Cynthia L. Atwood, Science Correspondent
Yale University Office of Public Affairs
433 Temple St.
New Haven, CT 06520
(203) 432-1326, fax (203) 432-1323
[email protected]
CONTACT: Cynthia L. Atwood
For Immediate Release: Nov. 22, 1996
Mysterious Glowing Bubbles To Be Dissected at Honolulu Meeting
As Possible Source of Pollution-Free Energy, Underwater Sonar
New Haven, CT -- Some scientists think the mysterious glowing water
bubbles at the center of the glass flask are caused by imploding shock
waves that produce miniature sonic booms. Others point to the same
processes that generate static electricity in thunderclouds, or to
some poorly understood quantum effect. Still others liken the glow
to sparks created by biting down on mint Lifesaver candies in the dark.
Whatever the mechanism, scientists agree that sound is somehow
converted into pulsating light by the simple expedient of bombarding
boiling water with sound waves. Called sonoluminescence, the enigma
has intrigued scientists since it was discovered in the 1930's. Today,
researchers are trying to harness the process for possible commercial
applications ranging from broad-band underwater sonar to pollution-free
energy.
Renewed interest in understanding sonoluminescence surfaced
recently when Robert E. Apfel, a mechanical engineer at Yale University,
challenged colleagues around the world to come up with new experiments
to test the growing list of theories. In an article in the Nov. 1
issue of the journal Science, he asked them to bring their ideas to a
joint meeting of the Acoustical Societies of America and Japan
scheduled Dec. 2-6 in Honolulu.
"There are just about as many theories for sonoluminescence as there
are theorists, but far too few experiments," said Professor Apfel, an
acoustics expert who designed experiments for the 1992 and 1995 space
shuttle Microgravity Missions to test how soap and other commercially
important surfactants react with the surface of water droplets. About
a dozen researchers have responded already to his challenge.
Brief Bursts of Visible Light
What is known thus far is that brief bursts of visible and
ultraviolet light are produced when bubbles generated by a heating
coil immersed in water are bombarded by ultrasound. The energy of the
sound waves is somehow concentrated into photons of light trillions of
times more energetic than the sound waves themselves. The phenomenon
is no "cold-fusion-in-a-jar" illusion, said Professor Apfel, referring
to claims made in 1989 but never realized of fusion energy produced in
a beaker of water. Sonoluminescence has been replicated in dozens of
labs, including his own.
Based on rare photographs made in his lab, the Yale engineer favors
a theory in which the walls of the bubbles rebound at supersonic speeds
when bombarded by ultrasound. The imploding shock waves generate
temperatures as high as 100,000 degrees Celsius, causing the gas to
become a hot, glowing plasma that emits pulses of light with clocklike
regularity. Each pulse lasts only trillionths of a second.
"The bubbles expand to about 100 microns in diameter during the
low-pressure phase of the sound wave, then collapse to a fraction of a
micron in size, at which time the flash occurs," Professor Apfel
explains. The implosion theory is supported by recent reports of a
popping sound, or a mini sonic boom. In fact, one team of scientists
plans to present computer simulations of these imploding sonic booms
at the Honolulu meeting.
If scientists could find a way to make bubbles as large as a
centimeter in diameter, energy storage would be boosted almost a
million times, he said. The collapse of a bubble that large would
produce a broad-band sound wave that could be useful in underwater
sonar for detecting hidden objects, perhaps even objects buried under
sand on the ocean floor.
First Photographs of Pulsating Bubbles
A technological challenge in sonoluminescence is to keep larger
bubbles from becoming elongated or distorted, since only spherical
bubbles produce a glow when they collapse, Professor Apfel and his
Yale colleagues discovered by capturing the first photographs ever
made of the pulsating bubbles.
The photos, which have a resolution of 5 microns, appear in the
Nov. 1 issue of the journal Science and will appear in the December
issue of the Journal of the Acoustical Society of America. Yale
graduate students Joseph Jankovsky and Jeffrey Ketterling participated
in the research along with former post-doctoral associate Yuren Tian.
While the shock-wave theory is viable, other theories for
sonoluminescence deserve a closer look, Professor Apfel said.
Proponents of the Lifesaver candy theory say a phenomenon called
fractoluminescence
produces flickers of light in much the same way that friction in
breaking
candy (or breaking glass) can produce sparks. Doubters of this theory
point to the fact that the light flashes occur only when nobel gases
such as xenon and argon -- normally found in air -- are dissolved in
the water. That would argue for some process that ionizes gas.
Other theorists argue that quantum fluctuations produce the glow
as traces of energy that exist in the ground state of all matter are
squeezed out of the bubbles. However, Alan Chodos, a senior research
physicist at Yale, has calculated that there probably is not enough
vacuum energy in the ground state to produce a visible flash.
One of the most intriguing possibilities is that sonoluminescence
could produce inertial (hot) fusion, which is a pathway to a boundless
source of energy that is free of pollution or radioactive waste.
Scientists at Lawrence Livermore Laboratories, for example, have
created inertial fusion by using high-energy lasers to bombard glass
pellets filled with deuterium gas. The compressed gas emits neutrons
that can produce heat, which can be converted to electricity.
Whether water bubbles enriched with deuterium and bombarded with
ultrasound would result in inertial fusion is an interesting question
scientists should explore, said Professor Apfel. "At the very least,
sonoluminescence produces a transient chemical reactor. The spectra of
emitted singles could provide a good analysis technique for extreme
pressure and temperature conditions," said the Yale engineer, who holds
two patents for neutron-detection devices.
###
Note to Editors: Robert E. Apfel, who is the Robert Higgin Professor
of Mechanical Engineering at Yale and past president of the Acoustical
Society of America, is a specialist in physical acoustics. He also
teaches architectural acoustics in the Yale School of Architecture.
This research was partially funded by a grant from NASA's Jet
Propulsion Laboratory. Professor Apfel is available for interviews at
(203) 432-4346.
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