Energy-Trapping Torsional-Mode Resonators for Liquid Sensing

Thickness-shear mode quartz crystal microbalance (QCM) has been widely used as liquid-phase sensors, such as viscometers and bio-detectors. However, due to coupling between the in-plane shear motion and the out-of-plane flexure, when used in contact with or immersed in a liquid, the out-of-plane motion generates compressional waves in the liquid that reflect off the liquid surface and return to the crystal. This interference effect causes depth-sensitive perturbations in the sensor response, often undesirable. In this study, we show that torsional-mode resonators may be used for liquid sensing without the depth effect. Samples in form of stepped plates, circular decals, and convex contoured faces are machined in elastic plates (e.g., cast aluminum, stainless steel, and brass). A non-contact electromagnetic acoustic transducer (EMAT) was employed to drive torsional-mode vibrations. Efficient energy trapping was observed for first-order torsional modes, leading to high quality factors. When placed in contact with water, the resonance frequency of the torsional mode was found to be independent of the water depth, in contrast to depth-dependent frequency oscillation for the thickness-shear mode. Finite element analyses are conducted to understand the torsional-mode vibrations as well as the effect of material anisotropy