## Sonoluminescence (Wiki)

Single-bubble sonoluminescence - A single, cavitating bubble.
Long exposure image of multi-bubble sonoluminescence created by a high-intensity ultrasonic horn immersed in a beaker of liquid
Another long exposure image of sonoluminescence in a beaker of water. Each bright blue dot is an individual bubble that is emitting light.

Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound.

## History

The Royal Navy employed the services of physicist Lord Rayleigh in the early 20th century to understand the degradation of ship propellers. Lord Rayleigh concluded that collapsing air bubbles were the cause.[1]

The sonoluminescence effect was first discovered at the University of Cologne in 1934 as a result of work on sonar. H. Frenzel and H. Schultes put an ultrasound transducer in a tank of photographic developer fluid. They hoped to speed up the development process. Instead, they noticed tiny dots on the film after developing and realized that the bubbles in the fluid were emitting light with the ultrasound turned on. It was too difficult to analyze the effect in early experiments because of the complex environment of a large number of short-lived bubbles. (This experiment is also ascribed to N. Marinesco and J.J. Trillat in 1933, which also credits them with independent discovery). This phenomenon is now referred to as multi-bubble sonoluminescence (MBSL).

In 1989 a major experimental advance was introduced by Felipe Gaitan and Lawrence Crum, who produced stable single-bubble sonoluminescence (SBSL). In SBSL, a single bubble trapped in an acoustic standing wave, emits a pulse of light with each compression of the bubble within the standing wave. This technique allowed a more systematic study of the phenomenon, because it isolated the complex effects into one stable, predictable bubble. It was realized that the temperature inside the bubble was hot enough to melt steel. Interest in sonoluminescence was renewed when an inner temperature of such a bubble well above one million kelvins was postulated. This temperature is thus far not conclusively proven, though recent experiments conducted by the University of Illinois at Urbana–Champaign indicate temperatures around 20,000 K. Research has also been carried out by Dr. Klaus Fritsch of John Carroll University, University Heights, Ohio.

## Properties

Sonoluminescence can occur when a sound wave of sufficient intensity induces a gaseous cavity within a liquid to collapse quickly. This cavity may take the form of a pre-existing bubble, or may be generated through a process known as cavitation. Sonoluminescence in the laboratory can be made to be stable, so that a single bubble will expand and collapse over and over again in a periodic fashion, emitting a burst of light each time it collapses. For this to occur, a standing acoustic wave is set up within a liquid, and the bubble will sit at a pressure anti-node of the standing wave. The frequencies of resonance depend on the shape and size of the container in which the bubble is contained.

• The light flashes from the bubbles are extremely short—between 35 and a few hundred picoseconds long—with peak intensities of the order of 1–10 mW.
• The bubbles are very small when they emit the light—about 1 micrometre in diameter—depending on the ambient fluid (e.g., water) and the gas content of the bubble (e.g., atmospheric air).
• Single-bubble sonoluminescence pulses can have very stable periods and positions. In fact, the frequency of light flashes can be more stable than the rated frequency stability of the oscillator making the sound waves driving them. However, the stability analyses of the bubble show that the bubble itself undergoes significant geometric instabilities, due to, for example, the Bjerknes forces andRayleigh–Taylor instabilities.
• The addition of a small amount of noble gas (such as helium, argon, or xenon) to the gas in the bubble increases the intensity of the emitted light.

Spectral measurements have given bubble temperatures in the range from 2,300 K to 5,100 K, the exact temperatures depending on experimental conditions including the composition of the liquid and gas.[2] Detection of very high bubble temperatures by spectral methods is limited due to the opacity of liquids to short wavelength light characteristic of very high temperatures.

Writing in Nature, chemists David J. Flannigan and Kenneth S. Suslick describe a method of determining temperatures based on the formation of plasmas. Using argon bubbles insulfuric acid, their data show the presence of ionized molecular oxygen O2+, sulfur monoxide, and atomic argon populating high-energy excited states, which confirms a hypothesis that the bubbles have a hot plasma core.[3] The ionization and excitation energy of dioxygenyl cations, which they observed, is 18 electronvolts. From this they conclude the core temperatures reaches at least 20,000 kelvin.[4]

## Rayleigh–Plesset equation

The dynamics of the motion of the bubble is characterized to a first approximation by the Rayleigh-Plesset equation (named after Lord Rayleigh and Milton Plesset):

 $R\ddot{R} + \frac{3}{2}\dot{R}^{2} = \frac{1}{\rho}\left(p_g - P_0 - P(t) - 4\mu\frac{\dot{R}}{R} - \frac{2\gamma}{R}\right)$

This is an approximate equation that is derived from the incompressible Navier-Stokes equations (written in spherical coordinate system) and describes the motion of the radius of the bubble R as a function of time t. Here, μ is the viscosity, p the pressure, and γ the surface tension. The over-dots represent time derivatives. This equation, though approximate, has been shown to give good estimates on the motion of the bubble under the acoustically driven field except during the final stages of collapse. Both simulation and experimental measurement show that during the critical final stages of collapse, the bubble wall velocity exceeds the speed of sound of the gas inside the bubble.[5] Thus a more detailed analysis of the bubble's motion is needed beyond Rayleigh-Plesset to explore the additional energy focusing that an internally formed shock wave might produce.

