What Causes Sonoluminescence and Acoustic Levitation?

[Serj]

I can't find it anywere, why does the bubble collapse? And could someone clarify acoustic levitation.

 

[Mk]

Sonoluminescence may occur when sound wave of sufficient intensity induces a gaseous cavity within a liquid to quickly collapse. This cavity may take the form of a pre-existing bubble, or may be generated through cavitation. Cavitation is a phenomenon very similar to boiling. It is when low pressure cavities explode. This happens most commonly in the popping/cracking of your knuckles.

Sonoluminescence in the laboratory can be made to be stable, so that a single bubble (about 1 µm diameter) will expand and collapse over and over again in a periodic fashion, emitting a burst of light (between 35-700 picoseconds)each time it collapses. For this to occur, an acoustic standing wave is created in a liquid, and the bubble will sit at a pressure anti-node of the standing wave. The frequencies of resonance depend on the shape and size of the container in which the bubble is contained.

Also another interesting thing is that the addition of a noble gas to the gas in the bubble increases the intensity of the emitted light dramatically.
The wavelength of emitted light is very short; the spectrum can reach into the ultraviolet. Light of shorter wavelengths has higher energy, and the measured spectrum of emitted light seems to indicate a temperature in the bubble of at least 10,000 kelvins, up to a possible temperature in excess of one megakelvin. Some estimates put the inside of the bubble at one gigakelvin!

Notice that this is hot enough and could initialize thermonuclear fusion. Theoretical fusion activated in this way is called bubble fusion. Recent experiments (2002, 2005) of R. P. Taleyarkhan, et.al., using deuterated acetone, show measurements of tritium and neutron output consistent with fusion, but these measurements have not been reproduced outside of the Taleyarkhan lab and remain controversial.

In Nature magazine, David J. Flannigan and Kenneth S. Suslick study argon bubbles in sulfuric acid and show that ionized oxygen, sulfur monoxide, and atomic argon populating high-energy excited states are present implying that the bubble has a hot plasma core. They point out that the ionization and excitation energy of dioxygenyl cation is 18 eV, and thus cannot be formed thermally; they suggested it was produced by high-energy electron impact from the hot opaque plasma at the center of the bubble.

The high compression of a small bubble of fluid is similar to the explosive compression of a pellet of material by laser beams, one of the methods proposed for creating nuclear fusion, which has not been very successful. Prosperetti and others think that it is impossible for a bubble to maintain a perfectly spherical shape as it compresses, with either the laser or acoustic compression method, ruling out the high temperatures required for nuclear fusion.
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http://upload.wikimedia.org/wikipedia/commons/5/5d/Sonoluminescence.png
1st slide: bubble
2nd slide: slow expansion
3rd slide: contraction!
4th slide: light!

Why do the bubbles pop? I thought the scientists make them pop!??

Here's a some pictures featuring sonic levitation: http://www.containerless.com/aal.htm

 

[Serj]

Thank you but that doesn't answer either of my questions. I will clarify. Why doesn't the bubble just keep expanding until it's so big it floats upward or destablizes and dissipate into many bubbles? How does an anitnode keep the bubble from rising?

 

[Q_Goest]

The Feb isssue of Scientific American had a good article on this that I kept. From that source:

This bubble is then trapped at the center of the cylindrical flask, where the buoyancy force that would make the bubble rise to the top is balanced by the force of the sound waves.

The buoyancy of such a small bubble is extremely slight. It would rise to the top very slowly, so the downward force similarly needs to be extremely slight. Note this is called "single bubble sonoluminescence". The phenomenon was originally found to occur when photographic plates were accidentally exposed to light emitted from large numbers of luminescent bubbles in an ultrasonic cleaning bath. It wasn't till decades later that a single bubble was isolated and held in position in the flask using sound.

Just a bit more info: the length of the sound wave is on the order of a centimeter, so the pressure on the outside of the bubble, all around the bubble, is relatively equal because the bubble is much smaller than a centimeter (ie: on the order of .01 mm). Also, the bubble doesn't simply expand and contract, it actually goes through a series of contractions that get smaller and smaller before the light is emitted.

 

[Serj]

thanks for clearing up levitation but what about the bubble collapse?

 

[Q_Goest]

Why does the bubble collapse? One can answer that on many levels, the highest one is that it is driven at close to its resonant frequency by the sound waves. But to really understand the phenomenon, you have to dig much deeper. I won't claim any expertise in this other than having studied it from the outside for a while.

There is a significant amount of chemistry taking place within the bubble. The bubble is not simply trapped air, although it can start out as an air bubble. From Nature, 25 July '02 (Yuri T. Didenko & Kenneth S. Suslick):

A bubble pulsating in water containing dissolved air is thought to contain primarily argon, because the N2, O2 and H2O that diffuse into the bubble during expansion should burn off to form soluble products during bubble compression: the "dissociation hypothesis". The expected initial products of chemical reactions inside the bubble include OH and nitrogen oxides (NOx). OH will react with organic compounds in the water or dimerize to H2O2, and NOx will react with water giving nitrite and nitrate ions.

So gasses that are disolved in the liquid can enter and exit the bubble, and can undergo chemical reactions. The force driving this is of course the pressure waves in the water which compress the bubble at roughly its resonant frequency. But how and why it collapses is also a function the chemical reactions taking place.

 

[Serj]

thank you. What occilates the bubble? Is it the transducers on the side or the one on the bottom?