A small stream of liquid flows quietly in mid-air, horizontally, as if inside an invisible tube. Nothing touches it. About five centimeters away, a cylinder lined with hundreds of tiny speakers surrounds the stream, yet there is no sound. The liquid simply keeps flowing.
"Of all the experimental setups I have seen, I would say this is the closest thing to magic," says Ari Salmi, Professor of Electronics at the University of Helsinki, who led the development of the massless tube.
It is not magic, however, but ultrasound. The University of Helsinki's Electronics Research Laboratory has a long tradition of ultrasound research. The stream of liquid is held in place by ultrasonic waves at a frequency of about 40 kilohertz. The waves are emitted by 256 small speakers surrounding the stream.
The ultrasonic tube could be used to transport corrosive or sticky substances, for example, which cause problems in ordinary pipes. Furthermore, it is extremely useful for transporting delicate, contamination-sensitive substances, as the absence of solid walls eliminates the risk of contamination.
The direction of the tube can also be changed on the fly, for example when the liquid needs to be redirected to a new destination. If needed, the liquid can be dispensed with single-drop precision.
"By changing how the speakers are controlled, we can reshape and turn the tube as needed," explains postdoctoral researcher Dmitry Nikolaev , who led the team on this project.
"Transporting a solid substance, for example, requires a different field configuration than transporting a liquid or a gas."
The tube can also guide an electric discharge along a desired route.
An idea born on an evening walk
Professor Salmi came up with the idea for the ultrasonic tube on an evening walk four years ago.
"We had been developing a device that cleaned the inner surfaces of pipes with ultrasound. At a lunch with a colleague, we discussed that it would be handy if a pipe had no surface for dirt to stick to."
The thought stayed with Salmi, and on his walk with his dog he came up with the idea of forming the walls of a tube with ultrasound.
However, getting from the idea to a working prototype was not easy. The University of Helsinki team, had to find through trial and error a way to make the speakers work in such a way that the tube would form. The work was carried out in collaboration with colleagues from the Public University of Navarre and the University of Michigan.
Eight centimetres, 50 watts
The prototype is now complete. It is only eight centimetres long, which is less than the device's diameter. According to Nikolaev, similar modules could easily be connected in a row, making it possible to build a tube of any length needed.
The eight-centimetre section of tube draws about 50 watts of power, roughly the same as an old-fashioned incandescent light bulb. And although humans cannot hear the ultrasound the tube produces, dogs might not like it.
"At the core of the device, the intensity of the ultrasound is comparable to a continuous roar of gunfire right next to your ear," Nikolaev says. "Even right here next to it, the sound could be compared to a jet engine if you were standing right beside it. But it is too high-pitched for the human ear."
The article Contactless continuous guidance of matter in its main states using curved elongated Bessel-like acoustic fields was published in PNAS.
