Energy-Trapping Torsional-Mode Resonators for Liquid Sensing

Abstract

Abstract-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.

Experimental verification of a quasi-trapped degenerate mode magnetic acoustic resonator

“…However, the anisotropic elasticity of the piezoelectric crystals would break the axisymmetry of the design required for degenerate resonant responses. Magnetic acoustic coupling provides an alternative excitation method which permits the use of elastically isotropic material thus maintaining the desired degeneracy [12][13][14][15][16][17]. The material must however be electrically conductive to support the generation of eddy currents essential for excitation via this method.…”

“…Electromagnetic excitation of acoustic waves was first demonstrated in the late 1960's [12] and has been explored in recent years as a means of realising high quality factor resonators for mass detection [13][14][15][16][17]. Figure 4 illustrates the device, the excitation and detection coils.…”

Recent advances in lateral field excited and monolithic spiral coil acoustic transduction bulk acoustic wave sensor platforms

“…Since then the applications have included inspecting the welds of sheet metal in a rolling mill, railroad rail inspection and a thickness gage for seamless stainless steel tubing [7]. In contrast to the earlier EMAT applications, which were on the macro scale, recent EMAT applications have been used on a micro scale for a broad array of sensing applications, which include biosensors [8] and liquid sensors [9].…”

Electromagnetically excited resonator sensors for remote mass detection and liquid phase sensing

“…These results have also been achieved by other groups, and similar conclusions could be drawn. Circular FSM or torsional plate resonators are suitable sensor devices for measuring liquid properties and distinguishing between different fluids [9].…”