## Mechanism of phenomenon

The mechanism of the phenomenon of sonoluminescence remains unsettled. Theories include: hotspot, bremsstrahlung radiation, collision-induced radiation and corona discharges, nonclassical light, proton tunneling, electrodynamic jets and fractoluminescent jets (now largely discredited due to contrary experimental evidence).

From left to right: apparition of bubble, slow expansion, quick and sudden contraction, emission of light

In 2002, M. Brenner, S. Hilgenfeldt, and D. Lohse published a 60-page review "Single bubble sonoluminescence" (Reviews of Modern Physics 74, 425) that contains a detailed explanation of the mechanism. An important factor is that the bubble contains mainly inert noble gas such as argon or xenon (air contains about 1% argon, and the amount dissolved in water is too great; for sonoluminescence to occur, the concentration must be reduced to 20–40% of its equilibrium value) and varying amounts of water vapor. Chemical reactions cause nitrogen and oxygen to be removed from the bubble after about one hundred expansion-collapse cycles. The bubble will then begin to emit light"Evidence for Gas Exchange in Single-Bubble Sonoluminescence", Matula and Crum, Phys. Rev. Lett. 80 (1998), 865-868). The light emission of highly compressed noble gas is exploited technologically in the argon flash devices.

During bubble collapse, the inertia of the surrounding water causes high pressure and high temperature, reaching around 10,000 kelvins in the interior of the bubble, causing the ionization of a small fraction of the noble gas present. The amount ionized is small enough for the bubble to remain transparent, allowing volume emission; surface emission would produce more intense light of longer duration, dependent on wavelength, contradicting experimental results. Electrons from ionized atoms interact mainly with neutral atoms, causing thermal bremsstrahlung radiation. As the wave hits a low energy trough, the pressure drops, allowing electrons to recombine with atoms and light emission to cease due to this lack of free electrons. This makes for a 160-picosecond light pulse for argon (even a small drop in temperature causes a large drop in ionization, due to the large ionization energy relative to photon energy). This description is simplified from the literature above, which details various steps of differing duration from 15 microseconds (expansion) to 100 picoseconds (emission).

Computations based on the theory presented in the review produce radiation parameters (intensity and duration time versus wavelength) that match experimental results with errors no larger than expected due to some simplifications (e.g., assuming a uniform temperature in the entire bubble), so it seems the phenomenon of sonoluminescence is at least roughly explained, although some details of the process remain obscure.

Any discussion of sonoluminescence must include a detailed analysis of metastability. Sonoluminescence in this respect is what is physically termed a bounded phenomenon meaning that the sonoluminescence exists in a bounded region of parameter space for the bubble; a coupled magnetic field being one such parameter. The magnetic aspects of sonoluminescence are very well documented.[6]

Exotic proposals

An unusually exotic theory of sonoluminescence, which has received much popular attention, is the Casimir energy theory suggested by noted physicist Julian Schwinger[7] and more thoroughly considered in a paper by Claudia Eberlein[8] of the University of Sussex. Eberlein's paper suggests that the light in sonoluminescence is generated by the vacuum within the bubble in a process similar to Hawking radiation, the radiation generated at the event horizon of black holes. According to this vacuum energy explanation, since quantum theory holds that vacuum contains virtual particles, the rapidly moving interface between water and gas converts virtual photons into real photons. This is related to theUnruh effect or the Casimir effect. If true, sonoluminescence may be the first observable example of quantum vacuum radiation. The argument has been made that sonoluminescence releases too large an amount of energy and releases the energy on too short a time scale to be consistent with the vacuum energy explanation,[9] although other credible sources argue the vacuum energy explanation might yet prove to be correct.[10]

## Nuclear reactions

Some have argued that the Rayleigh-Plesset equation described above is unreliable for predicting bubble temperatures and that actual temperatures in sonoluminescing systems can be far higher than 20,000 Kelvin. Some research claims to have measured temperatures as high as 100,000 Kelvin, and speculates temperatures could reach into the millions of Kelvin.[11] Temperatures this high could cause thermonuclear fusion. This possibility is sometimes referred to as bubble fusion and is likened to the implosion design used in the fusion component of thermonuclear weapons.

