Sonoluminescence

Sonoluminescence is the emission of points of light from bubbles of air trapped in water, which contains intense sound waves. The concept was first hypothesized in 1933 by Reinhardt Mecke of the University of Heidelberg from the observation that intense sound from military sonar systems could catalyze chemical reactions in water. German scientists H. Frenzel and H. Schultes at the University of Cologne (Germany) first observed it in 1934 when they placed an ultrasound transducer into a container of developing fluid (for photography). Bubbles in the fluid resulted, which they realized were emitting light. Modern experiments show it to be a result of heating in a bubble when the surrounding sound waves compress its volume by approximately a million-fold.

The precise details of light generation within an air bubble are not presently known. Certain general features of the process are well understood, however. Owing to water’s high degree of incompressibility, sound travels through it in the form of high speed, high pressure waves. Sound waves in air have smaller pressure, since air is highly compressible. The transmitted power in a wave is proportional to the product of its pressure and the amplitude of its vibrational motion. This means that the wave motion is strongly amplified when a sound wave travels from water to air.

Since the speed of sound is much greater in water than in air, a small bubble within water carrying sound waves will be subjected to essentially the same pressure at every point on its surface. Thus, the sound waves within the bubble will be nearly spherical.

Spherical symmetry, along with the large amplitude of displacement of the surface of the bubble, results in extreme compression of the air at the center of the bubble. This compression takes place adiabatically ; that is, with little loss of heat, until the air is at a high enough temperature to emit light. Temperatures within a sonoluminescing bubble range between 10,000 to 100,000 K (17,540 to 179,541°F; 9,727 to 99,727°C).

At the end of the 1980s,American scientists Lawrence Crum and Felipe Gaitan produced a single bubble in an acoustic standing wave, which produced a pulse of light whenever the bubble compressed inside the wave. Based on this technology, called single bubble sonoluminescence (SBSL), scientists are able to more effectively study sonoluminescence. In fact, the temperature range within a sonoluminescing bubble has been tentatively reduced to the lower range mentioned in the preceding paragraph, around 20,000K, although this temperature has not yet been independently confirmed.

In the 2000s, theorists are working on models of sonoluminescing bubbles in which the inward-traveling wave becomes a shock wave near the center of the bubble; this is thought to account for the extremely high temperatures there. Although the maximum temperatures within a sonoluminescing bubble are not known with any certainty, some researchers are investigating the possibility of using these imploding shock waves to obtain the million-degree temperatures needed for controlled nuclear fusion.