Fundamentals of Acoustic Cavitation and Sonochemistry

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

The observation that sonochemistry and luminescent activities are not necessarily coincident had never been reported and suggests that the conditions at the root of sonoluminescence and sonochemistry could be different in nature (or in intensity). Chemical species as tracers should serve to describe the liquid flows the knowledge of which is of prime importance in medical ultrasound (microstreaming, acoustic streaming and/or rectified acoustic streaming zy Even if synthetic exploitations are difficult to imagine, some fundamental aspects of sonochemistry and cavitation still to be explored are very probably involved, and in this respect deserve further development. 

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