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