Now this is one cool video from New Scientist and the American Physical Society. It’s truly amazing what you can do with some waves and some liquid!
This is a prime example of standing waves and resonant frequency.
Every material, every object, has a resonant base frequency: the lowest frequency that causes the material to shake and wiggle around. Vibrations happen in all objects all the time – a speaker shaking as it blasts out music, or your phone dancing off the table when it rings. But when the conditions are just right, this vibration is at its maximum – the energy transfer from the original wave to the object is at its peak. This is called the resonant frequency.
The resonant frequency also has harmonics – multiples of this original frequency. So if the base resonant frequency is 100 shakes per second (otherwise known as Hertz, or Hz) then 200 Hz will also cause some pretty intense wiggling.
Standing waves occur when an exact number of a wave’s peaks and troughs can fit into a space where a wave is transmitted. So if the distance taken up by each wave is 1 cm, then popping a wave into any 2 cm, 3 cm or any exact-centimetre-long space and reflecting it back and forth will result in a standing wave, where the wave is basically constant. Try this out with a slinky on the table, with someone holding the other end to you: you should notice that when you’re shaking it a certain number of times a second, it’ll move up and down around the same fixed points.
Popping something like a droplet of water in the middle of a standing wave which is operating at its resonant frequency is basically the optimum vibration conditions, which is why you see these awesome star shapes. Upping the frequency to the various higher harmonics increases the number of points on the water-stars.
There are some other seriously cool stuff you can do with sound waves and resonant frequencies: check out this seriously cool slo-mo video of paint dancing on a speaker.