(Photo : courtesy Dimos Poulikakos/ETH Zurich)
For years, scientists have known how to levitate small items such as fish and insects with sound waves - and now researchers in Switzerland have found how to move objects through air in the same way.
A study of the new findings was published online Monday in the Proceedings of the National Academy of Sciences, detailing what author and mechanical engineer Dimos Poulikakos of ETH Zurich, a science and technology university in Switzerland, calls an acoustic levitation breakthrough that will lead to "a huge amount of applications" in fields such as pharmaceutical and electronics manufacturing.
Sound waves always apply pressure to a surface when they hit it, but the effects are typically too small to leave any noticeable effects.
If, however, the intensity of sound is boosted high enough, then the waves can counteract the pull of gravity.
Poulikakos and his colleagues pushed the sound levels to about 160 decibels, which the research explains is even louder than a rocket launch and is enough to rupture a human eardrum.
The team experimented with a levitation device that looks somewhat like a chessboard, with penny-sized squares that each emit sound waves.
Placed directly above and a small distance away from the sound board, a clear plastic plate was used to reflect the sound; if strong enough, sounds waves can lift objects, causing them to hover and move around within the affected space.
Poulikakos's team performed a number of midair experiments, such as combining water droplets or chemical solutions, inserting DNA into cells and even making a tiny portion of instant coffee.
The scientists also lifted a wooden toothpick, a scientific first, then rotated it and moved it back and forth.
Daniele Foresti, another lead researcher and mechanical engineer at ETH Zurich, indicated balance was the key to not destroying the objects moved in the experiment.
Foresti said if the sound waves were too strong and pushed too hard, delicate objects like water drops would explode. Then again, if the waves weren't energetic enough, gravity would prevail and such droplets would fall.
The researchers discovered the way to move stuff though air was to slowly lower the sound intensity of the sound-emitting squares over which an object is floating while elevating that of the square "receiving" the object.
Poulikakos was quoted by The Washington Post comparing the ability to acoustically levitate an object without being able to move it like owning a luxury car permanently in park.
"We could walk around it and enjoy it, but we could not drive it," he said. "Now we can drive it."
Despite the intensity of the sound they were testing, the scientists were nevertheless able to work without ear protection and without risk of ear damage, since they used a sound with a frequency of about 24,000 hertz (Hz), well above the 20,000 Hz that represents the upper range of human hearing.