Acoustic Levitation of Liquids Looks Like Magic

No magic show: Real-world levitation to inspire better pharmaceuticals

The acoustic levitator uses two small speakers to generate sound waves at frequencies slightly above the audible range – roughly 22 kilohertz. When the top and bottom speakers are precisely aligned, they create two sets of sound waves that perfectly interfere with each other, setting up a phenomenon known as a standing wave.

The speed of sound is about 340 m/s, so a 22 kHz wave has a wavelength of about 1.5 cm; the nodes would thus be half that distance apart, but you wouldn’t have to fill every node. Notice the effect at 0:30, where the standing wave is adjusted — you can see the drops move up and down, in tandem.

This seems to be analogous to a dipole force trap in atomic physics. If that’s so, you’d have a radial pressure gradient also giving confinement, which might account for the somewhat strange look to the speakers.