Sonoluminescence

I believe the credit for this one goes to my lab partner.

At first sight, those three little pin-pricks of light may not seem very spectacular. However, the phenomenon that creates them (sonoluminescence) is really quite interesting. Some scientists once believed that the light was created by the ever elusive nuclear fusion, although this idea has since been almost completely rejected. We can get to the basic idea by breaking apart the name, sonoluminescence. The "sono" refers to sound, while "luminescence" is the glow. In essence, the lights are created by sound.

In the experiment above, we start with a rectangle container filled with purified water. The metal cylinder at the top that sticks into the water is a transducer; in other words, it will create the sound for us. Sound is a pressure wave; when it travels through the water (in this case), it causes the water molecules to oscillate back-and-forth causing regions of compression, where molecules are pushed together, and rarefaction, where particles are pulled away. The sound that we used was a higher pitch than humans can hear, known as ultrasound. If the frequency of the sound (that is, how many times it oscillates per second) is chosen correctly, a standing wave will form in the water. If you and a friend each hold one side of a rope and shake it in sync with one another, a standing wave will form. You'll know when one forms because there will be at least one point on the rope that doesn't seem to move. The stationary point is called a node. Between each node is an anti-node, the point that has the most motion. The anti-node is what we're interested in.

The next step is to make a very small bubble very near the anti-node. It is a difficult task, but if done correctly, the bubble move into the anti-node and get trapped there. Because the anti-node is where the wave motion varies the most, this point will rapidly oscillate between a high pressure state (compression) and a low pressure state (rarefaction). During the high pressure state, the air in the bubble will be squeezed into a smaller volume. The opposite happens during the low pressure state; that is, bubble's volume grows and the air inside can spread out more. This should help to explain why it is so difficult to trap a bubble. If you squeeze or stretch it just a little too much, the bubble will pop forcing you to start over.

So, we have a bubble in water that is changing volume due to being trapped in an anti-node of a sound wave. Now that the experiment is set-up, we can move to our initial question: why does the bubble light up? This is a difficult question to answer and, frankly, the exact answer remains a mystery of physics. However, we can make measurements that give us some insights into the process and allow us to speculate on what it could be. The light is emitted when the bubble is squeezed. At this point, the volume that air molecules have is very small. When the bubble is squeezed, the air molecules heat up. In essence, this heat causes the molecules to emit light. What isn't known is the process through which the light is emitted. The most accepted idea is that the heat energy pulls the molecules apart. When the molecules re-form, they emit light. However, this does not explain the spectrum (or variety) of light emitted. As the experiments continue, scientists hope to one day find the answer.

While this is a very neat experiment, it might not have much practical use. However, sonoluminescence does occur in nature. Have you ever heard of the beautiful mantis shrimp? Perhaps you've seen the comic about it that's been getting passed around lately. The mantis shrimp can close its claw so quickly that it creates a high enough pressure wave to form a cavitation bubble that can glow by sonoluminescence. Just be careful if you go looking for one. Not only do they create a large enough pressure wave to kill their prey, but it is even strong enough to break through the glass of an aquarium.


References and Further Reading
The Sonoluminescence Process by the American Institute of Physics
Sound is a Pressure Wave by The Physics Classroom
Sonoluminescence: Sound into Light (Scientific American article) by Seth Putterman
Mantis Shrimp by Chesapeake Bay Program
Why the Mantis Shrimp is my new favorite animal by The Oatmeal