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Sounding out chemical hot spots.

Sounding out chemical hot spots

Irradiating a liquid hydrocarbon with high-frequency, high-intensity sound waves can produce an effect much like the burning of fuels, say researchers at the University of Illinois at Urbana-Champaign. Their experiments show that ultrasonic irradiation of a cold liquid creates microscopic hot spots that emit light similar to that given off by high-temperature flames.

"Light emission under ultrasonic irradiation of water has been known for 30 or 40 years,' says chemist Kenneth S. Suslick. "But to our surprise, no one had examined the nature of the emissions from nonaqueous liquids.' Using sound waves inaudible to the human ear but considerably more intense than the sound generated by a jet engine, Suslick and colleague Edward B. Flint looked for evidence of "sonoluminescence' in liquids such as dodecane, tetrachloroethylene and nitroethane.

Ultrasonic irradiation of a liquid creates a swarm of tiny gas bubbles that grow, then suddenly collapse. This effect is known as acoustic cavitation (SN: 12/13/86, p.372). The rapid collapse produces intense local heating. As a result, vapor enclosed within imploding bubbles can reach temperatures as high as 5,000|C. At such temperatures, says Suslick, "we ought to see chemistry similar to flame chemistry.'

In fact, hydrocarbon vapor molecules are torn apart into highly reactive, chemically excited molecular fragments, just as they are in a flame. These excited fragments emit light that matches the blue color seen when hydrocarbons burn. If oxygen were present, the final products would be carbon dioxide and water. In the absence of oxygen, the long hydrocarbon chains originally present are cracked to form smaller hydrocarbons.

"This work is just the beginning of our sonoluminescence studies,' says Suslick. "We're interested in using sonoluminescence as a spectroscopic probe of what happens inside hot spots.' One possibility that the researchers would like to explore is the way emissions change when various substances are dissolved in the liquids.

Suslick's investigation of sonoluminescence complements other research he and his colleagues are doing on the effect of ultrasound on catalysis and the rates of chemical reactions (SN: 6/20/87, p.388). "Sonochemistry is a fundamental and different way of interacting energy with matter,' says Suslick. The high peak temperatures over short time periods produced by ultrasound provide reaction conditions and chemical information sometimes not obtainable in other ways.
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Title Annotation:sonoluminescence - ultrasonic irradiation of a cold liquid creates microscopic hot spots that emit light
Author:Peterson, Ivars
Publication:Science News
Date:Oct 10, 1987
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