Effects of Pressure on Ultrasonic Relaxation in Liquids

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Abstract
Measurements of sound absorption as a function of pressure in carbon disulfide and glycerol indicate that the relaxation effects causing the compressional losses in these liquids react differently to pressure changes. In CS2 the relaxation frequency increased with increasing pressure. In glycerol the relaxation frequency dropped as the pressure was increased. These results are consistent with the view that different relaxation mechanisms exist in associated and nonassociated liquids. The direction of the pressure changes in each case is in accord with the view that thermal relaxation is the cause of compressional loss in nonassociated liquids and that structural relaxation is the predominant factor in associated liquids. A detailed study of the thermal relaxation effects in CS2 showed that the variations in relaxation frequency with pressure could not be adequately explained by Kittel's theory of thermal relaxation in liquids. Theoretical considerations of the relaxational part of the specific heat indicated that it should be only slightly pressure dependent.

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