| Researchers say bubble fusion
more difficult to reproduce than once thought
By Charles Choi
United Press International
July 24, 2002
CHAMPAIGN-URBANA, Ill. - Although sound
waves can generate temperatures as hot as the surface of the sun simply by
squishing bubbles, the potential of tabletop nuclear "bubble fusion" raised
earlier this year may have been exaggerated, new calculations suggest.
Experimental findings reported last March suggested tiny bubbles could
trigger fusion reactions by collapsing in a neutron-loaded solution of acetone,
a common, naturally occurring solvent used to make plastic, fibers, drugs and
other chemicals. The research was led by Rusi Taleyarkhan at Oak Ridge
National Laboratory in Tennessee.
Bubbles in liquids trapped and energized by ultrasound beams tend to flare
with light in a phenomenon known as sonoluminescence, first observed in
1990. When bubbles inflated by sound waves collapse, the billionth-of-asecond-
long implosions generate incredible pressures normally found at the
bottom of the ocean along with temperatures of about 9,000 degrees
Fahrenheit.
Such intense pressure and heat led to speculation fusion could take place, in
which atomic nuclei are slammed together to liberate incredible forces with
little radioactive waste. Taleyarkhan's team said they detected chemical
byproducts of fusion in their souped-up paint thinner in a container the size of
three coffee mugs.
"Our results make Taleyarkhan's increasingly unlikely," Kenneth Suslick, a
chemist at the University of Illinois at Urbana-Champaign told United Press
International.
Suslick and colleague Yuri Didenko analyzed the chemical reactions from the
collapse of an isolated excited bubble, the byproducts formed. They reported
their findings in the July 25 issue of the British journal Nature.
"This energy is converted into light emission, chemical reactions and
mechanical energy," Suslick explained. "We were able to determine, for the
first time, how much of the energy goes into the chemistry of the bubble."
Suslick and Didenko generated a bubble about the size of a red blood cell and
trapped it in the center of a spherical container using soundwaves. They
adjusted the pressure in the container to expand the bubble to 1,000 times
its volume, then collapsed it repeatedly, using sensitive fluorescent chemicals
to monitor the byproducts created.
They found that volatile molecules such as water, nitrogen and oxygen were
ripped apart. Although less than one-thousandth of the energy involved fueled these chemical reactions, it was enough to eliminate the possibility
that fusion could occur, they said.
The new findings suggest sound-triggered fusion is improbable in highly
volatile fluids like acetone or water, Suslick said. However, "the possibility of
fusion occurring in low volatility fluids, such as liquid metals and molten salts,
cannot be ruled out at this time."
Tabletop fusion may be out of reach, but "there are other uses for
sonoluminescent bubbles," said physicist Detlef Lohse of the University of
Twente in Entschede, The Netherlands. For example, now that scientists
understand the chemical processes of sonoluminescence more thoroughly,
they might be able to harness it for applications in medicine and industry.
Suslick noted sonoluminescence is already helping to enhance the chemical
reactions used to make pharmaceuticals. Quoting Russian intellectual Leon
Trotsky, Suslick said the research should go "forward in all directions."
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