Coupling fluid attenuation

The echo from the surface of the lens can never be entirely eliminated, and because the reflection from the specimen must pass through the coupling fluid it will suffer attenuation that may well make it smaller than the lens surface... 

The coefficients k are given in Table 3.3 with T in °C. The polynomial expansion is useful in analysis programs to calculate the velocity from the temperature. The attenuation in water decreases with temperature, but the rate of decrease is less at higher temperatures. The attenuation and velocity are plotted against temperature in Fig. 3.1. For practical operation of an acoustic microscope with water as the coupling fluid, the smallest lens radius for routine operation is 40 pm (focal length 46pm). With water at 60°C a lens... 

The starting point for the design of lens for surface imaging is the choice of the frequency for which it is to be used. This determines the attenuation per unit distance in the coupling fluid (Table 3.1). In water the attenuation is about... 

In acoustic microscopy, viscous attenuation is most significant in the coupling fluid. In water (and all the other fluids in Table 3.1 except CS2 and superfluid 4He) at the frequencies used in acoustic microscopy, the relaxation time is much less than the period of the wave. Thus cot [Pg.77]

In most high-resolution lenses, Ft % 1 so that z0, and hence —zmin and zmax, are of the order of Ao. With the further notation that the wavenumber and attenuation in the coupling fluid are k and ( o, respectively, and that the response of the transducer to a uniform field of unit amplitude would be V0> the response to the geometrically reflected field is... 

The inversion procedure is most straightforward when attenuation in the coupling fluid is ignored. This may present problems in high-frequency applications. 

When the first edition was published in 1992, the resolution of the acoustic microscope techniques used at the time was controlled by the wavelength. In practice the frequency-dependent attenuation of the acoustic wave in the coupling fluid sets a lower limit to the wavelength, and therefore to the resolution, of about 1 pm for routine applications. Since then scanning probe techniques with nanometre scale resolution have been developed along the lines of the atomic force microscope. This has resulted in the development of the ultrasonic force microscopy techniques, in which the sample is excited by... 

Viscous coupling of plate modes to an adjacent fluid results in both attenuation of the plate mode and a change in propagation velocity. These can be estimated from a perturbation analysis and are given by [54]... 

To obtain the absolute sound attenuation in the coal slurry, the diffraction loss, the acoustic mismatch loss, the attenuation due to the Teflon window, and the oil coupling must be calculated. Thus, it is difficult to accurately determine the absolute attenuation. In practice, one measures the relative attenuation with respect to a standard. The attenuation of ultrasonic waves in a solid suspension is attributed to three major factors, namely, scattering, viscosity, and thermal effects. Although the presence of particles affects the fluid viscosity and thermal conductivity, the primary source of attenuation may be due to particle scattering. Hence, one may define the relative attenuation of the HYGAS coal slurry by comparing the slurry attenuation with that of the carrier fluid, i.e., the toluene/benzene mixture. This can be expressed by the equation... 

The fourth and last solution is a pressure-porosity wave in which fluid flow at the incompressible limit of fluid motions is coupled to elastic deformations of the matrix. The attenuation of this wave is highly frequency dependent, but it is quite conservative at low frequencies. Unlike the previous three solutions which each exist as an independent process, this solution is always coupled to porosity diffusion. It can leave behind an increase in pressure as it propagates, converting some of the inertial energy associated with the... 

One area where SQELS can make a significant contribution is that of polymers at liquid/liquid interfaces. Since the viscosity and density of the two fluids are much closer the coupling between dilation and capillary waves is severely attenuated and the prospects for obtaining dilational modulus and viscosity are much reduced or nonexistent. Oil/water interfaces are particularly interesting when block copolymers are placed at the interface. Only one such system has so far been discussed a block copolymer of styrene and ethylene oxide was placed at the interface between heptane and water (Sauer et al. 1987). There appears to be some evidence of a maximum in the damping of the interfacial capillary waves as the concentration of copolymer was increased. 

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