Zsolt Lazar
Enormous concentrations of energy, bluish and UV flashes of unmeasurably short duration, extremely high synchronicity, and all these achieved with an amazingly cheap experimental equipment; the name of this miracle is sonoluminescence.
As the name suggests this phenomenon (SL) has to do with both sound and light. In a nutshell it can be explained as follows: the energy of a sound wave can be transformed into light exhibiting remarkable features by the mediation of a bubble in a liquid. A typical experiment regarding the observation of SL is carried out in the following way: one single bubble the size of a few microns is levitated in water contained in a glass-made spherical vessel. Transducers are mounted on the vessel providing high intensity standing ultra-sound waves of a frequency of several tens of kHz. This way the experimentalists' organs of hearing do not get damaged. By light scattering techniques the reaction of the bubble to the varying local pressure can be observed. As long as the power of the transducers, consequently the amplitude of the pressure, does not exceed a certain threshold value determined by the type of the gas and liquid used, the bubble simply oscillates with a large amplitude (Rmax/Rmin 100). Above the threshold intensity the bubble starts radiating along the lower range of visible wavelengths, one, short, bluish flash each cycle.
Although the simple fact that one gets light from a bubble could be itself a challenging question to explain, this phenomenon exhibits some remarkable peculiarities that stirred the interest of several physicists.
- The spectrum of the emitted light can be well fitted by a black-body spectrum that is, the spectrum of a photon gas in equilibrium.
- The effective temperatures corresponding to the spectrum are of
orders of eV (1 eV 11600 K), i.e., ranging from a few thousands of K up
to beyond 50000
K depending on the preparation of the bubble. These temperatures correspond
to
an increase in the energy density, relative to the one characterizing the
sound
field, by a factor of 
.
- The flashes are extremely short and in many cases they occur in high synchronicity with the driving pressure.
- This new state of the bubble can be maintained for an unlimited time.
Sonoluminescence has been known for hundreds of years despite the variety of forms in which it manifested itself. More intensive investigations started only in the last decade when Felipe Gaitan discovered the conditions under which a single, stable cavitation bubble would produce SL each acoustic cycle (SBSL). So far many laboratories reproduced the same or similar experiments and as this allows for a good controllability, it facilitates the job of the theorists by providing the necessary data.
The main aspect studied was the one concerning the characteristics of the flash: the spectrum, duration and the phase of the acoustic cycle at which it occurs. The other aspect of great interest is the influence of various factors on these characteristics. Barber and Putterman managed to develop a powerful technic whereby the evolution of the bubble radius could be traced . The bubble was created either by introducing gas with a fine syringe or by the sudden heating of a tiny toaster wire immersed in the liquid. The created bubbles start dancing around the sound field, finally fusing and starting a highly nonlinear periodical radial motion made up of succesive expansions and collapses as the ambient pressure periodically falls and increases. One could clearly see how the bubble first expands to about ten times the initial size as the pressure decreases and then catastrophically collapses to a size of one tenth of the original followed by several `rebounces' of the radius in the second part of the acoustic cycle. The flash occurs close to the main collapse.
They noted several important facts:
- For a large number of various experimental parameters the pulse width is less than 50 ps (6 orders of magnitude faster than the acoustic frequency).
- Peak powers of the radiation are about 30mW.
- The emission is stable from the point of view of both intensity and phase of occurrance.
- Just before the minimum radius is reached the speed of the imploding gas-water interface exceeds the velocity of sound in the gas.
For an acoustic period of 37.7 
s this happens about 10 ns before Rmin.
- The SL light is also emitted prior to the minimum with about 5-10 ns.
- At low sound fields the bubble can be trapped in a non-light-emitting state where its bounces can be accurately described by fluid mechanics over the entire period of the motion. The intensity is a strongly increasing function of the pressure amplitude up to a treshold amplitude where the bubble is destroyed.
- The transition to the emitting state is characterized by the drop of the ambient (equilibrium) radius and of the `breathing bounces'.
So far we have learned the main features of the SL flash. Let us now turn
to
the examination of the dependence of these features on experimental
parameters,
and
see how they are related to quantities characterizing the motion of the
bubble.
The transition from the non-emitting phase to the emitting one and the
phenomena
accompanying it are not understood.
Not only the driving pressure dramatically the intensity of the SL
light but the number of photons per flash is strikingly sensitive to the
ambient
temperature. At 
C the purple light emitted by the bubble is so
bright
that it can be seen by the unaided eye even in the presence of external
lighting
but at 
C SL is barely visible in a darkened room.
Regarding the sensitivity of SL to external parameters it is not
surprising
that
the type of liquid and gas used in the experiment proved to be of crucial
importance. To date, no liquids other than water and glycerine-water
mixture
have been shown to demonstrate SBSL, although there is no a priori reason
why
it
should not exist in many liquids. It is possible to trap air bubbles also
in
nonaquos fluids, but at high drive levels these systems have resisted the
attempts to observe the transition to SL. However, the range of gases
that
exhibit
SL is quite large. Air produces remarkably intensive SL. One would expect
that
nitrogen should behave similarly since air is 80 
.
Surprisingly,
a bubble produces a very dim light even after adding the
proper
amount of oxygen. The key to the strong SL in air is the presence of
argon.
It was found that about 1 of it dramatically stabilizes the bubble
motion
and increases the light emision by over an order of magnitude to a value
that
exceeds the SL of either gas alone.
Heavy water has a strong effect on the spectrum of hydrogenic gases
yielding a
spectral peak well shifted from the one of a hydrogen bubble in water.
The partial pressure of the dissolved gas i.e., the concentration of the
gas
used in the experiment substantially influences the synchronicity of the
flashes.
