A prototype ultrasound tube guides liquid through midair

Water runs horizontally through the air, a thin stream with nothing touching it. Five centimeters away, a cylinder lined with hundreds of tiny speakers surrounds it, and no sound comes out. At the University of Helsinki, the 256 small speakers emit ultrasound, sound above the range of human hearing, at about 40 kilohertz, and those waves hold the stream in place. “Of all the experimental setups I have seen, I would say this is the closest thing to magic,” says Ari Salmi, professor of electronics, who led the development. It is not magic, but a long line of ultrasound research in the university's Electronics Research Laboratory. The results are published in PNAS.

The idea started with a cleaning problem. Four years ago, the team was building a device that scrubbed the inside of pipes with ultrasound. Over lunch, Salmi and a colleague agreed it would be handy if a pipe had no surface for dirt to stick to. On an evening walk with his dog, Salmi arrived at the answer: form the walls of a tube out of sound. Getting there was another matter. The team had to find, by trial and error, how to drive the speakers so that a tube would form, working with colleagues from the Public University of Navarre and the University of Michigan.

What the tube can carry is wider than liquid. The team describes moving powder, mist and liquids, and guiding an electric discharge along a chosen route. The direction can be changed on the fly, for instance when a liquid must be sent somewhere new, and it 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 project. Each substance needs its own pattern: “Transporting a solid substance, for example, requires a different field configuration than transporting a liquid or a gas.”

The prototype is small. Its working section is 8 centimeters long and draws about 50 watts, roughly what an old-fashioned incandescent bulb uses. According to Nikolaev, similar modules could easily be linked in a row to build a tube of any length. The sound is intense at the core, he says: “comparable to a continuous roar of gunfire right next to your ear.” Humans cannot hear it, because it is too high-pitched. Dogs, the article notes, might not appreciate it.

What changes, and for whom? The case for the tube is the pipe that causes trouble today. Corrosive or sticky substances make problems in ordinary pipes, and a tube with no solid wall gives them no wall to attack or to cling to. For delicate, contamination-sensitive materials, the absence of walls removes the risk of contamination. For those who move such products, the promise is a line that needs no cleaning. But the sources give no timeline, and no industrial installation has been announced. The present result is one 8-centimeter section, measured in a laboratory, and the chaining of modules into a longer tube is described as possible, not yet shown.