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 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 kelvins. Research has also been carried out by Dr. Klaus Fritsch of John Carroll University, University Heights, Ohio.
Some facts about sonoluminescence:
The wavelength of the emitted light is very short; the spectrum can reach into the ultraviolet. Light of shorter wavelength 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. 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. 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. In 2008, The Purdue University (where these experiments were performed) stripped R. P. Taleyarkhan of his professorship after accusing him of research misconduct, due to his controversial works on bubble fusion.
Writing in Nature, chemists David J. Flannigan and Kenneth S. Suslick study argon bubbles in sulfuric acid and show that ionized oxygen O2+, 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.
This is an approximate equation that is derived from the compressible Navier-Stokes equations and describes the motion of the radius of the bubble as a function of time . Here, is the viscosity, the pressure, and 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.
In 2002, M. Brenner, S. Hilgenfeldt, and D. Lohse published a 60-page review 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 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.
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. 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.