Surface acoustic wave, defined

A chemical microsensor can be defined as an extremely small device that detects components in gases or Hquids (52—55). Ideally, such a sensor generates a response which either varies with the nature or concentration of the material or is reversible for repeated cycles of exposure. Of the many types of microsensors that have been described (56), three are the most prominent the chemiresistor, the bulk-wave piezoelectric quartz crystal sensor, and the surface acoustic wave (saw) device (57). 

The reaction between the analjrte and the bioreceptor produces a physical or chemical output signal normally relayed to a transducer, which then generally converts it into an electrical signal, providing quantitative information of analytical interest. The transducers can be classified based on the technique utilized for measurement, being optical (absorption, luminescence, surface plasmon resonance), electrochemical, calorimetric, or mass sensitive measurements (microbalance, surface acoustic wave), etc. If the molecular recognition system and the physicochemical transducer are in direct spatial contact, the system can be defined as a biosensor [76]. A number of books have been published on this subject and they provide details concerning definitions, properties, and construction of these devices [77-82]. 

The power consumption for heating and cooling of samples can be reduced if the temperatures in the device are kept constant in three different regions and the sample is moved between these three regions. This movement can be operated at the surface of a thermostating chip, for example, by electrostatic manipulation or by surface acoustic waves (SAWs). Therefore fluidic transport paths (hydrophilic areas) are defined lithographically in a hydrophobic environment on a piezoactive substrate like lithium niobate. The excitation of SAWs is realized by microlithographically prepared thin-film electrodes. SAW systems are of particular interest... 

When the pressure amplitude of an acoustic wave in liquid or solid exceeds the ambient pressure (atmospheric pressure), the instantaneous pressure becomes negative during the rarefaction phase of an acoustic wave. Negative pressure is defined as the force acting on the surface of a liquid (or solid) element per surface area to expand the element [3,4]. For example, consider a closed cylinder filled with liquid... 

The analysis of propagating acoustic waves in an elastic medium allows its characterization by means of strain-stress relationships. The stress ay is defined as the ratio of an external force F parallel to a direction i (x,y or z) to a surface S perpendicular to the direction j. 

By means of method of visualization with the help of acoustic waves [1,2] we could get the microstructure images of steel samples on different depths from the surface. The analysis of acoustic images gave the possibility to calculate the dimensions of grains, to observe their transformation in the period of time or under external influences. In accordance with the theory of Hall - Peach there were defined the strength characteristics, for example flow limit ( Go,2) of the materials under study. The significance obtained o0 2 is in proper correspondence with values that are table one for the type of steel under consideration. 

TSM resonator, also known as quartz crystal microbalance (QCM), is the simplest and most widespread acoustic wave device today. TSM typically composes of a quartz plate sandwiched by electrodes on opposite faces. Electric field crosses through this plate when voltage is applied to the electrodes, resulting in a shear mechanical strain or displacement in the quartz. By oscillating the voltage frequency, a mechanical resonance can be generated, where the maximum displacement of crystal occurs at the surfaces. The resonant frequency, F, and the quality factor, Q, are the two resonance parameters. While Ej. is the mechanical thickness shear resonance as mentioned before, it is also defined as the frequency of the maximum value of the electrical conductance, Gei- Q is approximated mathematically from the electrical conductance resonance spectrum as 2 = Er/AEnw- or the ratio of resonant frequency to the half bandwidth [5]. 

Wang and Lee [1] define the so-called Langevin and Rayleigh radiation pressures, respectively, as the mean excess pressures that either depend upon the sound wave only (i. e., with C = 0), or on the sound wave together with a constraint which determines the constant C that contributes to the pressure. The concept of the radiation pressure enables the calculation of forces acting upon material surfaces, such as an interface between two fluids or the surface of a particle or a drop in a sound field. Strictly speaking, one should use the acoustic radiation stress tensor n to calculate such forces. However, in many situations, such as when the surface is rigid or when the velocity at a surface is normal to that surface, it is convenient to use the radiation pressure rather than the full stress. 

The specific acoustic impedance, Z, is the resistance of a medium to the propagation of a sound wave. It can be defined as the ratio of acoustic pressure to the so-called particle velocity at a single frequency (McClements, 1997). At an interface, the proportion of wave energy transmitted or reflected depends on the difference in impedance between the two media. Consequently, this difference determines the coupling between emitting surface and the treated medium. If the impedance difference is large, the proportion of energy reflected will be important and the ultrasound effects will be mainly localized at the interface. However, if the... 

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