Cavitation particulate matter

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. 

It has also been found that the presence of particulate matter, and more especially the occurrence of trapped vapour-gas nuclei in the crevices and recesses of these particles, also lowers the cavitation threshold. The way in which nucleation occurs at these sites (and from similar sites on the vessel walls) is shown in Fig. 2.10. 

The presence of particulate matter — especially that of trapped vapour gas nuclei in their crevices and recesses — has also been found to lower the cavitation threshold. This is required under these conditions are of paramount importance in the clinical field, where ultrasound Is widely used in applications such as high-intensity focused ultrasound (HIFU),... 

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