Ultrasonic Hysteresis in Viscous Liquids

Abstract

The ultrasonic absorption coefficients for longitudinal waves in glycerol and pentachlorobiphenyl are measured over the frequency range from 7.5 to 70 Mc. The measurements are made in the range of viscosities from 105 to 107 poise so that the ultrasonic frequencies used are well above the main relaxation frequencies associated with viscous flow processes in these liquids. The results indicate that the absorption coefficient per cm can be represented by the expression α = B plus; Hf, where B and H are constants and f is the ultrasonic frequency. The constants B and H in both liquids are found to decrease with increasing temperature, exhibiting an “apparent” activation energy of about 24 kcal/mole and 12 kcal/mole for the pentachlorobiphenyl and glycerol respectively. It is concluded that the results found here indicate that both terms B and Hf found in a could best be explained by assuming that one or more distributions of relaxation times exist. It is suggested that ultrasonic hysteresis in both liquids and solids have a common origin indicating that the hysteresis effect in liquids is not related to the viscous flow mechanism causing the absorption maximum at lower frequencies and viscosities. A possible mechanism offered for the hysteresis loss is the coupling of the acoustic energy into heat energy due to the anharmonicities in the lattice structure.