Generalized concept of shear horizontal acoustic plate mode and Love wave sensors

Abstract

An approach to mass and liquid sensitivity for both the phase velocity and insertion loss ofshear mode acoustic wave sensors based on the dispersion equations for layered systems isoutlined. The approach is sufficiently general to allow for viscoelastic guiding layers. Anequation for the phase velocity and insertion loss sensitivities is given which depends on theslope of the complex phase velocity dispersion curves. This equation contains the equivalentof the Sauerbrey and Kanazawa equations for loading of a quartz crystal microbalance byrigid mass and Newtonian liquids, respectively, and also describes surface loading byviscoelastic layers. The theoretical approach can be applied to a four-layer system, withany of the four layers being viscoelastic, so that mass deposition from a liquidcan also be modelled. The theoretical dispersion equation based approach tolayer-guided shear horizontal acoustic wave modes on finite substrates presented inthis work provides a unified view of Love wave and shear horizontal acousticplate mode (SH-APM) devices, which have been generally regarded as distinctin sensor research. It is argued that SH-APMs with guiding layers possessingshear acoustic speeds lower than that of the substrate and Love waves are twobranches of solution of the same dispersion equation. The layer guided SH-APMshave a phase velocity higher than that of the substrate and the Love waves aphase velocity lower than that of the substrate. Higher-order Love wave modes arecontinuations of the layer-guided SH-APMs. The generalized concept of SH-APMs andLove waves provides a basis for understanding the change in sensitivity withhigher-frequency operation and the relationship between multiple modes in Love wavesensors. It also explains why a relatively thick layer of a high-loss polymer canbe used as a waveguide layer and so extends the range of materials that can beconsidered experimentally. Moreover, it is predicted that a new type of sensor,a layer-guided SH-APM sensor, can be constructed in a manner analogous toa Love wave device. The sensitivity of such a device is predicted to approachthat of a Love wave sensor whilst retaining the advantage of the SH-APM ofusing the face opposite the one possessing the transducers as the sensing surface.