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A particle of metal sodium collides with a water droplet.

Watch Sodium and Water Levitate and Collide

Engineers manipulate sound frequencies to control particle movement.

Engineers in Switzerland have successfully levitated both a particle of instant coffee and a droplet of water using high frequency sounds to make them collide. The result was a very tiny cup of coffee.

But besides feeding a caffeine addiction for Lilliputians, the ability to acoustically control different matter in the air has other practical applications—like creating drugs or manufacturing materials. This new research highlights the potential of acoustic manipulation, a process physicists have been working with for at least 30 years. (Related: "Acoustic "Invisibility" Cloaks Possible, Study Says.")

High Frequency

Using sound waves is not the first or only form of levitation—magnetic fields can similarly force an object into the air. But this only works for magnetic materials.

Acoustic manipulation—the ability to levitate, translate, rotate, or vibrate substances with sound—permits researchers to handle an array of small particles such as DNA, toothpicks, or water droplets. It is also useful for moving matter without fear of contamination.

Researchers describe their new method in a study published this week in the journal Proceedings of the National Academy of Sciences.

Two cylinders, called a levitator, are placed facing each other. One cylinder emits a high frequency sound wave—similar to the frequency emitted when you whistle for your dog. The second cylinder reflects that sound back to the emitter, creating a stable sound wave that moves a particle resting on the first cylinder.

For this research, multiple levitators emitting various sound wave frequencies were set up in an arrangement similar to a chessboard, said Daniele Foresti, an engineer from the Swiss Federal Institute of Technology Zurich and lead author of the study.

By changing the frequency of the sound waves, Foresti and colleagues could move droplets from one square to another. The team could combine droplets by moving them to the same areas of their "chessboard."

Watch the Reaction

This video from Foresti's research shows a reaction between a particle of metal sodium and a water droplet. The two particles start off separately and slowly move toward one another. As they make contact, the sodium collides with the water droplet, reacting with oxygen. The ensuing violent, bubbling chemical reaction produces hydrogen. In reality, the reaction happens about 50 to 100 times faster than the collision in the video.

The researchers repeated the collision process with other substances including instant coffee and water. Foresti and colleagues also levitated a toothpick into the air, controlling its movement and causing it to spin.

Put to Practical Use

One area in which acoustic levitation can be helpful is the pharmaceutical industry. When two substances are being combined for drug production, contamination is always a concern because it could change the reaction. (Related: "'Sound Bullets' to Zap Off Tumors?")

"If you use a contactless approach, there is no contamination. You don't have to touch any surface or use a device, which can be contaminated after washing if some liquid sticks to the surface," Foresti said.

Glynn Holt, an associate professor of mechanical engineering at Boston University who was not involved in the current research, has done similar work involving acoustic manipulation.

And he has recently seen growing interest in levitating particles with sound as a way to help with oil spills. Instead of sweeping a fine net through water to catch oil droplets, acoustic levitation can draw the oil out of the water.

"The idea is old, but the application is timely," Holt said. "The oil droplets can coalesce and make bigger droplets, which you can more easily get out of the water."

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