Turn Sound Energy into Light
Hopkins scientist proposes new explanation
for sonoluminescence in bubbles
Prosperetti's theory appears in the April 1997 issue of the Journal of the Acoustical Society of America. His paper offers an alternative to the widely held view that the bubble glows because of shock waves that concentrate energy in its center as it shrinks.
His theory also deflates the hope among some researchers that sonoluminescence generates enough pressure and heat to produce nuclear fusion, a potential source of cheap, clean energy. Some scientists have speculated that bubble temperatures during sonoluminescence exceed 2 million degrees Fahrenheit, near the levels needed for fusion. This idea became a key plot point in the motion picture Chain Reaction, starring Keanu Reeves. But if Prosperetti's theory holds true, the heat inside the bubbles would peak at about 10,000 degrees F, the level found at the sun's surface. "It's enough to explain the chemical activity, but it's far below the amount needed to produce nuclear fusion," says Prosperetti, who is the Charles A. Miller, Jr. Distinguished Professor of Mechanical Engineering at Hopkins.
Sonoluminescence was discovered in 1934 by two German physicists who immersed powerful ultrasound generators in a vessel of water, creating a cloud of tiny bubbles that gave off a glow. Scientists were intrigued but found it was too difficult to study in detail the unwieldy mass of short-lived bubbles. In 1989, however, Lawrence Crum, then a professor at the University of Mississippi, and his graduate student, Felipe Gaitan, were able to induce sonoluminescence in a single bubble trapped within a sound field inside a cylinder of water. Since then, scientists have been able to study the phenomenon more closely. Much to their surprise, they realized that this "single-bubble" luminescence was different from the massive "multiple bubble" phenomenon first observed 60 years earlier and -- as it turns out -- far more mysterious. For example, the flash of light lasts an incredibly short time, a few tens of trillionths of a second. Also, the phenomenon is extremely sensitive to the nature, purity and temperature of the liquid and to the presence of dissolved gases in it.
Sound waves passing through the liquid cause the bubble to compress and expand repeatedly. At its largest point, the bubble's diameter is about that of a human hair. Scientists believe the sound energy is concentrated during the bubble's compression phase, then is released as light near the point where its size is smallest. But the exact mechanism has remained a mystery.
In his new paper, Prosperetti says it is unlikely that shock waves within the shrinking bubble trigger sonoluminescence because the bubble would need to maintain a near-perfect spherical shape. "I think it is absolutely impossible for the bubble to remain spherical," he says. "In a sound field, there is a very well-defined mechanism that will prevent this from happening. The fluid wants to push a jet, a finger of liquid, through the bubble, hitting the other side. What you see in sonoluminescence is the initial result of this 'hammer of water.'" This jet, moving at perhaps 4,000 miles per hour, or more than five times the speed of sound in air, strikes so quickly that water molecules do not have time to flow away from the point of impact. Instead, the fluid fractures. "This is what happens with Silly Putty, for instance," Prosperetti says. "If you pull slowly, it just stretches or flows. But if you pull it really hard, it snaps, and you get a brittle fracture."
Ice and even Wint-O-Green Lifesavers candy sometimes give off light when they crack, and water molecules could produce the same effect, the Hopkins researcher suggests. His theory holds the promise of explaining many facets of the phenomenon. For example, bright light emission requires tiny amounts of a noble gas such as xenon, argon or helium dissolved in the liquid because, Prosperetti believes, these inert atoms create flaws or weaknesses in water's crystal-like structure that provide a foothold where the fracture begins. In his paper, Prosperetti urges other researchers to test his theory. He suggests several lab experiments for this purpose, including the firing of a hyperfast bullet or fluid jet at water in a controlled setting to see if it produces luminescence.
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