Glimpses inside a tiny, flashing bubble.

Bombarded with an intense sound wave, a small gas bubble suspended in water can emit a string of extremely short, bright pulses of light. Known as sonoluminescence, this conversion of sound into light occurs during the rapid and violent contraction of the bubble as it oscillates in step with the sound wave (SN: 4/29/95, p. 266).

How the collapse of a bubble during contraction generates a flash of light, however, remains largely a mystery. Now researchers have obtained new experimental evidence that may illuminate key characteristics of the light source.

One group has discovered that the bubble wall separating the gas from the surrounding liquid may not always be perfectly spherical during collapse, as theorists had generally supposed. Another team has found that putting the bubble into a strong magnetic field can drastically decrease light emission.

Physicists Keith Weninger, Seth J. Putterman, and Bradley P. Barber of the University of California, Los Angeles used several photodetectors surrounding a flask of water to determine the uniformity of light emission from a sound-driven oscillating bubble. In some cases, they detected variations in light intensity in different directions, suggesting that the bubble wall was nonspherical-possibly squashed into a slightly ellipsoidal shape-during collapse. In other cases, the bubbles stayed spherical.

Their measurements, reported in the September Physical Review E, also indicated that ellipsoidal deformations could persist for up to 100 successive contractions and light flashes.

"This work is interesting because it establishes a new diagnostic [tool] that can be used to study a lot of different aspects of sonoluminescence," says Michael J. Moran of the Lawrence Livermore (Calif.) National Laboratory.

The pattern of light emission indicates that the nonspherical bubble wall refracts radiation coming from a small, spherical region of hot gas deep inside a bubble, Putterman and his colleagues conclude.

The observation that a collapsing bubble can actually appear perfectly spherical, at least sometimes, is surprising, Putterman says. "You've got a supersonic implosion, with the bubble wall collapsing at over four times the speed of sound. Yet, at the moment when the bubble hits its minimum radius and the light comes, we get a highly spherical state a large percentage of the time."

Though Putterman and his coworkers can now determine whether bubble collapse is spherical, they can't yet predict or control the shape in any given situation. "The system seems to hop between a spherical state and another state," Putterman notes. However, "we don't know what to do to force it from one state to the other."

Because electric charges accelerate during the generation of light flashes, studying the effect of an external magnetic field on the system could also lead to insights into sonoluminescence.

Woowon Kang and his coworkers at the University of Chicago have investigated bubble oscillations in uniform magnetic fields as strong as 20 teslas, more than 400,000 times Earth's magnetic field. Preliminary results indicate that a sufficiently strong magnetic field can suppress sonoluminescence, Kang says. "This is an exciting result," Putterman comments.

Kang is now interested in checking whether a weaker magnetic field that has a definite direction affects the distribution of light emitted by an oscillating bubble.