| Sonoluminescence:
Bubble Power
By Detlef Lohse
Nature
July 25, 2002
Gas bubbles in a liquid can convert sound energy into light. Detailed
measurements of a single bubble show that, in fact, most of the sound
energy goes into chemical reactions taking place inside this 'micro-reactor'.
'Single-bubble sonoluminescence' is
the remarkable phenomenon that
describes how a gas bubble in liquid,
exposed to a strong, standing sound wave,
collapses and emits light. First observed
12 years ago1, the basic physics of the process
seems to be understood2. That there is strong
and crucial chemical activity inside the
sonoluminescing bubble had already been
hypothesized3 and indirectly confirmed4,5.
Now Didenko and Suslick6 (page 394 of this
issue) have performed the first direct
measurements of the reaction rates inside
an individual bubble as it sonoluminesces.
Energy-wise, it seems that a sonoluminescing
bubble should be viewed not as a light
bulb, but rather as a high-temperature,
high-pressure, miniature reactor.
The process of sonoluminescence is
shown in Fig. 1, overleaf. First, at low sound
pressure, the micrometre-size bubble
expands, increasing its volume by a factor
of 1,000. When the pressure increases again,
the bubble eventually collapses dramatically,
shrinking to a radius that corresponds to
solid-state densities. The compression drives
up the temperature of the gas inside the
bubble - through this 'adiabatic' heating
the bubble interior is thought to reach
around 10,000-20,000 K. Consequently, the
gas becomes partly ionized and the recombination
of electrons and ions leads to the
emission of light.
As the bubble expands, gas dissolved in
the liquid enters the bubble. At the point of
adiabatic collapse, some of these gases are trapped inside the hot bubble and start to
react. For example, nitrogen molecules dissociate
into nitrogen radicals and then react to
form gases such as NH and NO. These highly
soluble gases re-dissolve in the surrounding
water when the bubble cools down. As the
bubble expansion begins again, the next reaction
cycle starts. Didenko and Suslick's calculations
of the energy budget of sonoluminescence
show that the amount of energy going
into endothermic chemical reactions inside
the bubble is two orders of magnitude higher
than that going into light emission.
However, one complication that still
remains is that the temperature inside the
bubble cannot be measured directly. It has to
be deduced either from the bubble dynamics
(for example, by Mie scattering1,7,8) or from
the properties of the light emitted (spectral
information, intensity and widths of the
light pulses9). Either way, assumptions have
to be made, whether in the modelling of the
bubble dynamics10 and the thermodynamics
of the heat and mass exchange between
the bubble and its surroundings, or in the
modelling of the plasma physical processes
to predict the observable light properties.
The information obtained in these two
ways should obviously be consistent in a
viable theory of sonoluminescence. Even
then, we can't be certain, as errors arising in
the modelling of the bubble interior and
light emission could compensate for each
other. But Didenko and Suslick's measurements
of the chemical reaction rates open up
a third experimental window on the process.
This extra constraint reduces the freedom in
modelling, leading towards further convergence
of the models.
It is astounding how many sub-disciplines
of physics and chemistry have played a role
in disentangling what happens in singlebubble
sonoluminescence. They range from
acoustics, fluid dynamics, plasma physics,
thermodynamics, atomic physics and spectroscopy,
to physical and analytical chemistry,
chemical kinetics, dynamical-system
theory and applied mathematics in general.
But nuclear and fusion physics are not on the
list: the final conclusion from Didenko and
Suslick's results is that it is the chemical reaction
rate within the bubble that limits the efficiency
of bubble collapse. So 'bubble fusion'
- an energy-generating fusion reaction in
the high-density, high-temperature interior
of the collapsing bubble11 - is most unlikely.
Although fusion may be out of reach, there
are other uses for sonoluminescent bubbles.
The extreme conditions inside the bubble are
adjustable through external parameters such
as forcing pressure or water temperature, so
the bubble can be considered as a controlled
high-temperature reaction chamber, offering
opportunities to measure reaction rates in
extreme temperature and pressure regimes.
But understanding single bubbles is not
enough. Before this knowledge can be applied
to sonochemistry12 - the enhancement of
chemical reactions through ultrasound in a
bubbly fluid - a better understanding of
bubble-bubble interactions will be needed.
Just as the hydrogen atom was the basic
model for larger atoms and molecules, so the
single bubble is the simplest building block
in the physics of a sound-driven bubbly fluid.
With the detailed understanding of the
hydrogen atom, atomic physics began to
flourish. By analogy, now that there is a basic
understanding of single-bubble sonoluminescence
and the chemical activity inside the
bubble, I expect also a flourishing of cavitation
physics.
Detlef Lohse is in the Department of Applied
Physics, University of Twente, 7500 AE Entschede,
The Netherlands.
e-mail: d.lohse@tn.utwente.nl
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