Cavitation threshold applied

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). 

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. 

The last piece for the model is the bubble-size distribution function and the limits for the rest radii of bubbles in the sound field. The cavitation thresholds as a function of applied sound pressure indicate the upper and lower size limits for bubbles in a cavitating sound field. A simplifying point of view would differentiate between a) transient bubbles, b) stable bubbles and c) dissolving bubbles. 

The pressure applied to the sonicated medium is directly involved in the resonance between the bubble vibration and the acoustic field, and modifies the cavitation threshold (Gh. 6, p. 251). By increasing this pressure, cavitation becomes more difficult, but the energy released by the collapse is greater. This effect was observed in a pressure range of 0.7-3 bar for the oxidation of iodide ions in water solution (the Weissler reaction, Ch. 8, p. 313) under an oxygen atmosphere, and for the oxidation of indene in a biphasic system. ... 

Cavitation begins at much smaller intensities when low sound frequencies are applied. Fig. 5 describes how the threshold intensity increases with increasing frequency. Drawing a vertical line at approximately 20 kHz, as one moves up this vertical line, wave intensity increases [W/cm2]. The first thing one encounters as the intensity is increased is the curve for aerated water, or water saturated with air. The intensity at this point is sufficient to produce cavitation as desorbed air contributes to bubble nucleation. As one continues to increase intensity, one will encounter the curve for degassed cavitation. This intensity is the absolute maximum intensity allowed (at standard conditions) for sound traveling in water at this frequency. Most of sonochemistry are performed at intensity levels between these two values. 

As we will see later, the Rayleigh-Plesset description closely matches the actual radial behavior of a bubble as long as the non-radial deformations are small (or of short duration). Since the behavior of a bubble depends on the applied acoustic pressure, Apfel estimated the threshold associated to transient cavitation. A part of this threshold is, of course, common to the Blake threshold (explosive growth of a cavitation nucleus. Fig. 14). 

During pressurization of a liquid, the Blake threshold pressure increases, which implies that higher acoustic pressures are needed to produce cavitations. Obviously, no cavitation occurs when the Blake threshold pressure exceeds the maximum acoustic pressure that can be applied with the currently available equipment. The vapor pressure of the liquid, however, can counteract the static... 

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