Engineers juggle objects with levitating sound waves

Alan Boyle, NBC News Science Editor
Tuesday, 16 Jul 2013 | 12:27 PM ET

It looks like "Harry Potter" magic, but it's just acoustic levitation: Researchers have created a device that uses sound waves to make liquid droplets and small solid objects float in the air and merge into each other on command.

The ultrasonic chessboard could someday be used for delicate chemical or pharmaceutical processes where contamination from a surface could spoil the reaction, said Dimos Poulikakos, an engineer at Switzerland's ETH Zurich. He is the senior author of a paper on the project appearing in the Proceedings of the National Academy of Sciences.

"Levitation is an old story," Poulikakos told NBC News. "It was really discovered 100 years ago."

What's new about the device that he and his colleagues constructed is that it uses a chessboard-like array of levitating devices to transport objects through the air with ultrasound. The research team made droplets of water and hydrocarbons float around the squares of the array and smash into one another.

Poulikakos said the method also worked on bits of instant coffee, a bubbling fleck of sodium, tiny steel balls and a wooden toothpick (which looked as if it was being twirled by an occult hand).

A photochemical liquid switch can be made by combining acoustically levitated drops.
Source: Dimos Poulikakos / ETH Zurich
A photochemical liquid switch can be made by combining acoustically levitated drops.

Surfing the wave

Each of the squares in the chessboard is an acoustic resonator, sending out sound waves at 24.3 kilohertz so that it's reflected off a precisely placed sheet of transparent plastic. The frequency is too high to be heard by human ears. But even at that frequency, the acoustic interference can be strong enough to create a standing wave between the resonators and the reflector, counteracting gravity's pull on the target object.

The levels on each of the resonators can be adjusted to transport an object from one of the squares on the chessboard to the next one over. "The particle can 'surf' along the resonators," Poulikakos said.

Tuning the power of the apparatus is a tricky thing. Too little power, and there's not enough force to manipulate a droplet. Too much, and the droplet goes poof. "Liquids are a lot more difficult to move than solids," he said.

The theoretical size limit for the device depends on the substance being levitated as well as the acoustic frequency. For a drop of water at 24 kilohertz, it's about a 10th of an inch (2.7 millimeters). But that's big enough for pharmaceutical applications, or even for applications that involve manipulating hazardous chemical or radioactive ingredients.

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The levitation game

The engineers at ETH Zurich aren't the only ones in the levitation game: Last year, researchers at Argonne National Laboratory in Illinois demonstrated an acoustic levitation setup that could hold liquids in a position for "containerless processing" and X-ray analysis.

Their teams have used magnetic fields rather than sound waves to levitate frogs, grasshoppers and mice, as well as fruit flies. Poulikakos and his colleagues said these techniques and others could be combined to add to the magic of contactless manipulation.

—Alan Boyle, NBC News Science Editor