Acoustic tractor beams use the power of sound to hold particles in mid-air, and unlike magnetic levitation, they can grab most solids or liquids. For the first time University of Bristol engineers have shown it is possible to stably trap objects larger than the wavelength of sound in an acoustic tractor beam.
Researchers previously thought that acoustic tractor beams were fundamentally limited to levitating small objects as all the previous attempts to trap particles larger than the wavelength had been unstable, with objects spinning uncontrollably.
The Bristol researchers discovered that the rate of rotation can be finely controlled by rapidly changing the twisting direction of the vortices, this stabilises the tractor beam.
Working with ultrasonic waves at a pitch of 40kHz, a similar pitch to that which only bats can hear, the researchers held a two-centimetre polystyrene sphere in the tractor beam.
This sphere measures over two acoustic wavelengths in size and is the largest yet trapped in a tractor beam.
Bruce Drinkwater, Professor of Ultrasonics from the Department of Mechanical Engineering, who supervised the work, added: “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.”