“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,” says Bruce Drinkwater, Professor of Ultrasonics at the University’s Department of Mechanical Engineering.

Current theory says these 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.

Instead the team used rapidly fluctuating acoustic vortices, which are similar to tornadoes of sound, to create a stable core. They were then able to increase the size of the silent core allowing it to hold larger objects. Working with ultrasonic waves at a pitch of 40kHz, the researchers held a 2cm polystyrene sphere in the tractor beam (pictured left).

This sphere is over twice the acoustic wavelength and is the largest yet trapped in a tractor beam. The research suggests that, in the future, much larger objects could be levitated in this way by boosting the power of the ultrasound.

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

Ultrasound is a key area for Bristol. The University team has previously shown smaller objects being moved around by the beams, while Ultrahaptics in the city is using focused ultrasound beams to provide the feeling of touch in mid-air.