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Cavitaion Seeds ■

"hot spot"

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Figure 3. Bubble growth and implosion in a liquid irradiated with ultrasound is the physical phenomenon responsible for most sonochemistry. Intense ultrasound waves generate large alternating stresses within a liquid by creating regions of positive pressure (dark color) and negative pressure (light color). A cavity can form and grow during the episodes of negative pressure. When the cavity attains a critical size, the cavity implodes, generating intense heat and tremendous pressure. Reprinted with permission from [16], K. S. Suslick, MRS Bull. 20, 29 (1995). © 1995, Materials Research Society.

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rapid than thermal transport, creating a short-lived localized hot spot. There is a nearly universal consensus that this hot spot is the source of homogeneous sonochemistry. Rayleigh's [10] early descriptions of a mathematical model for the collapse of activities in incompressible liquids predicted enormous local temperatures and pressures. Ten years later, Richards and Loomis [11] reported the first chemical effects of ultrasound. Cavitation was heavily studied during the 1950s and 1960s, culminating in a relatively detailed understanding [12-13].

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