Bristol team makes waves with acoustic tractor beam

Bristol team makes waves with acoustic tractor beam

It was previously believed that acoustic tractor beams were limited to levitating relatively small objects; only those approximately the same size as a wavelength of the sound.

Dr. Asier Marzo, the lead author on the paper from Bristol's Department of Mechanical Engineering, said that "Acoustic researchers had been frustrated by the size limit for years, so its satisfying to find a way to overcome it".

For the first time, scientists have developed a tractor beam capable for levitating objects larger than an acoustic wavelength. They could also transport delicate samples without containers, and even one day levitate humans with high-frequency - and therefore inaudible and safe - sound vibrations.

All up, then, acoustic tractor beams have represented little more than a neat physics parlour trick, a long way from the powerful attractors seen in, say, Star Trek: Voyager, fuzzing out from a Galaxy-class starship to hold a Delta Flyer spacecraft in place.

The concept of an acoustic tractor beam levitating a spherical object.

So far, the researchers have trapped a two-centimetre polystyrene sphere in the tractor beam.

The new approach, published in Physical Review Letters today [Monday 22 January], uses rapidly fluctuating acoustic vortices, which are similar to tornadoes of sound, made of a twister-like structure with loud sound surrounding a silent core.

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The key difference, is that, just like tornadoes, there is a silent core in which the object can hover.

Whereas earlier attempts saw objects spinning uncontrollably, researchers recently discovered that the rate of rotation can be controlled by rapidly changing the twisting direction of vortices.

Meanwhile, the supervisor of the project, Prof Bruce Drinkwater, added: "Acoustic tractor beams have huge potential in many applications".

Scientists believe the new technology can be eventually deployed to levitate even larger objects.

In addition, the technology could be used for manipulating micro-surgical instruments inside the body, transporting delicate objects or even levitating humans. "This was only thought to be possible using lower pitches making the experiment audible and risky for humans", said Senior Research Associate Dr. Mihai Caleap.

You won't be needing a spell to send people floating, but it does help if you have access to the world's most powerful acoustic tractor beam located at the University of Bristol. Touchless control is particularly exciting since it means that now fragile objects can be assembled without touching them.