Rate of shear of an ultrasonic oscillating rod viscosity probe

Ultrasonic oscillating rod probes have recently been used by researchers to measure viscosity and/or density in fluids. However, in order to use such probes to characterise the rheological properties of fluids, it is necessary to define the shear rate produced by the probe. This paper proposes an analytical solution to estimate the shear rate of ultrasonic oscillating rod viscosity probes and a method to measure their maximum operational shear rate. A relationship is developed which relates the torsional surface velocity of an oscillating cylindrical rigid body to the rate of shear in its vicinity. The surface displacement and torsional surface velocity of a torsional probe of length 1000 mm and diameter 1 mm were measured over the frequency range from 525 to 700 kHz using a laser interferometer and the maximum shear rate estimated. The reported work provides the basis for characterising shear rate for such probes, enabling their application for rheological investigations.