This page aims to give you a brief idea of the progression of theory and experiment surrounding sonoluminescence
Sonoluminescence is the art of creating light from sound. It was first observed by H. Fresnel and H. Schultes at the University of Cologne in 1943. During the Second World War there was a flurry of activity in the field of ultrasonics as there was a huge interest in developing radar systems. It was noticed that clouds of bubbles formed in these fields that gave off short bursts of radiation in an unpredictable manner. This effect was later named multi-bubble sonoluminescence (MBSL).
The periodic nature of the light emitted during sonoluminescence was first reported in two papers published in 1960 (Jarman and Negishi). Both these papers commented that the light was emitted during the last phase of the bubble collapse. Sakensa and Nyborg (1970) then studied the MBSL under more controlled experimental conditions and they found that emitted at the same frequency as the acoustic field i.e. one flash every acoustic cycle.
MBSL as a process is very difficult to understand, so to simplify the problem it was necessary that a single bubble could be trapped for analysis, this procedure was aptly named single-bubble sonoluminescence (SBSL). This was first achieved by Gaitan and Crum in 1989. He developed a method whereby a single bubble could be trapped in a standing resonant field and under certain conditions could be made to emit radiation.
Physicists then took up the challenge to find out as much as possible about this mysterious phenomenon. The exact theory still has to be agreed upon but what follows is a succinct account of the field as it stands.
If a standing resonant acoustic field is set up across a body of water a bubble can become trapped at the antinode due to a force called the Bjerknes force. This force is explained by the diagram below.
Due to the continuous nature of the field and the finite size of the bubble we can see that there is a pressure gradient across the bubbles. Since pressure is force per unit area this difference in pressure on either side of the bubble results in a force acting on the bubble towards the area of lower pressure. During the rarefaction phase this force is towards the antinode at the centre and during the compression phase this acts away from the antinode. However since the bubble expands during the rarefaction phase it is larger and the pressure gradient across the bubble is bigger than in the compression phase. This means that the force during the rarefaction is larger than during the compression. Over the whole acoustic cycle this results in a net force towards the antinode.
If the amplitude of an acoustic field on a liquid is greater than the ambient pressure it results in a negative pressure during the expansion phase that puts the liquid under tension. At a large enough tension the liquid breaks apart resulting in cavitation of clouds of bubbles in which the bubbles often organize themselves into dendrtitc structures (e.g. streamers, Neppiras, 1980). Gaitan (1989) found that as he reduced the driving pressure the streamers became thinner until only one bubble remained.
There are three ways of investigating this phenomenon.
The bubble dynamics tells us about how the size of the bubble changes in the acoustic cycle o more precisely the motion of the bubble walls. The method of observing the bubble dynamics is known as Mie scattering. This results in a bubble motion relative to the acoustic field as shown below.
As one can see the motion of the bubble wall is highly non linear with respect to the field. During the rarefaction phase the expansion is slow because of the opposition of atmospheric pressure and can be considered to be isothermal. At the maximum radius the bubble radius has increased by an order of magnitude resulting in a volumetric change of three orders of magnitude. During the expansion there is little gas inflow relative to the volume change and so at the maximum radius there is a very small internal pressure. At the switch in direction of pressure this results in a very rapid collapse during which the bubble walls reach speeds of four times the speed of sound. At the minimum radius there is a very sharp deceleration. The minimum radius is at nearly the Van der Waals hard core radius. During the collapse the centre of the bubble is heated and at about 200ps before the minimum radius a flash and a short burst of sound are emitted. The flash lasts between (50-300)ps and in the region of 105-107 photons are emitted in each flash. The minimum radius is followed by a series of after bounces as the bubble finds its way back to the ambient radius.
This cycle repeats for every acoustic cycle, which for a 100ml round bottomed flask is about 50ms, with an error in regularity of only 40ps. Bubbles can remain stable for long while depending on the conditions this is talked about later.
The diagram shows how the bubble behaviour changes as function of drive pressure.
On the whole the bubble dynamics are well described by theory. The Classical Rayleigh-Plesset equation equates the bubble wall motion to the mass/energy transfer. However one of the assumptions of this model is that the bubble wall does exceed the speed of sound in the liquid. This is not true for the last 10ns of the collapse and therefore the last stages of the collapse and the after bounces are not as well described. Theorists have come up with many clever extensions to the classical equation to make it more accurate e.g.
Sonoluminescence is very sensitive to experimental parameters and this results in a small window of stability for the sonoluminescence.
A threshold pressure is needed for a certain bubble radius in order for sonoluminescence to occur and as the drive pressure is increased the intensity of light output increases. However the maximum drive that produces stable long lasting sonoluminescence is limited due to instabilities in the bubble. There are two types of instability
Diffusive Instabilities. If the gas content of the liquid medium is too high then the acoustic field causes the gas to coalesce and the bubbles grow and become unstable. If the gas content of the water is too low then there is a high chemical potential between the bubble and its surroundings and the bubble dissolves. For the bubbles to be stable there must be no net mass flow across the bubble wall this usually means degassing the water of about 20% of its dissolved air at ambient temperature and pressure. This instability limits the drive pressure that any bubble can maintain.
