Ultrasound attenuation/velocity

Part II of the book deals with lesser known aspects of US for the analytical chemists such as its use as an energy source for detection purposes. Thus, ultrasound-based detection techniques viz. US spectrometry in its various modes including ultrasound attenuation, ultrasonic velocity, resonant ultrasound, laser-generated, ultrasound reflection and acoustic wave impedance spectroscopies) are dealt with in Chapter 9. Finally, Chapter 10 is devoted to seleoted applioations of US spectrometry — mostly non-analytical applications from whioh, however, analytical chemists can derive new, interesting analytical uses for ultrasound-based deteotion techniques. 

Applications of ultrasonic techniques to solid-gas systems rely on the fact that velocity and attenuation of US-waves in porous materials is closely related to pore size, porosity, tortuosity, permeability and flux resistivity. Thus, the flux resistivity of acoustic absorbents oan be related to US attenuation [118,119], while the velocity of slow longitudinal US is related to pore tortuosity and diffusion, and transport properties, of other porous materials [120]. Ultrasound attenuation is very sensitive to the presence of an external agent suoh as moisture in the pore space [121] and has been used to monitor wetting and drying prooesses [122] on the other hand, US velocity has been used to measure the elastic coefficients of different types of paper and correlate them with properties such as tensile breaking strength, compressive strength, etc. [123]. 

Active ultrasound uses a source of sound radiation, which is appHed to a process sample, with a detector placed such that modification to the signal can be detected and related to changes in the sample. Signal attenuation, velocity measurements and wavelength selective absorption provide the means of probing the sample. This approach promises to provide both chemical and physical information but as yet has not been used extensively. A number of on-line polymer-related studies have been reported in which polymer flow behaviour, viscosity, blend characterisation, and foaming-process monitoring have been examined [19]. 

In an early study, Greenleaf et al. [4] reported reconstructions of ultrasonic velocity from time-of-flight profiles. Since then there has been periodic activity in using ultrasound to determine the transmission properties attenuation or refractive index. 

P. V. Nelson, M. J. W. Povey, Y. Wang 2001, (An ultrasound velocity and attenuation scanner for viewing the temporal evolution of a dispersed phase in fluids), Rev. Sci. Instrum. 72, 4234. 

Ultrasound is used to obtain information about the properties of a material by measuring the interaction between a high frequency sound wave and the material through which it propagates. This interaction depends on the frequency and nature of the ultrasonic wave, as well as the composition and microstructure of the material. The parameters most commonly measured in an ultrasonic experiment are the velocity at which the wave travels and the extent by which it is attenuated. To understand how these parameters are related to the properties of foods it is useful to consider the propagation of ultrasonic waves in materials in general. 

In practice ultrasound is usually propagated through materials in the form of pulses rather than continuous sinusoidal waves. Pulses contain a spectrum of frequencies, and so if they are used to test materials that have frequency dependent properties the measured velocity and attenuation coefficient will be average values. This problem can be overcome by using Fourier Transform analysis of pulses to determine the frequency dependence of the ultrasonic properties. 

Side wall reflections. If the angle of diffraction of an ultrasonic wave leaving a transducer is large enough, reflections may occur from the side walls of the cell. This reflected ultrasound will interact with the ultrasound which has traveled directly through the sample and affects both velocity and attenuation measurements. It is therefore important to calculate the diffraction angle of the transducer and ensure that the side walls are far enough apart so that side-wall reflections do not interfere with the measurements [1]. 

Homogeneous liquids do not scatter ultrasound because they contain no discontinuities, and so the attenuation is due solely to absolution processes. If the attenuation coefficient is relatively low (a oVc) the velocity is given by the following equation ... 

In non-ideal mixtures, or systems where scattering of ultrasound is significant, the above equations are no longer applicable. In these systems the ultrasonic properties depend on the microstructure of the system, and the interactions between the various components, as well as the concentration. Mathematical descriptions of ultrasonic propagation in emulsions and suspensions have been derived which take into account the scattering of ultrasound by particles [20-21]. These theories relate the velocity and attenuation to the size (r), shape (x) and concentration (0) of the particles, as well as the ultrasonic frequency (co) and thermophysical properties of the component phases (TP). 

It is very interesting that large molecules, such as proteins, behave as particles and can be described by ultrasound scattering theory in the very long wavelength limit. Scattering theory is vindicated by the precise and repeatable nature of the data available for these molecules. In particular, it should be pointed out that the molecular adiabatic compressibility is insensitive to individual bonds and is the sum of the intrinsic compressibility of the primary structure (the amino acid sequence), cavities in the tertiary structure and interaction with the solvent (Kharakoz and Sarvazyan, 1993). Velocity and attenuation spectroscopy relate to different aspects of the molecule... 

