region indicated that a single relaxation time theory could not explain the results. Several possible explanations are discussed. The data at very high viscosities showed a frequency independent absorption loss per wavelength, which indicated the possible presence of a nonrelaxational type of hysteresis. The thermal coefficient of the high-frequency velocity was measured and found to be about twice the coefficient of the low-frequency velocity. The thermal coefficients of the instantaneous elastic moduli are compared to the low-frequency elastic moduli. It was concluded that the differences in the temperature dependence of these moduli could be related to the changing degree of order in the liquid.