Shape Instabilities. The bubble may move slightly in the field causing distortions or have small thermal fluctuations on its surface. this causes distortions that may lead to the annihilation of the bubble as it expands. this limits the size of the ambient radius of the bubble.
In order to upscale sonoluminescence we have to be able to expand the parameter range by reducing the effect of the instabilities.. Matula et al (2000) conducted sonoluminescence experiments in a parabolic flight and found that the light output was increased at all wavelengths in microgravity suggesting that gravity does indeed cause an instability. Also to be investigated is the so called "second stability branch" where, rather than just minimising both effects, the mass inflow during the rarefaction is made to balance the mass outflow during compression. This would be done by a suitable choice of liquid and gas content.
In a paper by Barber et al (1994) it was found that lowering the ambient pressure made the bubbles dimmer and they did not behave regularly. However if the pressure was increased there was little difference in the behaviour of the bubble however this may not be true for all liquids and increasing the ambient pressure may help to stabilise sonoluminescence in otherwise unsuitable liquids. Also in the same paper it was found that decreasing the temperature of the water increased the intensity of light emission at all wavelengths. In fact decreasing the temperature from 40oC to -6oC then the light emission increases by a factor of 100.
Didenko et al (2000) found, from their investigations into the chemistry taking place within the bubble, that liquids that would be good at supporting sonoluminescence should have low vapour pressures and a high content of atoms that form soluble products in the liquid.
Ashokkumar et al (2000) found that the intensity of the light was decreased by the addition of volatile solutes e.g. methanol or ethanol. They noticed that the radial dynamics of the bubble remained the same meaning that the decrease in light intensity was only due to energy absorption within the bubble. They claimed that the volatile solutes would dissolve into the bubble during the rarefaction and hinder heating.
Hiller and Putterman (1995) made an investigation into how the composition of gas within the bubble affected the sonoluminescence output. They found that neither 100% Nitrogen or 80% Nitrogen with 20% Oxygen resulted in sonoluminescence. It wasn't until trace amounts of noble gases were added that the sonoluminescence was seen. The brightest sonoluminescence was seen from 1% Xenon dissolved in Oxygen.
Krefting (2002) and Holzfuss(1998) have both managed to upscale sonoluminescence by switching on a second harmonic acoustic pressure at the last moment of collapse.
Generally the spectra given off by sonoluminescence is continuous with no spectral peaks but at really low drive amplitudes (where the light is not visible to the naked eye) Yasui et al (2001) saw the OH- line measurements taken over several days.
The intensity of the light increases as a function of acoustic drive pressure provided the bubble is within the stable regime. The intensity of the light also increases from the visible to the UV however a full spectrum is difficult to obtain because of the absorption of the UV wavelengths by the surrounding liquid.
The spectra has been successfully compared to a blackbody spectra at 40 000K and to Bremstrahlung radiation from a plasma at 100 000K (see below). All theories suggest a high temperature but there is not universal agreement about the correct theory. The likely sources of radiation are blackbody, Bremstrahlung, ion-electron recombination or a combination of all of them.
The time dependant spectra was measured using time correlated single photon counting methods (Gompf et al 1997) and a streak camera (Gompf et al 1998) and it was found that the flash lasted (50-300)ps and that all the wavelengths were switched on at the same time.
There have been many proposed explanations of this phenomena here I will only briefly discuss some of them;
None of the models explains all of the observations exactly the relatively long flash widths favour the adiabatic heating model because the hot spot would be short lived. However the fact that all of the wavelengths are "switched on" at the same time favours the hotspot model because the light emitting region in the adiabatic heating mode would be gradually heated up and we would expect the lower energy red wavelengths to be emitted first. On the other hand the shock wave would cause an instant jump in temperature "switching on" all the wavelengths at once.
This is the most recently attempted and least known about methods of investigating sonoluminescence. There is evidence to suggest that during the collapse the temperature reaches a sufficient level that the reaction rate f any chemically active substance is accelerated such that by the time of the sonoluminescence flash they are converted to soluble substances and dissolve out.
This is known as the "dissociation hypothesis" and was proposed by Lohse in 1997. The chemical reactions taking place are thought to be the following
These products are very soluble and dissolve readily in the water surrounding the bubble. this leaves a monatomic atmosphere inside the bubble. Didenko et al (2000) directly measured the change in concentration of OH radicals and NO in a small 15ml flask by reacting them with fluorescent dyes and measuring the fluorescence given off. The changes they found were in good agreement with predicted diffusion from the bubble.
They proposed therefore that high solubilities of these products made water such a good driving liquid for SBSL. They theorised that the presence of polyatomic products would hinder the heating. This should be considered when trying to upscale sonoluminescence or choosing other liquids that may be able to sustain SBSL.
At present there is much interest in this field due to recent, highly controversial claims from Taleyarkhan et al (2002), that they induced nuclear fusion in a sonoluminescing bubble. Attempts to reproduce his results failed but the search goes on...
SBSL