The sound of good music or a crying baby may move you emotionally. But sound can also move things physically – and twirl them and mix them and (accidentally) blow them up, using an acoustic wave device described in the Proceedings of the National Academy of Sciences.
A sound wave exerts a tiny amount of pressure, and with enough intensity, it can counteract the force of gravity. But so far, researchers have been able only to levitate small objects, and they haven’t yet been able to actually move them around.
Electric and magnetic fields can lift and transport things, but those objects generally need to have a magnetic field or certain material properties. It's a principle behind high-speed rail like Japan’s magnetic-levitation Linimo train.
Now, scientists at the Swiss Federal Institute of Technology in Zurich have built a device that uses sound to move and manipulate objects – one that can potentially be used on any material. Its checkerboard of 'Langevin piezoelectric transducers' – which are sort of like little speakers – gives off an acoustic wave that's reflected back by a panel placed above the checkerboard.
The sound wave reflects back between this floor and ceiling, forming a standing wave. Given enough power, it can hold an object suspended in one of its "nodes" -- points along the standing wave that remain in the same spot even as the wave oscillates.
The higher the wave's amplitude, the higher the intensity and thus, the louder the sound. Even cranked way up, the device was safe for human ears because it sent out waves at a frequency of 24,000 hertz, well above the limit of human hearing and around the same level as a dog whistle.
Levitation by high-frequency sound waves is a delicate dance. Too little force, and the object falls. Too much, and the sonic onslaught can cause it to explode, the authors explained.
The researchers found that by carefully reducing the amplitude blasted by one square "holding" an object in mid-air and simultaneously ratcheting up the volume in the square next door, they could lead an object in a waltz around the grid by letting it step from square to square.
They could even make materials interact, causing a sodium particle to hit a water droplet and fizz, mixing a tiny cup of Joe using an instant coffee granule, and even inserting DNA into cells. They were also able to twirl a toothpick – an object considered too long and oddly shaped for this kind of movement.
"We are not aware of any other method or technology able to perform such multidroplet transport and handling in a gaseous environment," the authors wrote.
The device can still lift only small items. But the technology could one day have all sorts of uses – for example, mixing chemicals in mid-air without fear of contamination from a container. That could also be handy when dealing with toxic waste.
"The contactless material handling can be extended to hazardous, chemical or radioactive samples," they wrote.