Probing the unknowns of sonoluminescence

Abstract

The passage of sound through a fluid with a trapped bubble leads to the clocklike emission of picosecond flashes of ultraviolet light. In this phenomenon, sonoluminescence (or SL), acoustic energy spontaneously focuses by over 12 orders of magnitude. SL is just one example of energy focusing in bubbly flows. Researchers studying cavitation from a fluid accelerating through a venturi tube in the 1960s observed subnanosecond flashes of light and had in fact at that time discovered the world’s fastest manmade source of light. As with many avenues of research on cavitation luminescence, not the least of which is described in Exodus 20:18 [‘‘And all the people saw the sounds’’] these wonderful insights were abandoned. SL is extremely sensitive to ambient temperature, strength of the acoustic drive, and doping with noble gases. Despite a plethora of theoretical publications the most basic aspects of SL remain unexplained. Neither the light emitting mechanism nor the size of the bubble nor the range of acoustic drives at which SL can be achieved are understood. It is also a mystery as to why water is the friendliest fluid for SL and it would be most valuable to understand why pure diatomic gas bubbles are very weak, unstable sources of SL. A reasonable picture of the energy concentrating mechanism starts from Rayleigh’s 1917 work on the high pressure developed in a collapsing bubble. Femtosecond light scattering from an SL bubble indicates that the collapse is strongly supersonic and suggests the formation of an imploding shock wave that further focuses the acoustic field. Measurements find that the flash of SL is emitted at the minimum radius where the acceleration exceeds 1011 g. The flash width is independent of wavelength from the ultraviolet to the infrared suggesting that the light is emitted when the gas is so stressed that it is in a new phase, perhaps a cold dense plasma. Methods of astronomy are being used to beat the diffraction limit and measure the size of the sonoluminescing ‘‘hot-spot.’’ The upper limit of energy focusing that can be achieved with this ‘‘star-in-a jar’’ has not yet been determined.