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

An approach to mass and liquid sensitivity for both the phase velocity and insertion loss of shear mode acoustic wave sensors based on the dispersion equations for layered systems is outlined. The approach is sufficiently general to allow for viscoelastic guiding layers. An equation for the phase velocity and insertion loss sensitivities is given which depends on the slope of the complex phase velocity dispersion curves. This equation contains the equivalent of the Sauerbrey and Kanazawa equations for loading of a quartz crystal microbalance by rigid mass and Newtonian liquids, respectively, and also describes surface loading by viscoelastic layers. The theoretical approach can be applied to a four-layer system, with any of the four layers being viscoelastic, so that mass deposition from a liquid can also be modelled. The theoretical dispersion equation based approach to layer-guided shear horizontal acoustic wave modes on finite substrates presented in this work provides a unified view of Love wave and shear horizontal acoustic plate mode (SH-APM) devices, which have been generally regarded as distinct in sensor research. It is argued that SH-APMs with guiding layers possessing shear acoustic speeds lower than that of the substrate and Love waves are two branches of solution of the same dispersion equation. The layer guided SH-APMs have a phase velocity higher than that of the substrate and the Love waves a phase velocity lower than that of the substrate. Higher-order Love wave modes are continuations of the layer-guided SH-APMs. The generalized concept of SH-APMs and Love waves provides a basis for understanding the change in sensitivity with higher-frequency operation and the relationship between multiple modes in Love wave sensors. It also explains why a relatively thick layer of a high-loss polymer can be used as a waveguide layer and so extends the range of materials that can be considered experimentally. Moreover, it is predicted that a new type of sensor, a layer-guided SH-APM sensor, can be constructed in a manner analogous to a Love wave device. The sensitivity of such a device is predicted to approach that of a Love wave sensor whilst retaining the advantage of the SH-APM of using the face opposite the one possessing the transducers as the sensing surface

Monitoring cell adhesion by piezo-resonators: impact of increasing oscillation amplitudes.

In recent years, the quartz crystal microbalance (QCM) has been established as a sensitive analytical tool to monitor the attachment and spreading of mammalian cells to in vitro surfaces. Due to its superior time resolution, the device is capable of reading even subtle differences in cell adhesion kinetics. However, thickness shear mode piezoresonators, which are the core component of the QCM approach, can be used not only as a sensor but also as an actuator when the oscillation amplitude of the crystal is increased so that molecular recognition at the solid-liquid interface is disturbed. In this study, we have addressed the impact of elevated lateral oscillation amplitudes on the adhesion kinetics of three mammalian cell lines. We used AT-cut piezoresonators with a fundamental resonance frequency of 5 MHz, and the analytical readout was performed by impedance analysis. Formation of stable cell-substrate contacts is retarded or entirely blocked when the lateral oscillation amplitude (in the center of the resonator) exceeds values higher than 20 nm. Shear oscillations of similar amplitude were, however, not sufficient to displace attached cells from the surface. Moreover, the experimental data prove that the normal QCM readout with oscillation amplitudes smaller than 1 nm is, indeed, non-invasive with respect to mammalian cells

Determination of the Physical Properties of Room Temperature Ionic Liquids Using a Love Wave Device

In this work, we have shown that a 100 MHz Love wave device can be used to determine whether room temperature ionic liquids (RTILs) are Newtonian fluids and have developed a technique that allows the determination of the density–viscosity product, ρη, of a Newtonian RTIL. In addition, a test for a Newtonian response was established by relating the phase change to insertion loss change. Five concentrations of a water-miscible RTIL and seven pure RTILs were measured. The changes in phase and insertion loss were found to vary linearly with the square root of the density–viscosity product for values up to (ρη)1/2 10 kg m–2 s–1/2. The square root of the density–viscosity product was deduced from the changes in either phase or insertion loss using glycerol as a calibration liquid. In both cases, the deduced values of ρη agree well with those measured using viscosity and density meters. Miniaturization of the device, beyond that achievable with the lower-frequency quartz crystal microbalance approach, to measure smaller volumes is possible. The ability to fabricate Love wave and other surface acoustic wave sensors using planar metallization technologies gives potential for future integration into lab-on-a-chip analytical systems for characterizing ionic liquids