If the Incredible Hulk were ever unleashed in the real world, guns and bombs would have absolutely no chance against him. Going by the 2008 movie, one of the few things that managed to hold the green giant back was sonic cannons that pinned him down with powerful sound waves. Scientists at the University of Helsinki, in collaboration with researchers from the Public University of Navarre and the University of Michigan, have now demonstrated a semblance of the technology behind that concept, with an ultrasonic tube that can physically confine and guide matter through the air.
The invisible, massless tube, made of nothing but ultrasonic sound waves from 256 miniature speakers, can guide liquids, gases, powders, and even plasma, making it look as though the substances are floating through thin air. Interestingly, everything happens without any audible sound. Changing the flow direction is as simple as reshaping the invisible tube by adjusting how the speakers operate.
“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 and project leader.
The concept was inspired by an earlier project in which the researchers used ultrasound to clean substances stuck to the inner surfaces of pipes. Building on this idea, they wondered whether they could eliminate the problem entirely by removing the pipe’s physical walls, so there was nothing to stick to in the first place.
To actualize the idea, the researchers ran several rounds of trial and error with different speaker configurations.
The resulting prototype uses 256 miniature ultrasonic transducers arranged around a cylindrical structure, emitting sound waves at about 40 kilohertz (kHz). By carefully controlling the timing of these waves, the researchers can make them interact to produce an acoustic field that exerts forces on passing materials, keeping them confined to a particular path. Adjusting the field allows the team to bend or redirect that path without moving any physical pipe. The prototype tube measures just 8 cm (3.1 inches) long and draws about 50 watts of electricity, about as much as an old-fashioned incandescent light bulb.
While this is only a prototype, the research shows the possibility of continuously guiding different states of matter through the air using sound alone. Previous acoustic manipulation systems have largely focused on suspending and moving individual particles or droplets, but the new system can handle continuous streams. In their experiments, the researchers successfully guided flowing water, directed powders into different containers, steered mist and gas streams, and even controlled the path of electrical discharges.
As far as potential applications go, the team envisions contactless transportation of substances, particularly corrosive or sticky materials that can damage conventional pipes or build up on their inner surfaces. The technology could also be useful for handling highly sensitive substances that are easily contaminated. The ability to redirect the flow electronically also opens up possibilities for delivering materials precisely where they're needed.
For now, though, a considerable gap remains between this prototype and full-scale ultrasonic pipes. An obvious limitation is the tube’s size compared to what it takes to produce. A total of 256 speakers to generate a pipe just 8 cm long is a lot of hardware. The researchers believe they could connect similar modules in series to create much longer tubes.
Then there's the sound's intensity. According to the researchers, the ultrasound at the center of the tube has an intensity comparable to the continuous roar of gunfire right next to your ear. Even outside the device, the acoustic intensity is comparable to standing beside a jet engine. Thankfully, at 40 kHz, the sound is beyond the normal range of human hearing, so you wouldn't actually hear all that racket. Just make sure there are no dogs nearby.
The research was published in the journal PNAS.
Source: University of Helsinki
