| New Bubble Reaction Findings Make Fusion Claims Unlikely
By Sarah Graham
Scientific American
July 25, 2002
Sonoluminescence, the process in which light is created when
sound waves move through a
liquid and cause bubbles to
expand and collapse, was at the
center of a contentious scientific
debate earlier this year. A team
of researchers reported that
they had exploited the
phenomenon to achieve nuclear
fusion using a tabletop
apparatus, an assertion that
quickly met with skepticism.
Now new research casts further
doubt on those claims.
According to a report published
today in the journal Nature,
scientists have directly
measured the reaction rates
inside a single bubble as it
sonoluminesces and the
findings suggest so-called
bubble fusion is "most unlikely."
Yuri T. Didenko and Kenneth S. Suslick of the University of Illinois at
Urbana-Champaign studied single bubbles in water subjected to
ultrasound and, for the first time, established an energy inventory for
the collapsing spheres. The team calculated that most of the sonic
energy is converted into mechanical energy, which creates motion in
the liquid surrounding the bubble. Less than one millionth of the
sound energy gets converted into light and one thousand times that
amount powers chemical reactions occurring within the sacs.
Specifically, the scientists measured the yields of hydroxyl radicals
and nitrite ions produced by the so-called acoustic cavitation of the
bubbles. Because energy is required to power these chemical
transformations, there is less available to raise the temperatures
inside the bubble to the intensity required for nuclear fusion to occur.
"Some researchers have suggested that conditions within a
cavitating bubble might be hot enough and have high enough
pressure to generate nuclear fusion," Suslick says. "But we've shown
that chemistry occurs within a collapsing bubble, and that it limits the
energy available during the cavitation." He adds, however, that
bubbles in certain liquids, such as molten salts or liquid metals, could
conceivably reach the sky-high temperatures required for fusion to
occur and the possibility of sonofusion "cannot be ruled out at this
time."
In an accompanying commentary, Detlef Lohse of the University of
Twente in the Netherlands writes that "although fusion may be out of
reach, there are other uses for sonoluminescent bubbles." In particular, he notes that because the temperatures within the
bubbles approximate those on the surface of the sun and the
pressures are as high as those near the bottom of the ocean, the
bubbles could be used as controlled high-temperature reaction
chambers to study reaction rates under extreme conditions.
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