Effects Of Ultrasound 21 Physical Effects of Ultrasound

Intensive sonication of liquids can generate cavitation bubbles. The bubbles collapse violently, and in doing so, generate high temperatures and pressures within the gaseous contents. There are three different theories about cavitation—the hot spot, the electrical, and the plasma theory. The most popular one is the hot-spot theory. Thus, it has been experimentally shown that the cavitational collapse creates drastic conditions inside the medium for an extremely short time generating temperatures of 20005000 K and pressures up to 1800 atm inside the collapsing cavity. This can lead to a reactive chemical environment, as well as the often-observed pulse of light near the end of the collapse phase of the bubble oscillation. As a result, emission of light often accompanies sonochemistry. Such sonolumi-nescence provides an extremely useful spectroscopic probe of the conditions created during cavitation bubble collapse [1-3] (Fig. 1). As with sonochemistry, sonoluminescence is a consequence of acoustic cavitation. The collapse of bubbles caused by cavitation produces intense local heating and high pressures, with very short lifetimes (Fig. 2). The collapse of bubbles in a multibubble cavitation field produces hot spots with effective temperatures of ca. 1000 atm, and heating and cooling rates above 10 K • s-1. In a single-bubble cavitation, conditions may be even more extreme [4, 5]. Thus, cavitation can create extraordinary physical and chemical conditions in otherwise cold liquids. Research studies have concentrated on either an isolated single bubble or clouds of many bubbles. The acoustic cavitation bubble is likely to be near spherical in single-bubble sonoluminescence (SBSL), with almost an indefinite number of oscillations being observed, while in multibubble sonoluminescence (MBSL), it will more likely be nonspherical, with only a small number of oscillations before it breaks up. High temperatures of 104-105 K

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Figure 1. Light resembling that from a gas flame is generated when cold hydrocarbon liquids are exposed to ultrasound; the phenomenon is known as sonoluminescence. The graphs show the spectrum produced by the sonoluminescence of dodecane, C12H24 (top), and the combustion of methane, CH4 (bottom). The similarities between the spectra are due to the formation and emissions of diatomic carbon in both cases. Reprinted with permission from [3], K. S. Suslick, Sci. Am. 82 (1989). © 1989, Andrew Christic, Slim Films.

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Wavelength (nm)

Figure 1. Light resembling that from a gas flame is generated when cold hydrocarbon liquids are exposed to ultrasound; the phenomenon is known as sonoluminescence. The graphs show the spectrum produced by the sonoluminescence of dodecane, C12H24 (top), and the combustion of methane, CH4 (bottom). The similarities between the spectra are due to the formation and emissions of diatomic carbon in both cases. Reprinted with permission from [3], K. S. Suslick, Sci. Am. 82 (1989). © 1989, Andrew Christic, Slim Films.

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