Bristol team makes waves with acoustic tractor beam

In what is claimed to be a world first, engineers at Bristol University have developed an acoustic tractor beam that can stably trap objects larger than the beam’s soundwaves.

The device works by creating acoustic vortices, tiny tornado-like structures with loud sound surrounding a silent core. It had previously been thought these vortices were limited to supporting objects no bigger than their wavelengths, with larger objects spun out of the field by the rotational motion imparted to them. However, by constantly altering the direction the vortices spin, the rotation of the objects in the beam can be controlled, allowing larger objects to be levitated with stability.

tractor beam

Published in Physical Review Letters, the study details how the team used ultrasonic waves at a pitch of 40kHz, a frequency similar to that which only bats can hear. Rapidly changing the twisting direction of the vortices allowed the researchers to expand the size of the silent core, which supported larger objects.

“Using acoustics to levitate people may even be feasible”

A 2cm diameter polystyrene sphere – over two acoustic wavelengths in size – is so far the largest object successfully suspended in the tractor beam.

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

tractor beam

Those potential applications include new methods for drug delivery and micro-surgery, as well as contactless production lines where delicate objects are assembled without needing to be touched. As the technology advances, using acoustics to levitate people may even be feasible.  

“In the future, with more acoustic power it will be possible to hold even larger objects,” said senior research associate Dr Mihai Caleap, who developed the simulations for the device.

“This was only thought to be possible using lower pitches making the experiment audible and dangerous for humans.”