| Researchers Probe Chemistry of Bubbles
By Daniel S. Burgess
Photonics Technology News
September 2002
Since luminescence in single cavitating bubbles was described more than a decade
ago, the phenomenon has attracted much attention from scientists. Earlier this year,
interest was piqued all the more, when a team at Oak Ridge National Laboratory in
Tennessee claimed to have observed neutron emissions from the bubbles that
indicated fusion processes. Now researchers at the University of Illinois at Urbana-
Champaign have analyzed the chemistry of single bubbles, opening the door to a
better understanding of the event and calling into question the claims of fusion.
Sonoluminescence occurs in the
bubbles in an acoustic wave in a
liquid. In a low-pressure region of
the wave, a bubble expands a
thousandfold, trapping gases that
were dissolved in the liquid. When
exposed to a high-pressure region,
the bubble collapses violently,
producing temperatures that may
be as high as 20,000 K and
ionizing the gases within.
At this stage, it emits a 50- to 500-ps
pulse of broadband radiation, and the
products of the reaction dissolve back
into the liquid. The process begins again
as the bubble is exposed once more to a
low-pressure region of the wave.
The university researchers created their
60-µm-diameter bubbles in a 15-ml glass
cell that contained water at 3 or 22°C. A
titanium rod vibrating at 28 or 52 kHz
induced the acoustic waves, and a Jobin
Yvon monochromator and a 1024 X 256-pixel CCD detector collected the spectra
from individual cavitating bubbles. The team employed a long-pass filter at
wavelengths longer than 400 nm to minimize second-order radiation.
Energy conversion
The scientists monitored the production of ions, radicals and photons in a given
collapse event. Together with measurements of the size of the bubbles, these counts
enabled them to estimate the efficiency with which a bubble converts its potential
energy into mechanical energy, heat, chemical reactions and light.
They discovered that about 1/10,000 of the energy in a collapsing bubble was
used to form nitrite ions and hydroxyl radicals from the nitrogen that diffused
into it in the expansion phase. The energy of the photons released in
luminescence was two orders of magnitude smaller. The results indirectly
challenge the claim of fusion in perdeuterated acetone, noted Kenneth S.
Suslick, who conducted the study with Yuri T. Didenko. The volatility of
acetone should lead to strongly endothermic chemical reactions of polyatomic
molecules in the bubbles that would readily consume the available energy
and thereby limit the maximum temperature on collapse.
The researchers note, however, that the possibility of fusion by cavitation
cannot be rejected in substances with extremely low volatility, such as polar
organic liquids, liquid metals and molten salts.
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