Sound Into Light -- Tiny Bubbles That Give Off Flashes, Intense Heat May Illuminate Study Of Energy

------------------------------------------------------------------ Physics: The process in which high-pitched sounds can be converted to light has puzzled scientists for years. Some recent developments in this science of sonoluminescence offer some clues, which could result in a better understanding of specific high-temperature, high-pressure phenomena. ------------------------------------------------------------------

DALLAS - There's no reason to be embarrassed if you've never heard of sonoluminescence.

Even Webster's New World Dictionary, which delivers on such wonders as anamorphosis and endomixis, is mum on the phenomenon.

Not to fear. Sonoluminescence is fairly straightforward. Under the right conditions, sending a high-pitched tone through water produces an eerie glow.

"You actually do convert sound into light," said physicist Lawrence Crum. "It's really quite remarkable."

Researchers discovered sonoluminescence in 1934. Six decades later, physicists still can't explain it. But some recent experiments have shed light on the process.

"We're just learning all of the things that you might be able to do with it," said Crum, a professor in the Applied Physics Laboratory at the University of Washington.

The discovery of a new type of sonoluminescence six years ago rejuvenated the field. A special condition known as single-bubble sonoluminescence makes the whole phenomenon easier to study and could lead to practical applications - if not of sonoluminescence itself, then of some related process.

Physicists are eager to learn how sonoluminescence works because it packs an incredible punch. Depending on conditions, the flash of light can be as short as 50-trillionths of a second (in the most recently discovered form of the phenomenon) and coincides with a release of energy that can heat a tiny space to more than 8,500 degrees Fahrenheit and amplify pressures thousands of times.

"It's very difficult to get those temperatures and pressures in a specific spot on Earth," Crum said.

Producing intense temperatures and pressures makes possible chemical reactions that would be impossible under less extreme conditions. A whole field of chemistry, known as sonochemistry, has developed around that endeavor. It doesn't necessarily use sonoluminescence to generate chemical reactions. But sonoluminescence might help chemists understand what's going on in their experiments.

Kenneth Suslick and Kathleen Kemper, chemists at the University of Illinois, figured out how much pressure is produced when a sound wave makes a flash of light.

In the book "Bubble Dynamics and Interface Phenomena," published this year, they reported that one type of sonoluminescence produces pressures 1,700 times that of Earth's atmosphere at sea level. That sort of information applies not just to sonoluminescence but to many related processes.

The key to sonoluminescence is the same as the secret to good champagne - tiny bubbles. As sound waves pass through water or any material, they alternately compress and expand it. The expansion sometimes allows microscopic bubbles to form, and the subsequent compression smashes them.

"The critical part of this is to realize that when you compress gas you get heating," Suslick said.

In the form of sonoluminescence discovered in the 1930s, the compressing bubbles reach temperatures as hot as the surface of the sun and more pressurized than the deepest parts of the ocean. Such intense conditions can cause gases in the bubble to glow. But scientists still don't completely understand the process.

In the old form, known as multiple-bubble sonoluminescence, the sound wave travels through a water chamber as millions of bubbles form and collapse to produce a faint glow.

In the new form, discovered in 1988 by University of Mississippi graduate student Felipe Gaitan, a single bubble in a small chamber repeatedly produces flashes of light.

A remarkable discovery has been that single-bubble sonoluminescence and multiple-bubble sonoluminescence seem to occur by completely different mechanisms.

"They're related phenomena, but they're not the same phenomenon. And that in and of itself is a little bit of a surprise," Suslick said.

Single-bubble sonoluminescence produces temperatures far beyond those of multiple-bubble sonoluminescence. Estimates go as high as 53,000 degrees Fahrenheit. But researchers warn that such measurements rely on many assumptions and may not mean much.

Researchers reason that to be so much more intense than multiple-bubble sonoluminescence, single-bubble sonoluminescence must work by a different mechanism. They hypothesize that a shock wave is produced as the bubble collapses. In a shock wave, material moves faster than the speed of sound traveling through it.

"When that shock wave hits the center, you get incredible heating," Suslick said.

The bubble gets squeezed down to thousandths of its original size. But for some reason, it stops collapsing when it reaches about one-25,000th of an inch across - smaller than a human red blood cell. The bubble makes a light flash, then rebounds to its full size.

Under the right conditions, a bubble can give off flash after flash, emitting 25,000 pulses of light a second. Crum and his colleagues have no idea how a bubble can do such a thing.

A clue to the bubble's durability during single-bubble sonoluminescence may lie in a paper published last month in the journal Science.

Researchers at the University of California, Los Angeles, noted that the intensity of a sonoluminescent flash depends on the composition of the air in the bubble. A tiny amount of a noble gas - including helium, argon and xenon and the other elements that lie on the right edge of the periodic table - added to nitrogen produces a flash more than 10 times as intense as either nitrogen or a noble gas by itself.

"This is a mystery. We don't understand it at all," Putterman said.

It is curious, however, that plain old air is about 1 percent argon. That's just the right proportion to enhance the sonoluminescence effect.

"The presence of the argon in the air turns it from being a phenomenon that might have been overlooked, or not discovered, to a bubble which is sufficiently bright to be seen with the unaided eye," Putterman said.

Understanding the details of sonoluminescence could lead to practical applications. In one sonochemical application, Suslick makes iron with no crystalline structure. He and two University of Illinois colleagues, Mark Grinstaff and Myron Salamon, described the properties of amorphous iron last year in the journal Physical Review B. The material is "useful in any situation where you have a rapidly oscillating magnetic field," Suslick said, such as audio tape recording.

The idea in making amorphous iron is to cool molten-iron droplets quickly. Even if the conditions aren't right for sonoluminescence, sound waves can make and then squash bubbles extremely quickly.

"You get cooling rates of billions of degrees per second," Suslick said. "They (the iron droplets) freeze so quick that they don't have time to form an organized crystalline array."

Suslick believes sonoluminescence is important to understanding how sound waves concentrate energy by creating and destroying bubbles in fluids.

"It's a useful probe of the conditions and chemical species formed in bubble collapse," Suslick said.