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Making light of sound in solitary bubbles.

Trapped in an intense sound wave, a tiny gas bubble in water can emit a string of flashes bright enough to be visible in an undarkened room. Producing a startling sound-and-light show on an intriguingly small scale, this simple system serves as a remarkable microlaboratory for physics and chemistry.

Now, researchers have demonstrated that slight changes in the composition of the gas inside such a bubble can strongly influence the intensity and wavelengths of the light that escapes. For example, adding a small amount of argon, xenon, or helium to a nitrogen bubble substantially increases the intensity of ultraviolet light emission.

Physicists Robert Hiller, Keith Weninger, Seth J. Putterman, and Bradley P. Barber of the University of California, Los Angeles, describe their findings in the Oct. 14 SCIENCE.

When an intense sound beam travels through water, it creates microscopic cavities that immediately fill with gas originally dissolved in the liquid. Such bubbles alternately expand and contract in step with regular changes in the sound wave's pressure.

During the contraction phase, a bubble can collapse so violently and rapidly that it concentrates the sound energy sufficiently to heat the enclosed gas to temperatures exceeding 10,000 kelvins. The heated gas luminesces, giving off an extremely bright flash of visible and ultraviolet light lasting less than 50 picoseconds.

Although researchers have known about this effect -- called sonoluminescence -- since the 1930s, they still do not have a complete understanding of how it works (SN: 10/23/93, p.271). The experiments of Hiller and his coworkers represent one attempt to elucidate the process.

The researchers found that raising the noble gas content of a nitrogen bubble in water to 1 percent dramatically stabilizes the bubble's motion. It also increases the intensity of light emission by a factor of at least 10.

At the same time, the spectrum of light generated by a bubble depends strongly on the gas inside the cavity. A bubble containing argon produces ultraviolet light that peaks at a wavelength of 300 nanometers. However, a helium-laced bubble shows no such peak.

"Some exciting atomic physics may be occurring within the collapsing cavitation bubble that gives rise to [singlebubble sonoluminescence]," Lawrence A. Crum and Ronald A. Roy of the University of Washington in Seattle comment in the same issue of SCIENCE. "However, many of the results [Hiller and his colleagues] present are also anomalous and defy immediate explanation."

Clearly, further investigations are necessary to pin down how sonoluminescence occurs. At the same time, the new results suggest the possibility of using gas impurities for improved control of the characteristics of light emissions from collapsing bubbles.
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Title Annotation:composition of gas inside sound wave affects the intensity and wavelength of the emitted light
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Oct 15, 1994
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