Embargoed until 2 p.m. EDT
NSF PR 02-63 - July 24, 2002
Light From Gas Bubbles: Sonoluminescence Measured
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A cloud of gas bubbles in a liquid excited
by ultrasound (generated by a titanium
rod vibrating 20,000 times a second) can
emit flashes of light (sonoluminescence)
due to extreme temperatures inside the
bubbles as they collapse.
Image credit: K. S. Suslick and K.
J. Kolbeck, University of Illinois
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Gas bubbles form and collapse when a liquid
is energized by ultrasound.
Image credit: K. S. Suslick and K.
J. Kolbeck, University of Illinois
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(Size: 400KB)
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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).
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