Chaotic flashes of bubble-light.

It sounds more like magic than physics: Sound waves traveling through water momentarily compress an air bubble to produce a brief, bright flash of light -- one flash with each cycle of the sound wave. Researchers have known about this pehnomenon, called sonoluminescence, for more than 50 years, but they have yet to come up with a complete, convincing explanation of how the temporary collapse of a bubble can concentrate the energy of a sound wave more than a trillion times and excite atoms and molecules into producing light (SN: 5/11/92, p. 292).

Generally, these flashes occur at a specific sound wave frequency and pressure for a given bubble diameter, and researchers have founded they can generate an extremely steady train of flashes under these conditions. Now R. Glynn Holt of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his co-workers have probed the effect of slightly shifting the sound wave frequency and pressure away from their optimal values for producing flashes. They observed changes in the timing of the flashes, suggesting that acoustically driven bubble collapse may involve chaotic dynamics.

Holt and his colleagues studied this effect by using a special electrical circuit to measure slight variations in the timing of the flashes coming from a single bubble. They discovered that by altering the sound wave frequency and pressure appropriately, they could produce a pattern of flashes in which every second flash was slightly delayed. For other values of frequency and pressure, they observed timing delays that appeared to vary randomly. "We believe we have chaos going on here," Holt says.

Holt and his colleagues argue that bubble-wall motion by itself is insufficient ot produce the observed effects. By studying how timing variations depend on pressure and frequency, researchers may have a better chance of pinning down the mechansim responsible for sonoluminescence.