Sensing shear mode

As an example of the use of array methodology to study chemical sensor properties let us consider the thirteen molecular structures reported in Figure 5. To investigate the sensing properties of these molecules we studied the behaviour of the response of thickness shear mode resonators (TSMR) sensors, each coated with a molecular film, to different concentration of various volatile compounds (VOC). Analyte compounds were chosen in order to have different expected interaction mechanisms. 

Devices based on piezoelectric crystals, which allow transduction between electrical and acoustic energies, have been constructed in a number of conrigurations for sensor applications and materials characterization. This cluqtter examines those devices most commonly utilized for sensing a( licatithickness-shear mode (TSM) resonator, the surface acoustic wave (SAW) device, the acoustic plate mode (APM) device, and the flexural plate wave (FPW) device. Each of these devices, shown schematically in Figure 3.1, uses a unique acoustic mode. 

Figure 43. Schematic diagram of a quartz crystal augmented with electrodes and sensing layers, illustrating the way in which the species to be detected is absorbed into the sensing layer (A) and the ideal thickness shear mode of a quartz plate (B) [230], [203]...

Fig. 13.1 Ditferent types of acoustic devices that can be used for sensing application FBAR film bulk acoustic resonators, TSM thickness shear mode, SMR solidly mounted resonators. Other designations are in the text...

In Section 2.5.1, the various beam actuation concepts are presented using the classification illustrated in Figure 2.10. Analogously applying to sensing, we distinguish between normal (lengthwise) and shear mode actuation in the plane of the wall as well as between its consistent and sectorial application across the plane of the cross-section. 

Nanofiber films are used as a sensing interface for thickness shear mode (TSM) piezoelectric sensors. TSM sensors coated with nanofiber films made of poly-lactic acid-co-glycolic acid (PLAGA) polymers were studied under various ambient conditions and were reported to possess better sensitivities than their thin film counterparts [2]. For TSM resonators, the resolution varies linearly with the surface area of the sensing interface. Hence, polymer nanofiber would be an ideal material for this purpose. 

Some of the most important early experimental observations were of transitions from the quasi-brittle crazing deformation mode to the ductile shear deformation mechanisms with changes in the experimental conditions, such as temperature and strain rate, as well as in polymer variables, such as polymer backbone architecture, blend composition, crosslinking and physical aging state of the polymer glass. One of the strengths of the model of craze growth outlined above is that it allows one to make sense out of some experimentally observed craze-to-shear transitions that had previously defied explanation . The idea behind this explanation is quite simple One writes an expression for the shear yield stress, viz ... 

In this book we shall begin with an examination of hydrodynamic lubrication. If there is a mode of real lubrication behavior which approaches the ideal, it is simple hydrodynamic lubrication. There is no contact of the solid surfaces and therefore no wear in the generally accepted sense the friction is that arising from the viscous shear of the lubricating fluid. The part that the viscosity and the density of the lubricating fluid plays in hydrodynamic lubrication leads us from engineering physics to the physical chemistry of liquids. 

Martin F., Newton M. L, McHale G., Melzak K. A., and Gizeli E., Pulse mode shear horizontal-surface acoustic wave (SH-SAW) system for liquid based sensing applications, Biosensors and Bioelectronics, 19, 627-632, 2004. 

The response of piezoelectric devices propagating shear horizontal acoustic plate modes (SH-APMs) has been modeled and experimentally characterized for variations in surface mass, liquid rheological properties, and solution dielectric coefficient and electrical conductivity. The nature of the SH-APM and its propagation characteristics are outlined and used to describe a range of Interactions at the solid/liquid interface. Sensitivity to sub-monolayer mass changes is demonstrated and a Cu sensor is described. The APM device is compared to the surface acoustic wave device and the quartz crystal microbalance for liquid sensing applications. 

Chemical sensors based on acoustic wave (AW) devices have been studied for a number of sensing applications, the majority of which fall in the category of gas and vapor detection (1-8). Recently, the use of these sensors in liquid environments has been explored (9-13). AW sensors utilize various types of acoustic waves, including the surface acoustic wave (SAW), the shear-horizontal acoustic plate mode (SH-APM) (10-13), and the Lamb wave (also a plate mode) (3.14). Even though most studies of these piezoelectric sensors have centered on SAW devices (1.2.4-8), differences in the propagation characteristics of the various acoustic modes make some better suited than others for a given sensing application. 

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