Seeing something float magically in nothing but air is always a breathtaking sight, but there aren’t too many ways to do it. You could use a really thin string, like a magician, or maybe some powerful electromagnets. But what if you could levitate objects with nothing more than the air itself?
Acoustic levitation works by creating a standing wave between the transducer (like a speaker) and a reflector (or two transducers, as in the above video), where there are fixed points of strong oscillation between compressed and rarefied air—but also fixed points where the air pressure is stable. That’s where a droplet can float, pushed up repeatedly by the strongly compressed air beneath and above it. It’d be like bouncing in place on top of a balloon that’s inflating and deflating really quickly.
But wait, there’s more! Not only can you levitate a droplet of water with sound, you can change its shape!
If you turn up the volume of your transducers, you can flatten your droplet into a disc. Then, if you change the frequency to a harmonic—meaning 1/2, 1/3, 1/4… of the original frequency—you get these wonderful star-shaped patterns as a result of new waves on the surface of the droplet itself. Full details in the researchers’ paper.