On January 27, 2006, researchers at Rensselaer Polytechnic Institute claimed to have produced fusion in sonoluminescence experiments.[12][13]

Experiments (2002, 2005) of R. P. Taleyarkhan using deuterated acetone showed measurements of tritium and neutron output consistent with fusion but the low quality of the papers and the doubts cast by the report into his scientific misconduct had made him lose all credibility that he formerly had amongst the scientific community.[14] On August 27, 2008 he was stripped of his named Arden Bement Jr. Professorship, and forbidden to be a thesis advisor for graduate students for at least the next 3 years.[15][16]

## Biological sonoluminescence

Pistol shrimp (also called snapping shrimp) produce a type of sonoluminescence from a collapsing bubble caused by quickly snapping a specialized claw. The light produced is of lower intensity than the light produced by typical sonoluminescence and is not visible to the naked eye. The light and heat produced may have no direct significance, as it is the shockwave produced by the rapidly collapsing bubble which these shrimp use to stun or kill prey. However, it is the first known instance of an animal producing light by this effect and was whimsically dubbed "shrimpoluminescence" upon its discovery in 2001.[17] It has subsequently been discovered that another group of crustaceans, the mantis shrimp, contains species whose club-like forelimbs can strike so quickly and with such force as to induce sonoluminescent cavitation bubbles upon impact.[18]

## References

1. ^ Didenko, Y.T.; McNamara, III, W.B.; Suslick, K.S. (January 2000). "Effect of Noble Gases on Sonoluminescence Temperatures during Multibubble Cavitation". Physical Review Letters 84(4): 777–780. Bibcode:2000PhRvL..84..777D. doi:10.1103/PhysRevLett.84.777. PMID 11017370.
2. ^ David J. Flannigan and Kenneth S. Suslick (2005). "Plasma formation and temperature measurement during single-bubble cavitation". Nature 434 (7029): 52–55.Bibcode:2005Natur.434...52F. doi:10.1038/nature03361. PMID 15744295.
3. ^ "Temperature inside collapsing bubble four times that of sun | Archives | News Bureau | University of Illinois". News.illinois.edu. 2005-02-03. Retrieved 2012-11-14.
4. ^ Bradley P. Barber and Seth J. Putterman, "Light Scattering Measurements of the Repetitive Supersonic Implosion of a Sonoluminescing Bubble," Phys Rev Lett 69, 3839-3842 (1992)
5. ^ "Within article "Cold Fusion: A History of Mine"". Infinite-energy.com. 1989-03-23. Retrieved 2012-11-14.
6. ^ Phys. Rev. Lett. 76, 3842 - 3845 (1996); http://arxiv.org/abs/quant-ph/9506024v1
7. ^ K.A. Milton, “Dimensional and dynamical aspect of the Casimir effect: understanding the reality and significance of vacuum energy”, hep-th/0009173 (2000) http://arxiv.org/abs/hep-th/0009173
8. ^ S.Liberati, F.Belgiorno, Matt Visser, "Comment on Dimensional and dynamical aspects of the Casimir effect: understanding the reality and significance of vacuum energy", hep-th/0010140v1 (2000) http://arxiv.org/abs/hep-th/0010140v1
9. ^ Nature China (2008-10-15). "Sonoluminescence: Baking bubbles : Article : Nature China". Nature.com. Retrieved 2012-11-14.
10. ^ "RPI: News & Events - New Sonofusion Experiment Produces Results Without External Neutron Source". News.rpi.edu. 2006-01-27. Retrieved 2012-11-14.
11. ^ "Using Sound Waves To Induce Nuclear Fusion With No External Neutron Source". Sciencedaily.com. 2006-01-31. Retrieved 2012-11-14.
12. ^ Purdue physicist found guilty of misconduct, Los Angeles Times, July 19, 2008, Thomas H. Maugh II
13. ^ Jayaraman, K. S. (2008). "Bubble fusion discoverer says his science is vindicated". Nature India. doi:10.1038/nindia.2008.271.
14. ^ Press, Associated (August 27, 2008). "Purdue reprimands fusion scientist for misconduct". USA Today. Retrieved 2010-12-28.
15. ^ Detlef Lohse, Barbara Schmitz and Michel Versluis (2001). "Snapping shrimp make flashing bubbles". Nature 413 (6855): 477–478. doi:10.1038/35097152. PMID 11586346.
16. ^ S. N. Patek and R. L. Caldwell (2005). "Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp". Journal of Experimental Biology 208 (Pt 19): 3655–3664. doi:10.1242/jeb.01831. PMID 16169943.

## General references

Newer research papers largely ruling out the vacuum energy explanation