Following the historical path of SL studies it can be concluded that the
parameter space
of multiple bubble sonoluminescence (MBSL) is larger. The majority of the
measurements led to the same results as in the case of SBSL, but more
knowledge
about the possible influence of various factors could be gathered. Of
special
importance is the possibility of using other liquids than water.
Jarman studied MBSL for 15 liquids with widely different properties. The
best
correlation of the MBSL intensity seemed to be with the quantity
of the liquidwhere 
and 
are the coefficient of the surface
tension, and respectively the partial pressure of the vapor. Young found
that all the liquid metals produced MBSL and it can be well correlated
with the
thermal diffusivities. One of the most intriguing puzzle related to MBSL
is
that its spectrum is not always smooth but contains lines characteristic
to the
bubble content. Presently SBSL constitutes the exclusive object of study
for
scientist engaged with sonoluminescence because its parameter space is
extremely rich possibly having several surprizes in store. Its
exploration has
just begun.
The possibility of conversion of acoustic energy into electro-magnetic
energy
with the help of bubbles is an intriguing problem in itself, but it is
not
condamnable that the large area of applicability of the phenomenon
enhanced
considerably the interest of physicists. The amazing robustness of SBSL
suggests
that there may be technologicalapplications in a variety of disciplines.
Consider the measurements of the synchronicity of this phenomenon which
demonstrated
that the stability of the system is of the order of 5 parts in 
.
Those measurements were made without knowledge of the origin of this
stability
and there were no serious efforts to improve it. That suggests that it
might be
possible to develop a cheap precision frequency source based on SBSL.
This phenomenonis primarily a diagnostic indicator of the enormous energy
concentration that can arise from the implosive collapse of a cavitation
bubble. The
technological use of this energy concentration has great promise. There
is
considerablepotential for influencing chemical reactions in an extended
region of
violent cavitation activity in MBSL.
Research in the relatively new discipline of sonochemistry suggests that
many
chemical reactions can be influenced by ultrasound - a technology whose
potential for industrial applications is gradually
being recognized. The necessary time for certain reactions can be
decreased
from several hours to a few milliseconds by this method.
Industrial-sized reactors that take advantage of these gains are being
developed for commercial use.The production of amorphous
(noncrystallized) iron,
is of considerable commercial interest for its catalytic capabilities.
It is difficult to cool a liquid metal rapidly enough to prevent
crystallization.
However, in the chemical reactor within acavitation bubble, ferrous
compounds
can be decomposed into free atoms and quenched on such short time scales
that
solidification of the iron can occur before crystallization.
Amorphous iron is easily produced on a laboratory scale by this technique.
To date, SBSL has been demonstrated only in water and mixtures of
glycerin
and water. MBSL is known to occur in liquid metals such as mercury. If
SBSL
could
be demonstrated in mercury, and the scaling parameter
holds,
then one should expect sonoluminescence intensities
nearly 10000 times greater than what one finds for water.
Energy concentrations of , temperatures exceeding 50000 K,
optical
pulse synchronicities to a few parts in , pulse durations of 50
ps,
production of exotic chemical species and imploding shockwaves-all this
from a
simple mechanical system costing a hundred dollars to construct!
From a practical perspective, the sensitivity of SL to gas content and
ambient
temperature suggests that further substantial improvements in the
characteristics of the emitted radiation are possible. Although the
temperatures encountered so
far are low from the point of view of nuclear physics, the idea of the
possibility of achieving nuclear fusion in a sonoluminescing bubble came
up.
Calculations suggest that temperatures as high as 
K are to be
expected .
This result has prompted calculations of the possibility of inertial
confinement fusion with a deuterium-tritium gas mixture, which yield a
qualified estimate
of 40 neutrons per second under ideal conditions.
The importance of understanding SL becomes even more evident if we regard sonodynamics. Sonodynamic therapy is a promising new modality for cancer treatment, based on the synergistic effect on tumor cell.The combination of a drug (typically a photosensitizer) and ultrasound has a killing effect on the cell. The mechanism of sonodynamic action was said to involve the photoexcitation of the sensitizer by SL light.
Last but not least, mention must be made of the idea advanced by Chodos. He suggests that SL may provide the tentative first step toward a new method of particle acceleration. But what is going on in a sonoluminescing bubble? More than half a dozen different theories have been put forward, yet experiments are well ahead theory. Even the radiation mechanism has not been clarified up to now. The most acceptable theory so far is the one attributing the light to the high concentration of energy in the centre of the bubble due to the implosion of spherical shock-waves that are formed as a result of the violent collapse of the bubble. Hydrodynamic calculations that try to reproduce the processes taking place in the bubble show that, indeed, during the collapse a shock-wave is launched toward the centre which at the moment of the "self-collision" leads to very high temperatures and pressures in the centre. The results are in good agreement with the data. However, reliability of these calculations can be subjected to serious objections as they don't take into consideration some molecular effects beyond hydrodynamics which clearly have a determining role in this phenomenon. A second question to ask is about the strange behaviour of the bubble and the dependence of SL on the different factors. For the understanding of this aspect one should get deeply involved with bubble dynamics which even up to the present has not given the answer to almost any of the puzzling problems related to SL. Sonoluminescence is one of the most controversial problems in contemporary physics due to the fabulous complexity of the phenomenon. If we recall the already realized and imaginably achievable applications mentioned above we can conclude that sonoluminescence provides an outstandingly promising and exciting area of research.
BIBLIOGRAFY
L.A.Crum, Phys. Today, Sept (1994) 22.
S.J.Putterman, Scien. Amer. Febr (1995).