Scientists have been able to use the power of sound to levitate small items — including insects and fish — for decades. But now researchers from Switzerland have figured out how to move objects around in midair, according to a new study.
The breakthrough in acoustic levitation will allow scientists to unlock “a huge amount of applications for this very powerful method,” including in pharmaceutical and electronics manufacturing, said author and mechanical engineer Dimos Poulikakos of ETH Zurich, a science and technology university in Switzerland.
The study was published online Monday in the Proceedings of the National Academy of Sciences.
Poulikakos’ 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. They also levitated a wooden toothpick — something that had never been done before — while rotating it and moving it forward and backward.
Sound waves exert pressure when they hit a surface, but the effects are usually too small to notice. But if the intensity is cranked up high enough, sound has the ability to counteract the effects of gravity.
Poulikakos and his colleagues used levels of about 160 decibels; that’s louder than standing near a rocket launch and is enough to rupture a human eardrum. But they were able to work without ear protection.
They took advantage of the fact that the frequency of sound — the physical property that gives it a pitch — also matters. Using 24,000 Hertz (Hz), a level comparable to a dog whistle, they were unaffected by the noise. The upper range of human hearing is about 20,000 Hz.
Their levitation device looks similar to a chessboard, with each penny-size square emitting its own sound. A large, clear plastic plate is placed a small distance above the chessboard to reflect the sound; if the sounds waves are strong enough, objects can hover and move around within the space.
The tricky part was figuring out how to move things from square to square carefully, without damaging them, said lead author Daniele Foresti, also a mechanical engineer at ETH Zurich.
Foresti found that balance was the key. Push too hard, and the sound waves will cause a water droplet to explode. Don’t push hard enough, and the droplet will fall and gravity wins. Eventually, he found that the way to succeed was to slowly lower the sound intensity of the “giving” square while ramping up that of the “receiving” one.
Think of a table with a row of light bulbs, each with its own dimmer switch, Foresti said. If you carefully lower the brightness of one bulb while increasing that of the next, you will keep a steady level of light in the room. This concept allowed him to carefully adjust the levels of each sound-emitting square via computer software in order to move hovering objects around the chessboard remotely.
Poulikakos compares the previous state of acoustic levitation — without the airborne motion control — to a luxury car kept permanently in park. “We could walk around it and enjoy it, but we could not drive it,” he said. “Now we can drive it.”
Physicist Rick Weber of Argonne National Laboratory outside Chicago commended the authors for developing “a highly innovative approach” that furthers the capabilities of acoustic levitation. A YouTube video of Weber’s own experiment went viral late last year. It showed the drop-by-drop suspension of a liquid using Argonne’s acoustic levitator. But Weber was not able to transport the drops from place to place.
Poulikakos’ advance over motionless levitation will probably be useful to the pharmaceutical industry, allowing scientists to mix solutions in midair without the potential for contamination from a container, Weber said.
For now, scientists are able to use acoustic levitation to move only small, lightweight objects. Poulikakos said he will soon release new results that show his team moving heavier and more dense objects, such as steel balls.