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.

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. They 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 degrees Celsius was postulated, making it a possible source for nuclear fusion energy.

Properties

The bubble must be sitting in a pressure antinode of the standing wave in order to be trapped in position and sonoluminesce. A bubble nearby to one of these antinodes will tend to migrate towards it. The frequencies of resonance depend on the shape and size of the container.

Here are some facts about sonoluminescence:

• 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 micrometer 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 reaching even 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 kelvin, up to a possible temperature in excess of one million degrees. Some estimates put the inside of the bubble at one billion degrees . 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 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 confirmed and are highly debated.

Theories

The mystery of how a low-energy-density sound wave can concentrate enough energy in a small enough volume to cause the emission of light is still unsolved. It requires a concentration of energy by about a factor of 10¹² (one trillion).

There are two prominent theories to account for the light generated by the collapsing bubble:

1. Dr. Claudia Eberlein, a physicist at the University of Sussex, suggests 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 believes 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 kilometers per hour). Water ice and Wint-O-Green Lifesavers can give off light when they crack (called triboluminescence), 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.

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.

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

The achievement of fusion through sonoluminescence was fictionalized in the movie Chain Reaction, starring Keanu Reeves and Morgan Freeman.

Shrimpoluminesence

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.

References

• Putterman, S. J. "Sonoluminescence: Sound into Light," Scientific American, Feb. 1995, p.46.
• 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)

See also

• Cold fusion
• Bubble fusion