Sonochemistry nucleation

Increases in the appHed static pressure increase the acoustic intensity necessary for cavitation, but if equal number of cavitation events occur, the coUapse should be more intense. In contrast, as the ambient pressure is reduced, eventuaUy the gas-fiUed crevices of particulate matter which serve as nucleation sites for the formation of cavitation in even "pure" Hquids, wiU be deactivated, and therefore the observed sonochemistry wiU be diminished. 

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. 

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. 

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