It is far easier to measure sound velocity and attenuation in fluids than solids. The reason for this is that sample presentation is much simpler in the case of fluids because they flow around the transducers, which may be held static or even moved through the sample. This is not generally the case with solids. For an example of the in-line application of ultrasound to the measurement of solid fat content in margarine during manufacture, see Figure 21.2. 

Holmes, A.K., Challis, R.E., Wedlock, D.J. 1993. A wide bandwidth study of ultrasound velocity and attenuation in suspensions comparison of theory and experimental measurements. J. Coll. Interface Sci. 156, 261-268. 

Ultrasound reflective spectroscopy application in the analysis of bubbly liquids has been known for many years. This technique is based on the transmission of ultrasound waves through a dispersion, and measuring the velocity and attenuation spectra. 

Homogeneous liquids do not scatter ultrasound because they do not contain any discontinuities. Attenuation in these systems is solely due to absorption caused by thermodynamic relaxation processes. In a pure homogeneous liquid, which is not highly attenuating ( .e. a adiabatic compressibility and the density by the equation... 

Ultrasound resonance spectroscopic measurements are commonly obtained by applying a broad-pulse and selecting the frequency at which the sample enters into resonance at the frequency concerned. Under these conditions, the velocity, attenuation, impedance and other characteristics of US can be measured and subsequently processed as required. 

As with any analytical technique, it is important for US spectrometry users to have a thorough understanding of its underlying physical principles and of potential sources of errors adversely affecting measurements. The basis of ultrasonic analyses in a number of fields (particularly in food analysis) is the relationship between the measurable ultrasonic properties (velocity, attenuation and impedance, mainly) and the physicochemical properties of the sample (e.g. composition, structure, physical state). Such a relationship can be established empirically from a calibration curve that relates the property of interest to the measured ultrasonic property, or theoretically from equations describing the propagation of ultrasound through materials. 

Ultrasound-based detection is an excellent choice for food industries, where its use is in continuous expansion. Most studies in this area have used ultrasonic velocity to extract information about products, probably because it is the simplest, most reliable type of ultrasonic measurement. Thus, it has been used to determine the chemical structure (including chain length and degree of unsaturation) of various oils [94], oil composition and adulteration [95] the correlation of the proportion of polar compounds to ultrasonic velocity and attenuation in fried olive oil [96]. 

In most of the cases, an ultrasonic wave propagates adiabatically, so the (20) looks more naturally its right-hand side represents the adiabatic (non-relaxed) modulus and non-adiabatic contribution to the dynamic modulus. Recall that the relaxed (or isothermal) modulus should be regarded as quasi-static one. Figure 1 shows the frequency-dependent factor of non-adiabatic contribution as function of cox. One can see that transformation from isothermal-like to adiabatic-like propagation occurs in the vicinity cox = 1. The velocity of ultrasound is increased in this region, while the attenuation reaches its maximum value. 

Now we will overview some experiments that reveal the specificities of the Jahn-Teller effect in diluted crystals. First of all, we will discuss a justification of their relaxation origin. We have mentioned before that the first experiments were done on the crystals of aluminum oxide (corundum), yttrium aluminum garnet, yttrium iron garnet, and lithium gallium spinel doped with a number of 3d ions [10,11]. The main result was the discovery of attenuation maximum which was considered to be observed at cot 1 and reconstruction of the relaxation time temperature dependence. In some experiments reported later both the velocity and attenuation of ultrasound were measured as functions of the temperature. They were done on ZnSe and ZnTe crystals doped with transition metals. These crystals have the zinc-blende structure with the Jahn-Teller ion in tetrahedral coordination. The following... 

Fig. 5 Temperature dependences of velocity [vi (T) - vi(4.2)/vt (4.2)] open circles) and attenuation of ultrasound (filled circles) with respect to the level at T = 4.2 K obtained in ZnSe Ct + at 54.4 MHz. Concentration of the impurity cr = 10 cm l Longitudinal wave, ultrasound passage I = 0.717 cm, propagation direction [110]. After Fig. 1 in [17]...

Acoustic Methods Ultrasonic attenuation spectroscopy is a method well suited to measuring the PSD of colloids, dispersions, slurries, and emulsions (Fig. 21-15). The basic concept is to measure the frequency-dependent attenuation or velocity of the ultrasound as it passes through the sample. The attenuation includes... 

The degree of emulsification in such materials can also be estimated by the measurement of ultrasound velocity in conjunction with attenuation [4]. It is possible to determine factors such as the degree of creaming (or settling ) of a sample, i.e. the movement of solid particles/fat droplets to the surface (or to the base) [5], Such information gives details, for example, of the long-term stability of fruit juices and the stability of emulsions such as mayonnaise. The combination of velocity and attenuation measurements shows promise as a method for the analysis of edible fats and oils [6], and for the determination of the extent of crystallization and melting in dispersed emulsion droplets [7]. 

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