Tiny air bubbles blasted with sound waves may act as crucibles for chemical reactions. Scientists have known for more than 60 years that as these bubbles rhythmically collapse and expand, they release flashes of light--a phenomenon called sonoluminescence. Now a report in the 26 January Physical Review Letters suggests that searing hot temperatures generated inside the bubbles drive out nitrogen and oxygen, leaving behind a stunning light show produced by the trace gas argon.
A bubble of pure argon shines radiantly as soon as high-frequency sound waves begin to make it pulse, while a nitrogen bubble never sheds more than a dim glow. Because sonoluminescing air bubbles start out dim and grow brighter, some physicists had suspected that bubbles may become richer in argon the longer they pulse. Thomas Matula and Lawrence Crum of the University of Washington, Seattle, decided to test whether chemistry was behind this brightening.
The team bombarded air bubbles with ultrasound. After getting a bubble to expand and pulse, they turned the sound down, stopping the sonoluminescence before it had started to brighten. But the researchers elicited an immediate flash after jacking up the sound waves to get the bubble to sonoluminesce a second time. Matula says this sudden illumination happens because during the first bout of sonoluminescence, temperatures in the bubble, which can be as high as several hundred thousand degrees--hotter than the sun's surface--do something to nitrogen and oxygen to make them form compounds such as nitrous oxide that dissolve in the surrounding water. Argon, an inert, "noble" gas, then lights up in the next round of pulsing.
Theory and computer simulations had both predicted that sonoluminescence in an air bubble is actually fireworks from argon in disguise. "What was missing was the experiment," says William Moss of the Lawrence Livermore National Laboratory in California.