Single-Bubble Microreactors
By Steve Ritter
Chemical & Engineering News
July 29, 2002
Chemistry quantified in a cavitating bubble; fusion likely
out of reach
Using sensitive fluorescence spectroscopy, chemists at the
University of Illinois, Urbana-Champaign, have made the first direct
measurements of energy dissipation and reaction rates inside an
isolated bubble in water driven to violent size oscillations by highintensity
ultrasound. The results of postdoc Yuri T. Didenko and
chemistry professor Kenneth S. Suslick suggest that a cavitating
bubble could be thought of as a high-temperature, high-pressure
microreactor and have important implications for future work on
ultrasound-driven chemistry [Nature, 418, 394 (2002)].
However, Didenko and
Suslick conclude that the
endothermic reactions
limit the temperatures that
can be achieved inside a
cavitating bubble. The
maximum temperature for
single-bubble cavitation is
generally expected to be
below 20,000 K--far less
than the 1 million K
needed for the tabletop
"bubble fusion" in a
cavitating deuteroacetone
system that was reported
in a controversial Science
paper earlier this year
(C&EN, March 11, page
11). The extraordinary
conditions needed to
initiate nuclear fusion will
be very difficult to obtain
by single-bubble cavitation
in a volatile liquid such as water or acetone, Suslick
says, although the
possibility of fusion in
molten salts or liquid
metals cannot be ruled out.
As cavitating bubbles collapse, they emit flashes of light, a process
known as sonoluminescence. In addition, a flurry of chemical
activity has been hypothesized to occur inside. Although chemical
reactions have been indirectly observed, quantitative analysis of the
products has proven difficult because of the tiny amount of reacting
gas inside a single bubble. The Illinois chemists' experiments have
now overcome this obstacle.
Gases that diffuse into the expanding bubble from the surrounding
liquid are thought to become ionized under the high temperature
generated by the bubble's collapse. Excited-state molecules and
the recombination of the separated electrons and ions give rise to
the light emission. During this recombination, the original N2, O2,
and H2O molecules present have been predicted to re-form as
hydroxyl radicals, nitrogen oxides, and other species. The hydroxyl
radicals, in turn, are expected to react with organic molecules in the
water or to dimerize to H2O2.
By direct correlation from the fluorescence spectra, Didenko and
Suslick were able to measure the number of photons emitted and
the yield of OH and NO2
- in a 10-µm cavitating bubble. The data
were used to calculate the energy balance of the collapsing bubble.
Most of the system's potential energy is converted into mechanical
energy that is imparted to the liquid, the researchers find. The
remaining energy is converted to heat, chemical reactions, and
light, with the energy going into the chemical reactions 100 times
greater than that going into light emission.
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