On the interpretation of shear viscosity ultrasonic measurements

Abstract

This article discusses the possibility of using the measurements of ultrasonic wave parameters to estimate numerical values of shear and bulk viscosities on-line. It is noted that to increase the reliability of such estimates, it is necessary to introduce classification criteria for liquids. The on-line classification of liquids can be carried out through the measurement and comparison of the real and imaginary parts of liquid acoustic shear impedances. When the measured real and imaginary parts of the impedance are equal, the calculated ultrasonic viscosity values do not differ from the shear viscosity measured by a capillary viscometer. This occurs in liquids with a shear viscosities of less than 0.015 … 0.02 Pa ∗ s (i.e., low-viscous liquids). If the real and imaginary components of the liquid shear impedance are not equal, however, the proportionality coefficient between the viscous tensor and the shear strain rate becomes a complex number, and the calculated shear viscosity values are significantly different from values obtained by the capillary viscometer. This holds true in viscous liquids. The interpretation of the results in viscous non-polymeric liquids is considered in detail within the framework of the Maxwell model, and also within the framework of a proposed shear viscosity relaxation model. The influence of the relaxation process on the measured values of ultrasonic shear viscosity is considered. As an example, the model takes into account the presence of small molecular clusters in viscous liquids. The propagating viscous wave breaks the equilibrium distribution of the clusters, which in turn causes the relaxation process.It is shown that the shear viscosity of such liquids calculated from the ultrasonic data becomes a complex quantity depending on the frequency. A scheme for bulk viscosity measurements in the on-line mode is proposed. To find numerical values of the bulk viscosity, it is necessary to measure the real and imaginary components of the liquid shear impedance and the absorption coefficient of the longitudinal waves (if the thermal conductivity contribution can otherwise be neglected). Then the Stokes sound absorption coefficient and, accordingly, the numerical value of the bulk viscosity coefficient can be calculated.