CHAMPAIGN, Ill. -- When liquids are irradiated with ultrasound, miniature bubbles are formed and compressed. During bubble collapse, hot spots are created that have temperatures nearly as high as the surface of the sun. As reported in the Oct. 21 issue of Nature, chemists at the University of Illinois have been able to control and measure these temperatures for the first time. By looking at the spectra of light emitted from these hot spots, the scientists were able to determine the temperatures in exactly the way that astronomers measure the temperature of stars.
Generating light from high-intensity sound is called sonoluminescence. Professor Ken Suslick and his students Yuri Didenko and William B. McNamara III have examined sonoluminescence in solutions containing volatile metal compounds and observed the emission from excited metal atoms. Emission from metal atoms is responsible for the color of fireworks and the yellow color of most flames. The relative intensity of the light at different wavelengths is an excellent thermometer of the temperature of the hot metal atoms.
"It is amazing that in an otherwise cold liquid sitting on a desktop, we can create small hot spots with temperatures as high as a star surface, pressures as great as the ocean bottom, and lifetimes shorter than a lightning strike," said Suslick, the William H. and Janet Lycan Professor of Chemistry at the U. of I. "The hot spots are microscopic furnaces that can drive high-energy chemical reactions."
Because the temperatures inside the collapsing bubble are controlled by the bubble contents, scientists can run sonochemical reactions under whatever conditions they desire. Suslick has previously used high-intensity ultrasound in liquids to destroy unwanted chemical waste and to create materials with desirable properties, depending on what chemicals are being irradiated.
The process of bubble formation and collapse is called cavitation, and it provides a unique interaction between matter and energy. The process of cavitation also is responsible for submarine propeller noise, for erosion of turbines, and for the noise that boiling water makes on the stove.
"The field of sonochemistry has undergone a renaissance during the past few years, but still remains in its infancy," Suslick said. "As our understanding of the nature of the chemical effects of ultrasound has grown, so too has the impact of sonochemistry on the chemical community and on a wide range of the physical sciences."
Ultrasound consists of sound waves above about 18,000 cycles per second, too high-pitched to be audible to human ears. Scientists can make narrow beams of "silent" ultrasound far more intense than the roar of a jet engine.
"By understanding the conditions inside the bubbles, we can learn to control cavitation and its chemical and physical effects," Suslick said.
The work was funded by the U.S. Department of Energy and the Defense Advanced Projects Agency.
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