Sensing the properties of liquids with doubly rotated resonators

Immersion of a resonator into a liquid results in changes in the resonator´s frequency and impedance. These changes have been used to characterize liquid properties. Frequency can be measured with far higher accuracy than impedance or any other quantity. The goal of the project described in this paper was to investigate whether or not it is possible to measure a liquid´s properties by means of frequency measurements alone. When a doubly rotated resonator (θ≈35° and φ>0°) is operated in a liquid, the displacement of the surface is partly out of the plane of the plate. By controlling the φ angle, one may control the ratio of in-plane to out-of-plane displacements. The out-of-plane component of the displacement propagates a damped compressional wave into the liquid, while the in-plane component propagates a damped shear wave. The frequency changes of doubly rotated resonators have been measured in glycerol solutions of a variety of concentrations. At each concentration, the frequency change was found to increase with increasing φ angle. A method is proposed for the determination of a fluid´s density, viscosity and acoustic wave velocity