Acoustic cavitation bubbles nucleation

Generation Spontaneous generation of gas bubbles within a homogeneous liquid is theoreticaUy impossible (Bikerman, Foams Theoiy and Industrial Applications, Reinhold, New York, 1953, p. 10). The appearance of a bubble requires a gas nucleus as avoid in the liquid. The nucleus may be in the form of a small bubble or of a solid carrying adsorbed gas, examples of the latter being dust particles, boiling chips, and a solid wall. A void can result from cavitation, mechan-ic ly or acoustically induced. Blander and Katz [AlChE J., 21, 833 (1975)] have thoroughly reviewed bubble nucleation in liquids. 

How is a bubble created in acoustic cavitation There are three mechanisms in nucleation of a bubble in acoustic cavitation [14], One is the nucleation at the surface of solids such as a liquid container, motes or particles in liquid, if present. Nucleation takes place especially at crevices of motes, particles or a liquid container (Fig. 1.3). 

The third mechanism for nucleation is the fragmentation of active cavitation bubbles [16]. A shape unstable bubble is fragmented into several daughter bubbles which are new nuclei for cavitation bubbles. Shape instability of a bubble is mostly induced by an asymmetric acoustic environment such as the presence of a neighboring bubble, solid object, liquid surface, or a traveling ultrasound, or an asymmetric liquid container etc. [25-27] Under some condition, a bubble jets many tiny bubbles which are new nuclei [6, 28]. This mechanism is important after acoustic cavitation is fully started. 

Acoustic cavitation can be considered to involve at least three discrete stages nucleation, bubble growth, and, under proper conditions, implosive collapse. The dynamics of cavity growth and collapse are strikingly dependent on local environment we therefore will consider separately cavitation in a homogeneous liquid and cavitation near a liquid-solid interface. 

It is well known that some amounts of cavities or small bubbles are present in rubber during any type of mbber processing (Kasner and Meinecke, 1996). The formation of bubbles can be nucleated by precursor cavities of appropriate size (Gent and Tompkins, 1969). The proposed models (Isayev et al., 1996a,c,d Yashin and Isayev, 1999,2000) were based upon a mechanism of rubber network breakdown caused by cavitation, which is created by high intensity ultrasonic waves in the presence of pressure and heat. Driven by ultrasound, the cavities pulsate with amplitude depending mostly upon the ratio between ambient and ultrasonic pressures (acoustic cavitation). 

The tensile strength of a pure liquid is determined by the attractive intermolecular forces which maintain its liquid state the calculated tensile strength of water, for example, is in excess of -1000 atmospheres (7). In practice however, the measured threshold for initiation of cavitation is never more than a small fraction of that. Indeed, if the observed tensile strengths of liquids did approach their theoretical limits, the acoustic intensities required to initiate cavitation would be well beyond that generally available, and no sonochemistry would be observed in homogeneous media Cavitation is initiated at a nucleation site where the tensile strength is dramatically lowered, such as small gas bubbles and gas filled crevices in particulate matter, which are present in the liquid. 

Acoustic intensity has a dramatic influence on the observed rates of sonochemical reactions. Below a threshold value, the amplitude of the sound field is too small to induce nucleation or bubble growth. Above the cavitation threshold, increased intensity of irradiation (from an immersion horn, for example) will increase the effective volume of the zone of liquid which will cavitate, and thus, increase the observed sonochemical rate. 

The effect of the bulk solution temperature lies primarily m its influence on the bubble content before collapse. With increasing temperature, in general, sonochemical reaction rates are slower. This reflects the dramatic influence which solvent vapor pressure has on the cavitation event the greater the solvent vapor pressure found within a bubble prior to collapse, the less effective the collapse. Increases in the applied static pressure increase the acoustic intensity necessary for cavitation, but if equal numbers of cavitation events occui. the collapse should be nioie intense. In contiast, as die ambient pressure is reduced, eventually the gas-filled crevices of paniculate matter which serve as nucleation sites for the formation of cavitation in even pure liquids, will be deactivated, and therefore the observed sonochemistry will be diminished. 

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