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# Sonoluminescence

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

## History

The 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).

More than 50 years later, in 1989, a major advancement in research was introduced by Felipe Gaitan and Lawrence Crum, who were able to produce 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 deduced the temperature at about 20,000 kelvins.

## Properties

Sonoluminescence may or may not occur whenever a sound wave of sufficient intensity induces a gaseous cavity within a liquid to quickly collapse. 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 analysis of the bubble show that the bubble itself undergoes significant geometric instabilities, due to, for example, the Bjerknes forces and Rayleigh-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.

The wavelength of emitted light is very short; the spectrum can reach into the ultraviolet. Light of shorter wavelengths has higher energy, and the measured spectrum of emitted light seems to indicate a temperature in the bubble of at least 20,000 kelvins, up to a possible temperature in excess of one megakelvin. The veracity of these estimates is hindered by the fact that water, for example, absorbs nearly all wavelengths below 200 nm. This has led to differing estimates on the temperatures in the bubble, since they are extrapolated from the emission spectra taken during collapse, or estimated using a modified Rayleigh-Plesset equation (see below). Some estimates put the inside of the bubble at one gigakelvin [1]. These estimates are based on models which cannot be verified at present, and may include too many unsupported assumptions.

Temperatures this high make the study of sonoluminescence especially interesting for the possibility that it might produce a method for achieving thermonuclear fusion. If the bubble is hot enough, and the pressure in it is high enough, fusion reactions like those that occur in the Sun and other stars could be produced within these tiny bubbles. This possibility is sometimes referred to as bubble fusion.

On January 27, 2006, researchers at Rensselaer Polytechnic Institute claimed to have produced fusion reactions by sonoluminescence, without an external neutron source, according to a paper published in Physical Review Letters [2] [3]. To date, these results have not been reproduced by other members of the scientific community.

Recent experiments (2002, 2005) of R. P. Taleyarkhan, et.al., using deuterated acetone, show measurements of tritium and neutron output consistent with fusion, but these measurements have not been reproduced outside of the Taleyarkhan lab and remain controversial. Brian Naranjo of the University of California, Los Angeles, has recently completed an analysis of the Taleyarkhan results claiming that Taleyarkhan had most likely misinterpreted the radioactive decay of standard lab materials for the byproducts of nuclear fusion.

Writing in Nature, chemists David J. Flannigan [4] and Kenneth S. Suslick [5] study argon bubbles in sulfuric acid and show that ionized oxygen $\mbox{O}_2^+$, sulfur monoxide, and atomic argon populating high-energy excited states are present implying that the bubble has a hot plasma core. They point out that the ionization and excitation energy of dioxygenyl cation is 18 electronvolts, and thus cannot be formed thermally; they suggested it was produced by high-energy electron impact from the hot opaque plasma at the center of the bubble (Nature 434, 52 - 55 (03 March 2005); doi:10.1038/nature03361).

## Fluid Mechanics

The dynamics of the motion of the bubble is characterized to a first approximation by the Rayleigh-Plesset equation

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

This is an approximate equation that is derived from the compressible Navier-Stokes equations, and describes the motion of the radius of the bubble R as a function of time t. Here, η is the viscosity, p the pressure, and γ the surface tension. This equation, though approximate, has been shown to give good estimates on the motion of the bubble under the acoustically driven pressure collapse of the bubble.

## Mechanism of phenomenon

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

In 2002 M. Brenner, S. Hilgenfeldt, and D. Lohse, published a 60 page review "Single bubble sonoluminescence" (Reviews of Modern Physics 74, 425) which 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).

During bubble collapse, the inertia of the surrounding water causes high speed and high pressure, reaching around 10000 K 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.

## Exotic proposals

An unusually exotic theory of sonoluminescence, which has received much popular attention, yet is considered to have a marginal effect on the mechanism of SBSL by the scientific community at large, is the Casimir energy theory proposed by Claudia Eberlein, a physicist at the University of Sussex. In 1996, it was suggested that the light in sonoluminescence is generated by the vacuum around the bubble in a process similar to Hawking radiation, the radiation generated by the edges of black holes. Quantum theory holds that a vacuum is filled with virtual particles, and the rapidly moving interface between water and air converts virtual photons into real photons. This is related to the Unruh effect or the Casimir effect. If true, sonoluminescence may be the first observable example of quantum vacuum radiation. It is, however, argued that the mechanism leading to the above effects do not occur on the proper time scales to describe the observed spectrum of SBSL, which is thought to likely obey a classical cavitation collapse; and thus the Casimir model has been largely relegated to the position of an ancillary remnant of the field at large.

## 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. It most likely has no biological significance, and is merely a byproduct of the shock wave, 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 October of 2001. [6] It has subsequently been discovered that another group of shrimp, 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.[1]

## Cultural references

• In the X-Men comics, Alison Blaire a.k.a. Dazzler has this as her "Mutant Power".
• Sonoluminescence was featured in the movie Chain Reaction, starring Keanu Reeves and Morgan Freeman.
• Sonoluminescence is used by artists Evelina Domnitch and Dmitry Gelfand to create a three-dimensional installation; footage of this artwork is featured on a DVD, along with music by artists such as Alva Noto and Taylor Deupree, released on the Line label.[7]
• A cold fusion bomb based on sonoluminescence is featured in The Outer Limits episode Final Exam.
• Sonoluminescence is the reason for a glowing character in the Eureka episode "God Is In The Details."
• Sonoluminescence is featured as a science fair project by secondary characters Glynis & Friedman in the Joan of Arcadia episode "Jump", and is mentioned a number of times in the episodes leading up to the science fair.

## References

• Putterman, S. J. "Sonoluminescence: Sound into Light," Scientific American, Feb. 1995, p.46. (Available Online)
• H. Frenzel and H. Schultes, Z. Phys. Chem. B27, 421 (1934)
• D. F. Gaitan, L. A. Crum, R. A. Roy, and C. C. Church, J. Acoust. Soc. Am. 91, 3166 (1992)
• M. Brenner, S. Hilgenfeldt, and D. Lohse, "Single bubble sonoluminescence", Rev. Mod. Phys., April (2002).
• R. P. Taleyarkhan, C. D. West, J. S. Cho, R. T. Lahey, Jr. R. Nigmatulin, and R. C. Block, "Evidence for Nuclear Emissions During Acoustic Cavitation," Science 295, 1868 (2002). (see bubble fusion article for direct link)
• "Tiny Bubbles Implode With the Heat of a Star", New York Times article, registration and small fee may be required
1. ^ 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: 3655-3664. doi:10.1242/jeb.01831.