June 22, 1998 COLUMBUS, Ohio -- An Ohio State University physicist may have uncovered the atomic process behind sonoluminescence, an effect in which ultrasonic waves break against the surface of a water bubble and heat the atoms inside until they glow.
This new explanation may assist the emerging field of sonochemistry, where scientists use ultrasound to accelerate and enhance chemical reactions -- for instance, in the creation of new materials. It may also hold applications in the field of optics.
Sanjay Khare, a postdoctoral researcher in physics at Ohio State, said that even though scientists know a great deal about the motions of bubbles and ultrasonic waves, nobody knows exactly how sonoluminescence works on an atomic level.
“We’re talking about the physical conditions inside an ordinary bubble in water,” said Khare, “but still this is a phenomenon that we don’t fully understand.”
Khare and Pritiraj Mohanty, a graduate student of physics at the University of Maryland, College Park, found a possible clue to the atomic source of sonoluminescence when they considered that the ultrasound-stimulated bubbles emit light in very short pulses, as short as 10 parts in a trillionth of a second.
“We knew that any single atom of the gas inside a bubble would take much longer to decay and emit light,” said Khare. “And we knew that when many atoms decay together they sometimes decay faster.”
Khare and Mohanty hit upon the idea that if the many atoms inside the bubble decayed at the same time, then the light waves would emerge in step with each other and at the same frequency. That would account for the short pulses.
In a paper that appeared in a recent issue of the journal Physical Review Letters, the researchers proposed that stimulated, or excited, atoms decaying in unison could emit the kind of light seen in sonoluminescence.
The researchers also hypothesized in the paper that the type of gas inside the bubble will affect the reaction. They cited previous studies in which minute traces of specific gases inside bubbles intensified light emission.
If correct, Khare and Mohanty’s idea will explain sonoluminescence at an atomic or microscopic level. Scientists have known of the phenomenon for over 60 years, but none have yet found a satisfactory explanation.
Khare said that even though sonoluminescence was discovered in 1934, the effect gathered real interest among physicists only eight years ago when researchers at the University of Mississippi developed a technique for maintaining a single bubble during experiments, which afforded researchers more control of the process.
During sonoluminescence, the gas inside the bubble may reach temperatures as high as 10,000 degrees Celsius -- almost twice the temperature on the surface of the sun. Sonochemists believe they may be able to use the high temperatures to fuse atoms and form new materials.
In a possible optics-related application, sonoluminescence may act as a source of ultra-short light pulses, which scientists use to study very short physical processes such as atomic excitation.
Khare and Mohanty plan to continue this work, which was sponsored by the National Science Foundation and the Department of Energy Basic Energy Sciences, Division of Materials Sciences.
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