Acoustic Bubble Collapse and Light
Date: Fall 2012
If you collapse a bubble underwater with a sound-wave how and why does it produce light?
There is pretty good evidence that the light is simple thermal luminescence. The vapor in the bubbles gets so hot upon the collapse that it emits light.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
The phenomenon you refer to is called “cavitation”. It has many various applications and descriptions, depending upon how the system is set up. In simplest terms a bubble of a gas is generated by a rapid change in the pressure of the liquid. When the pressure is changed from greater than to less than the vapor pressure, a bubble is formed. However, when the pressure is rapidly reduced, the bubble collapses. This results in an explosive decompression so rapidly that the local pressure and temperature increases so rapidly, a very unequal condition occurs. The pressure and temperature increase to such an extent that the bubble the vapor within the bubble gets so hot that it emits light. Other things can also happen also. For example, if this decompression occurs on the blades of a propeller, the blade can be damaged.
Cavitation is a large topic both in basic science and engineering as well. For a more detailed treatment, see the Wikipedia title “cavitation”. It is a very interesting and large topic.
That is one good question! I believe you are describing sonoluminescence.
Sonoluminescence can occur when a sound wave of sufficient intensity induces a gaseous cavitation(a bubble) within a liquid to collapse quickly. The sound wave propagating through water can be observed to spontaneously focus its energy by over a factor of one trillion, generating 20,000 K degrees and collapse. Upon collapse, a very short flash of ultraviolet light is emitted.
The motion of the bubble under sonic conditions is approximated by the Rayleigh-Plesset equation up until the collapse of the bubble:
Where R, radius of the bubble and time derivative dotR. ì is the viscosity, p the pressure, and ã the surface tension.
The focusing of energy during the formation of the shock wave upon collapse has something to do with the emission of light. But the details are unclear. The mechanism for the emission of light is highly debated and is currently under investigation.
Hoping that helps! Peter E. Hughes, Ph.D. Milford, NH
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Update: November 2011