Threshold ultrasonic intensity

Sehgal, C. M. and Wang, S. Y. J. 1981. Threshold intensities and kinetics of sonoreac-tion of thymine in aqueous solutions at low ultrasonic intensities. J. Am. Chem. Soc. 103 6606-6611. 

The ultrasonic intensity defines the acoustic pressure amplitude, which determines the threshold necessary to produce cavitation. As the acoustic pressure amplitude is increased, both the number of bubbles and their maximum dze increases, resulting in an increased overall cavitation activity. Okuyama (69) demonstrated that the intenaty of collapse of a cavitation bubble does not strongly depend on the ultrasonic intensity. The main effect of increasing ultrasonic intensity is that a larger number of cavitation bubbles are formed. Therefore, to alter cavitation intensity... 

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). 

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 formation of free radial OH and H in a naturally air-saturated aqueous solution exposed to traveling ultrasonic wave of 820 kHz was investigated using a spin-trapping agent, 5,5-dimethyl-l-pyrroline-l-oxide (DMPO) and ESR techniques [75]. It was shown that the cavitation threshold occurred at 0.537-0.632 W cm-2, and no further increase was observed above 3 W cm-2. At a fixed sound intensity the yield of OH increased linearly with the sonication time. 

The chemical effects of820-kHz diagnostic ultrasonic cavitation was studied and it was reported that the OH radical produced under cavitation in water could react with non-fluorescent terephthalic acid (TA) to produce fluorescent hydroxy tere-phthalate (HTA) [76-78]. Thus cavitation induced by ultrasound could be detected through measuring the HTA fluorescence value produced. When the sound intensity was above the threshold value of ultrasonic cavitation, the yield of HTA increased rapidly with the sound intensity and the yield increased approximately linearly with exposure time. When the sound intensity reached a certain value, the yield of HTA reached saturation. The relationship between the yield of HTA and sonication period was described and explained by an equation given by the authors. 

As an example in processing, Kardashev has reported some very interesting ideas aimed at reducing the time required for crystallization 56 The best results were obtained by cycling the temperature and shifting the ultrasonic power intensity above and below the cavitation threshold. By this means, the duration of the crystallization process could be reduced by a factor of 3 to 4. 

For reference meaairements, the sound intensity/2 is taken dose to the threshold of audibility of the human ear, which is proximately 10 watts/cm. The average intensity of ultrasonic waves used in laboratory investigations of liquids, which is in the order of 10 watts/cm, is therefore many orders of magnitude greater than the sound intensity of the most powerful loudspeakers. As an example let us... 

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