This is another video from astronaut and chemist, Dr. Don Pettit, on the International Space Station. He’s the same guy who made the Bubbles and Anti-Bubbles in Space video. This time he’s experimenting with water droplets on speakers. He’s playing tones through the speakers and sweeping through different frequencies to observe their effects on the droplets. The interesting thing about this is that there is no gravity to dissipate the waves from the surface of the water. The water droplets are able to hold some very interesting shapes depending on their size and the tone frequency.

To break this down even further, you have to take a look at wave behavior. Lets take what we’re seeing on the surface of those water droplets and break it down into a 2D scenario. This is a crash course on standing wave behavior by Professor Brian Cox with help from Simon Pegg. Before you watch it, remember that the surface of water has elastic properties due to surface tension. That’s to say that you can put energy into it by stretching it and it will use that energy to return to its original shape.

Now to bring it all together, the spring in the second video is the equivalent of the surface of the water droplets in the first video. How big the water droplet it effectively changes the spring rate, or how stiff the spring is. The two guys moving the ends of the spring in the second video are the equivalent of the speaker making tones in the first video. If you expand the spring into a three dimensional dome shape, you can start to see why the water droplets look the way they do in the first video. When the spring is excited slowly, you get a small number of very large waves (2D) or ripples (3D) that move slowly. As the frequency increases, the excitation is moving faster and you get a lot more small waves and ripples that are moving at much higher speed. As the frequency gets very high, the surface of the droplets become chaotic and that’s when it starts to shoot the droplets off. Knowing what you now know about wave behavior, what do you think the answer to Don’s Challenge Question is?

Sources: Physics Central APS and The BBC on YouTube.