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An extensive survey is made of the chemical effects of ultrasonic waves. Increased understanding of the mechanism of these effects is sought through a consideration of the significance of certain experimental variables such as frequency, intensity, pressure, and temperature. Also considered is the role of cavitation. NTENSE ultrasonic waves produce unusual chemical, physical, and biological phenomena. Although a fully satisfying explanation of the mechanisms involved is not yet available (even after 25 years of study by laboratories in many countries), much relevant information has been obtained. CAVITATION Practically all of the observed chemical effects of ultrasonics in liquid systems have been attributed to "cavitation," which is the formation and violent collapse of small bubbles or cavities in the liquid as a result of pressure changes2 -7 In the usual case of water saturated with air, the negative pressure portion of the sound wave causes some of the air to come out of solution as minute bubbles, which act as weak spots for the further tearing apart of the liquid to form larger cavities. Then when the pressure increases, as in the other half of the sound wave cycle, the cavities collapse with a violent hammering action which may generate local pressures of thousands of atmospheres and local temperatures of several hundred degrees. 8 Electrical discharges also occur, as the result of the electric potential built up between opposite walls of the cavity. 9-•4 The ultrasonic intensity re suired to produce cavitation in.ordinary distilled water is about 0.03 watt/cm 2, equivalent to a pressure variation of =t=0.3 atmosphere25 * This article is based on a paper presented at

Molecular fragmentation of organic liquids was produced by cavitation due to ultrasound waves, even in the absence of water. The sonolysis of acetonitrile under argon yielded N(2), CH(4), and H(2); but under oxygen the products were N(2), CO, CO(2), and H(2)O. Pure nonaqueous carbon tetrachloride also underwent sonolytic decomposition under either argon or oxygen, with the production of elemental chlorine.

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Contrary to current belief, cavitation has been found responsible for the depolymerizing effect of intense ultrasonic waves. This was demonstrated by irradiating two portions of a 1 percent polystyrene solution in toluene under conditions identical except for the following. The first portion was given no special prior treatment, showed many cavitation bubbles during irradiation, and decreased in molecular weight (as measured by the intrinsic viscosity) to about one-tenth of the initial value. The second portion was given a preliminary treatment of degassing by boiling under vacuum, showed no cavitation bubbles during the irradiation, and underwent no appreciable change in molecular weight. Similar experiments with solutions of hydroxyethyl cellulose in water showed that, in this case also, cavitation is necessary for depolymerization. The opposite conclusion of earlier investigators is attributed to their inadequate method for eliminating cavitation. Oxidants known to be produced by ultrasonic waves in solutions containing dissolved oxygen or nitrogen cannot be responsible for the degradation, because substantially the same amount of depolymerization occurs even when helium is the only gas present.

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