Polystyrene ball levitating using acoustics

Engineers from the University of Bristol have trapped objects larger than the wavelength of sound in an acoustic tractor beam and kept them there stably.

Acoustic tractor beams use sound to hold particles in mid-air. Unlike magnetic levitation, they can grab most solids or liquids.

The research, which uses ultrasonic waves, could lead to the development of ways of moving drug capsules or micro-surgical implements within the body, the container-less transportation of delicate large samples and even levitating humans.

In the past acoustic tractor beams have been restricted to levitating small objects. Larger objects would spin uncontrollably because the rotating sound field transfers some of its spinning motion to the objects, causing them to orbit faster and faster until they are ejected.

However, the Bristol engineers’ approach, which has been published in Physical Review Letters this January, uses rapidly fluctuating acoustic vortices. These resemble tornadoes of loud sound that surround a silent core.

The sound tornados that stabilise the object in the tractor beam
The sound tornados that stabilise the object in the tractor beam

The technique rapidly changes the twisting direction of the vortices to finely control the rate of rotation of an object, stabilising the tractor beam. The control enables the size of the silent core to be increased, so the tractor beam can hold larger objects.

Working with ultrasonic waves at a pitch of 40kHz, a similar pitch to that which only bats can hear, the researchers held a 2cm polystyrene sphere in the tractor beam. This sphere measures over two acoustic wavelengths in size and is the largest object yet trapped in a tractor beam.

Dr Asier Marzo, lead author on the paper from Bristol’s Department of Mechanical Engineering, said: “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.”

Dr Mihai Caleap, the senior research associate who developed the simulations, said: “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.”

Bruce Drinkwater, Professor of Ultrasonics from the Department of Mechanical Engineering at Bristol University said: “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.”

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