# Sonoluminescence

File:Sonoluminescence.jpg
Long exposure image of multi-bubble sonoluminescence created by a high intensity ultrasonic horn immersed in a beaker of liquid.

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)

More than 50 years later, in 1989, a major advancement in research was introduced by Felipe Gaitan (or Felip Caitan) and Lawrence Crum, who were able to produce single bubble sonoluminescence (SBSL). In SBSL, a single bubble, trapped in the acoustic standing wave, emits a pulse of light with each compression of 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 megakelvin was postulated. This temperature is thus far not conclusively proven.

## Properties

Sonoluminescence may 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 setup 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.
• The bubbles are very small when they emit the light—about 1 micrometre in diameter.
• 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.
• For unknown reasons, 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 dramatically.

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 10,000 kelvins, up to a possible temperature in excess of one megakelvin. 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 idealizations.

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 could be produced within these tiny bubbles. This possibility is sometimes referred to as bubble fusion. 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.

Writing in Nature, chemists David J. Flannigan and Kenneth S. Suslick study argon bubbles in sulfuric acid and show that ionized oxygen ${\displaystyle {\mbox{O}}_{2}^{+}}$, 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).

## Mechanism of phenomenon

The high compression of a small bubble of fluid is similar to the explosive compression of a pellet of material by laser beams, one of the methods proposed for creating nuclear fusion, which has not been very successful. Prosperetti and others think that it is impossible for a bubble to maintain a perfectly spherical shape as it compresses, with either the laser or acoustic compression method, ruling out the high temperatures required for nuclear fusion.

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

Other theories include hotspot, bremsstrahlung radiation, collision induced radiation and corona discharges to non-classical light.

Two largely discredited theories of sonoluminescence which have none the less gained a disproportionately large dissemination throughout the general public are:

1. Claudia Eberlein, a physicist at the University of Sussex, suggested in 1996 that the light 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. The fact that addition of inert gases changes the properties of light emission, when emission should only depend on the properties of the vacuum and interface movement, is evidence against this theory.
2. Andrea Prosperetti, a professor of mechanical engineering at Johns Hopkins University, posited that the light is generated as a jet of liquid shoots from one side of the bubble to the other at very high speed (around 6000 km/h). Water ice and Wint-O-Green Lifesavers can give off light when they crack (called fractoluminescence), and it is thought that the high pressures inside the bubble cause the water to form ice-like structures. As the jet hits the other side, the water "fractures" in the same way that silly putty and other non-Newtonian fluids behave like solids when subjected to sudden stresses. The fracture causes a release of photons. Prosperetti believes that an introduction of noble gases changes the way the water molecules align themselves, creating flaws in the crystal-like structure that enhance the fracturing effect. This theory may be tested by firing hyper-fast jets of water to see if it produces light without the acoustic cavitation.

Both theories are currently seen in the scientific community as being drastically overcomplicated and implausible, with the Hawking radiation theory being the most objectionable of the two, bordering on pseudoscience. Prosperetti himself no longer believes that his original theory is applicable to stable single bubble sonoluminescence, as high end, high speed cameras have recorded the phenomena and have shown no liquid jets present.

## Shrimpoluminescence

Pistol shrimp (also called snapping shrimp) produce sonoluminescence from a collapsing bubble caused by snapping a specialized claw quickly closed. 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. [2]

## Cultural references

Sonoluminescence was featured in the movie Chain Reaction, starring Keanu Reeves and Morgan Freeman.