A tube without walls: ultrasound guides liquids, solids and gases through the air
Researchers at the University of Helsinki move substances through the air without contact: 256 ultrasound speakers form an invisible tube whose path can be changed electronically.
Archive image · Photo: Yartsa / Wikimedia Commons, CC BY-SA 4.0 (cropped and resized)
A University of Helsinki team has developed a kind of pipeline without walls. A cylinder with 256 speakers emits ultrasound at around 40 kilohertz; the sound fields form an elongated, invisible tube that can guide solids, liquids and gases. The work was published in the journal PNAS, with the Public University of Navarre and the University of Michigan also involved.
The prototype section is about eight centimetres long, the liquid stream runs about five centimetres from the speakers, and the setup draws about 50 watts, similar to an incandescent bulb. “By changing how the speakers are controlled, we can reshape and turn the tube as needed,” says project lead Dmitry Nikolaev. Solids require a different field configuration than liquids or gases.
According to Professor Ari Salmi, the idea goes back to a lunch conversation about four years ago: a pipe with no surface for dirt to stick to would be handy. On an evening walk, he then thought of walls made of ultrasound. “Of all the experimental setups I have seen, I would say this is the closest thing to magic,” Salmi says.