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Sound waves can levitate tiny particles, droplets

By Kelly Servick
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A tragic opera may move you to tears, but here’s a more literal application of the moving power of sound. Sound waves with frequencies just above human hearing can levitate tiny particles and liquid droplets and even move them around, a team of engineers has demonstrated. The advance could open up new ways to handle delicate materials or mix pharmaceuticals.

Researchers have already developed several levitation methods. For example, electrostatic or magnetic fields can exert a concentrated force on an object to counteract gravity. But these fields work only on metallic substances or materials with magnetic properties. Sound waves don’t discriminate, however, and physicists worked out the basic principle of "acoustic levitation" nearly a century ago. A vibrating plate generates a sound wave that bounces against another surface to create a stable standing wave. The points of lower pressure in this static pattern can trap a particle. Scientists have learned how to hold increasingly heavy particles including superdense iridium and even liquid droplets in this acoustic sweet spot.

But until now, that was pretty much the extent of the trick, said mechanical engineer Dimos Poulikakos of the Swiss Federal Institute of Technology in Zurich. “It’s like we had a car which we made fancier and fancier, but it stayed parked. We were never able to drive.” Moving a liquid with sound is a delicate balancing act, he said. As you vary the acoustic force to push the droplet around, you run the risk of shattering it with too much pressure.

Poulikakos’ team spent four years trying to budge their floating droplets from a standstill. Finally, they conceived of a chessboard-style setup with multiple vibrating plates, each generating its own sound frequency. By varying the frequency that each plate emits, they can move the acoustic field and the object trapped inside. Their new design, described in the Proceedings of the National Academy of Sciences, can precisely control the lateral movement of liquid droplets while keeping them floating smoothly in midair. It can also move them toward one another. When two droplets are forced into the same place, they may coalesce into a single droplet or react dramatically.

In the new research, the team merges liquids with solids, dissolving coffee in a water droplet, and also uses the setup to lift and spin a toothpick. Previously, no one had been able to control objects larger than a few millimeters in diameter, said physicist Chris Benmore of Argonne National Laboratory in Lemont, Ill., who was not involved in the work. The technique is limited to objects about three times the density of water, but the team is now working to push its limits. By changing the shape of the reflecting surface to create a stronger acoustic force, they expect to move denser materials, such as steel.

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