Light from gas bubbles: Sonoluminescence measured

July 24, 2002

A gas bubble excited by ultrasound turns a tiny fraction of the sound energy into light. This phenomenon, called sonoluminescence, has been observed for decades.

Now, chemists supported by the National Science Foundation have, for the first time, measured the chemical reactions and light emission from a single water bubble excited by sound waves. The researchers, Ken Suslick and Yuri Didenko of the University of Illinois, report their findings in the July 25 Nature.

Ultrasound applied to a liquid causes the formation, growth, compression and collapse of microscopic bubbles in a process called cavitation. These small oscillations can cause intense heat and pressure, similar to the conditions produced on a large scale by explosions or shock waves. This excitation also can cause emission of short flashes of light.

The ability of ultrasound to induce high-energy chemical reactions has been studied for potential industrial and medical applications, such as the breakdown of pollutants and development of medical imaging agents. To harness this process, however, scientists needed to quantify the energy and molecular particles released within a single, isolated bubble.

The Illinois experiment showed that, as pulsating water bubbles collapse, they create temperatures high enough to break water molecules apart.

Less than one millionth of the sound energy is converted into light. A thousand times more energy goes into the formation of atoms, molecular fragments and ions. The largest part of the sonic energy is converted into mechanical energy, causing shock waves and motion in the liquid surrounding the gas bubble.

"Cavitation, which drives the implosive collapse of these bubbles, creates temperatures resembling those at the surface of the sun and pressures like those at the bottom of the ocean," Suslick said. "This phenomenon offers a means of concentrating the diffuse energy of sound into a chemically useful form."

Possible applications include making catalysts to clean fuels, removing sulfur from gasoline, and enhancing the chemical reactions used to make pharmaceuticals. The process has already been used to make new chemical catalysts for industrial use and biomedical agents for magnetic resonance imaging (MRI).
For images see:

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