The torsional waveguide viscosity probe: design and anomalous behavior

This paper is concerned with the design of viscosity sensors based on a torsional waveguide. The advantages of using guided wave attenuation instead of speed for viscosity estimation are established. The effects of probe material, dimensions and operating frequency on viscosity measurement are discussed in the context of a requirement to match the measured attenuation to the range of viscosity values that are required to be measured, given the constraints on measurability imposed by the overall signal and noise conditions. A prototype probe is shown to work well with Newtonian liquids but to appreciably underestimate the viscosities of polymeric oils; these anomalies are explained quantitatively on the basis of a model of intramolecular relaxation. The probe was unsuccessful when applied to slurries, and a basic explanation is given

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