Ultrasonic threshold

Automatic Methods with the help of appropriate algorithms permit the calculation of the threshold to segment ultrasonic images. It is this work, this method has been adopted. 

The ultrasonic images that we want to process are HF- type, (i.e., huilt from High Frequency signals. Fig. 4). Consequently, the noise is situated in the eentral part of the matrix. In order that we define two thresholds tl and t2. These last will be determined by using one of some measures quoted in the bibliography. 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

The first requirement for the level of ultrasonic power required to cause chemical effects in a reaction is that sufficient acoustic energy must be supplied to overcome the cavitation threshold of the medium. Once this has been exceeded then the region of... 

In Chapter 2 we explained why there existed a cavitation threshold i. e. a limit of sound intensity below which cavitation could not be produced in a liquid. We suggested that only when the applied acoustic amplitude (P ) of the ultrasonic wave was sufficiently large to overcome the cohesive forces within the liquid could the liquid be tom apart and produce cavitation bubbles. If degradation is due to cavitation then it is expected that degradation will only occur when the cavitation threshold is exceeded. This is confirmed by Weissler who investigated the degradation of hydroxycellulose and observed that the start of degradation coincided with the onset of cavitation (Fig. 5.21). 

The maximum ultrasonic amplitude must be chosen so that the average threshold amplitude occurs at roughly three-quarters of the reference period. 

UFM detection is obtained by measuring the cantilever deflection as the ultrasound amplitude is modulated (Fig. 13.3). The ultrasonic excitation from a longitudinal wave transducer fixed to the bottom of the sample causes normal vibration of its surface. As the ultrasonic amplitude is increased, contact is eventually broken at the pull-off point (aI = hi), giving a discontinuity in the time-averaged displacement. We refer to this ultrasonic amplitude as the threshold amplitude, and the corresponding inflection in the displacement... 

The shape of the force versus indentation curve depends on surface adhesive and elastic properties. Variations in these parameters affect the ultrasonically induced deflection. Conversely, the variations in the shape of the ultrasonically induced normal deflection contain information on surface adhesive and elastic properties. Figure 13.3 illustrates how the threshold amplitude should depend on the normal force value. If the normal force is set at a higher value F2 > Fi, then the threshold amplitude a2 = h2) needed to reach the pull-off point should be higher than the threshold amplitude (fli = hi) for Fi. If the threshold amplitude values (fli and a2) are measured for two different normal force values (Fi and F2), the contact stiffness is... 

It is possible to identify a threshold amplitude, defined as the amplitude at which the contact breaks, and pull-off occurs, for part of the ultrasonic cycle. It can be identified as the amplitude at which the inflection occurs in the normal deflection signal. 

As mentioned above, an important factor that controls the performance and especially the electrical properties of CNTs-reinforced composites is the state of dispersion of CNTs. Ultrasonication has been shown to be more effective in dispersing the nanotubes without the need for surfactants or other chemical treatments. Figure 12.5b presents electrical results of samples prepared by using a different composite processing. MWNTS were dispersed in this case in cyclohexane by ultrasonication and the MWNTs suspension was then mixed into a cyclohexane solution of SBR. Mixing was achieved by a further sonication for 30 minutes. Cyclohexane has been chosen in this case on account of the solubility of the rubbers in this solvent. As revealed in Figure 12.5b, the percolation threshold is shifted to a lower nanotube content and from this point of view, measurements of electrical resistivity appears as an indirect tool to evaluate the state of dispersion. 

The use of ultrasound to enhance performances of titania (or preferably Ti02-loaded zeolites and mesoporous materials) have been instead reviewed recently by Smirniotis et al. It was found that the presence of an ultrasonic field enhances the rate of photodegradation. A threshold of specific energy that should be provided in the reaction medium exists. This threshold is about 0.1 kW/1 and operation under this volume will ensure structural stability of these catalysts used. The use of ultrasound also avoids the formation of toxic intermediates observed when photocatalytic degradation of phenol alone is used. ... 

Fig. 1 illustrates the two mechanisms proposed for the processes of liquid disintegration and aerosol generation within ultrasonic nebulizers. The capillary-wave theory relates to the production of capillary waves in the bulk liquid. These waves constructively interfere to form peaks and a central geyser. When the amplitude of the applied energy is sufficiently high, the crests of the capillary waves break off, and droplets are formed. The rate of generation of capillary waves is dependent on both the physicochemical properties of the nebulized fluid and the intensity of the ultrasonic vibration. Mercer used Eq. (1) to calculate the threshold amplitude for the generation of capillary waves ... 

A very important point occurs in the transmission of acoustic power into a liquid which is termed the cavitation threshold. When very low power ultrasound is passed through a liquid and the power is gradually increased, a point is reached at which the intensity of sonication is sufficient to cause cavitation in the fluid. It is only at powers above the cavitation threshold that the majority of sonochemical effects occur because only then can the great energies associated with cavitational collapse be released into the fluid. In the medical profession, where the use of ultrasonic scanning techniques is widespread, keeping scanning intensities below the cavitation threshold is of vital importance. As soon as the irradiation power used in the medical scan rises above this critical value, cavitation is induced and, as a consequence, unwanted even possibly hazardous chemical reactions may occur in the body. Thus, for both chemical and medical reasons there is a considerable drive towards the determination of the exact point at which cavitation occurs in liquid media, particularly in aqueous systems. Historically, therefore, the determination of the cavitation threshold was one of the major drives in dosimetry. 

The presence of undissolved gas and of cavitation bubbles affects the transparency and refractive index of a liquid. Thus when a sonicated liquid is irradiated with light, X-rays, y-rays, or even high-frequency ultrasound, the attenuation and (or) refraction of the wave can be used to detect both the cavitation threshold and bubble density, and their variation with time. This is possible even within a very short period of the order of one ultrasonic cycle [138,139]. 

These methods have several advantages over the methods previously described including (a) the absence of distortions of the ultrasonic field which might be engendered by an invasive probe system (b) they can be used in a wide range of frequency and ultrasonic power, below or above the cavitation threshold and (c) they can even be used with solid materials by studying the reflected beam at the surface of the material [140]. 

McLean and Mortimer [187] have studied the variations in HO free radical production during the sonication of aqueous solutions at different powers at 970 kHz. A typical curve is given in Figure 36. From this it is clear that a threshold exists for radical production, after which there is a linear correlation with acoustic power up to a limiting value which probably corresponds with surface cavitation . Acoustic power was calibrated with a radiation balance and a PVDF hydrophone. Repeatability on experiments performed on the same day was less than 15%, but day-to-day variations could be as much as 50%, probably mainly due to small uncontrolled changes in the alignment of the reaction chamber (a test tube dipped in a water tank) with the ultrasonic source which was an acoustic horn. 

All the effects reflect the physical phenomenon of acoustic or ultrasonic cavitation. With this, to initiate the cavitation one must apply a certain threshold sound pressure designated as a cavitation threshold and determine the cavitation strength of a liquid. 

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