Picture a bubble in water. Its gaseous contents are enclosed by the surface tension of its spherical walls. Its shiny surface is evident to our eyes by the strong refractive mismatch across its boundary. Left on its own, gravity pushes the bubble upward, eventually breaching the water-air surface and releasing its contents. We have all observed this story countless times. What you may not have witnessed is the story of a bubble emitting what appears to be a continuous stream of light. Such is the phenomenon known as Sonoluminescence, translated as “sound into light”.
To understand this marvel of nature, again picture a bubble in water. Now imagine the surrounding water is singing with a frequency that pulls on the bubble from all directions. The bubble expands with the increasing sound, eventually growing 1000 times its size when the sound reaches its crescendo. The music then reverses course and squeezes the bubble onto itself. What follows is a violent inward collapse reaching Mach speeds and a million fold volume compression! What was once room temperature gas within the bubble is transformed into a 10,000K highly-ionized dense plasma. For the brief moment of its existence, the plasma emits a brilliant flash of light that can be seen with the unaided eye (photograph above). After this radiant climax is reached, the bubble expands back to its original size and cools to its surroundings…ready for the next encore performance.
Sonoluminescence is a well-developed field of study with a history dating back to the early 1930s. And yet many questions remain. What exactly is this dense plasma during light emission and what are its properties? How can an object smaller than a grain of sand achieve the ionization required for its blackbody radiation? Can we make Sonoluminescence hot enough to achieve nuclear fusion? The story of Sonoluminescence is far from over as scientists around the world continue to uncover its mysteries.
Would you like to know more?
- https://www.sciencenewsforstudents.org/article/cool-jobs-world-aglow – Great educational website dedicated to bringing science developments to a young audience. Scroll down to “Bright Bubbles” for section on Sonoluminescence.
- http://acoustics-research.physics.ucla.edu/sonoluminescence/ – Research website of Professor Seth Putterman at UCLA covering Sonoluminescence, dense microplasmas, and much more.
- http://www.scs.illinois.edu/suslick/# – Excellent research group led by Professor Kenneth Suslick studying the properties and applications of Sonoluminescence
- Sonoluminescence: Sound into Light
S. Putterman, Scientific American, February 1995
- Bataller, A., Kappus, B., Camara, C., and Putterman, S. “Collision Time Measurements in a Sonoluminescence Plasma with a Large Plasma Parameter.” Physical Review Letters, 113(2), 024301 (2014).
Profiled as a Focus Story in Physics, 7(72), (2014) and online
- Kappus, B., Bataller, A., and Putterman, S. “Energy Balance for a Sonoluminescence Bubble Yields a Measure of Ionization Potential Lowering.” Physical Review Letters, 111(23), 234301 (2013).