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Why fiery bubbles live in a waterworld.

The prospect of carrying out chemical reactions at temperatures hotter than the sun has driven scientists to explore the process known as sonoluminescence (SN: 04/29/95, p.266). In a new study, scientists at the University of Chicago offer a theory about why it occurs in water but not in other liquids.

Sonoluminescence takes place when sound waves pass through water, forming bubbles that can concentrate huge amounts of energy. When the bubble walls collapse, they compress the gas inside, heating it tens of thousands of degrees and creating the glow that gives the phenomenon its name.

But sonoluminescence doesn't work in more viscous liquids. To find out why, Michael P. Brenner and his colleagues looked at the interplay between two instability mechanisms at work in the bubble. Their analysis appears in the July 31 Physical Review Letters.

One mechanism, called parametric instability, occurs when the bubble wall oscillates, distorting the bubble's spherical shape. Given enough time, these slow fluctuations would overwhelm the bubble and cause it to collapse in on itself.

The other mechanism, Rayleigh-Taylor instability, kicks in when a bubble is at its smallest. Then, the relatively larger movement of molecules on the surface can cause it to implode. "You're a poor little bubble, and you get hit over the head [with these molecular movements]," Brenner says.

Parametric instability, ironically, provides some protection against the Rayleigh-Taylor effect. The diagram shows how a bubble's radius responds to increasing pressure. The slow parametric oscillations (jagged line) give the bubble some time to adjust its size before it hits the Rayleigh-Taylor instability region (solid line). When the pressure reaches 1.15 atmospheres, the bubble wall collapses.

In a thicker liquid, though, the parametric instability threshold is higher than that of the Rayleigh-Taylor. Thus the Rayleigh-Taylor instability takes its quick and deadly effect at pressures too low to create sonoluminescence.
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Title Annotation:theory proposed on why sonoluminescence occurs in water but not other liquids
Publication:Science News
Article Type:Brief Article
Date:Aug 19, 1995
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