Scientists use sound tornadoes to create the world’s biggest tractor beam

In the past researchers thought that acoustic tractor beams were limited to levitating only small objects because all the previous attempts by scientists around the world, including the team from New York University who recently created tractor beams using both light and sound, to levitate objects larger than the wavelength of the light or sound waves they were using ultimately sent their objects spinning out of control. This is because the rotating sound fields the researchers used transferred some of its spinning motion to the objects causing them to orbit faster and faster until they are ejected from the tractor beam.

Now a team from the University of Bristol in the UK have discovered a new approach using ultrasound and they’ve broken the "wave" barrier. Details of their work has been published in Physical Review Letters.

The new approach uses rapidly fluctuating acoustic vortices, which are similar to tornadoes of sound, that make a twister like structure with loud sound surrounding a silent core, and in some respects you can think of the core in the same way you think of an eye of a hurricane, calm.

See It In Action

The team discovered that the rate of rotation of these twisters can be finely controlled by rapidly changing the twisting direction of the vortices, and this helped the team stabilise their tractor beam. They were then able to increase the size of the silent core allowing it to hold larger objects and this was the breakthrough – all of a sudden they weren’t limited to just levitating small objects, now they were able to levitate much larger objects, something that noone has ever done before, and something that noone thought possible.

Working with ultrasonic waves at a pitch of 40kHz, a similar pitch to that which only bats can hear, the team held a 2cm polystyrene sphere in their tractor beam. This sphere measures over two acoustic wavelengths in size and is the largest object ever to be trapped in a tractor beam, and now the research suggests that in the future much larger objects, perhaps even humans, could be levitated in this way, however it would take a huge rig to levitate a human and no doubt there will be some challenges involved, such as them getting pulled apart in basically something that resembles a tornado…

“Acoustic researchers had been frustrated by the size limit for years, so its satisfying to find a way to overcome it. I think it opens the door to many new applications,” said Dr Asier Marzo, lead author on the paper from Bristol’s Department of Mechanical Engineering.

“In the future, with more acoustic power it will be possible to hold even larger objects. This was only thought to be possible using lower pitches making the experiment audible and dangerous for humans,” added Dr Mihai Caleap, Senior Research Associate, who developed the simulations.

“Acoustic tractor beams have huge potential in many applications. I’m particularly excited by the idea of contactless production lines where delicate objects are assembled without touching them,” said Bruce Drinkwater, Professor of Ultrasonics from the Department of Mechanical Engineering, who supervised the work, and it’s funny he should say that because that’s precisely what a team in Lithuania from a company called Neurotechnology Group who recently created the world’s first Ultrasonic 3D Printer have done – they’re now using tractor beams to help manufacture and assemble electronics products. But that’s another story, and actually it’s now kind of old news, so maybe you could read that one next... there’s even a video! Oh, and if you want to create your own tractor beam then check out the video below… but remember, people might get mad if you start levitating their pets without permission.

Make Your Own Tractor Beam