Sodium and water are pushed together by sound waves.

SOURCE: REF. 1

Water droplets, coffee granules, fragments of polystyrene and even a toothpick are among the items that have been flying around in a Swiss laboratory lately — all of them kept in the air by sound waves. The device that achieves this acoustic levitation is the first to be capable of handling several objects simultaneously. It is described today in the Proceedings of the National Academy of Sciences1.

Typically, levitation techniques make use of electromagnetism; magnetic forces have even been used to levitate frogs2. It has long been known that sound waves could counter gravity, too, but so far the method has lacked practical application because it could do little more than keep an object in place.

To also move and manipulate levitating objects, Dimos Poulikakos, a mechanical engineer at the Swiss Federal Institute of Technology (ETH) in Zurich, and his colleagues built sound-making platforms using piezoelectric crystals, which shrink or stretch depending on the voltage applied to them. Each platform is the size of a pinky nail.

The platforms emit sound waves which move upward until they reach surface lying above, where they bounce back. When the downward-moving reflected waves overlap with the upward-moving source waves, the two ‘cancel out’ in the middle, at so-called node points. Objects placed there remain stuck in place because of the pressure of sound waves coming from both directions (see picture below).

A liquid droplet is levitated in the space between sound-emitting platforms (bottom) and a sound-reflecting surface (top).