Researchers from the Department of Mechanical Engineering at the University of Bristol, UK, have developed an acoustic tractor beam that’s so powerful that it could lead to the levitation of drug capsules or micro-surgical implements within the body and even humans. They believe it’s the “world’s most powerful acoustic tractor beam.”

The study is published in the journal Physical Review Letters.

An acoustic tractor beam uses the power of sound to hold particles in mid-air, and can grab liquids, most solids, or small objects like insects and food.

Until now, researchers thought only small objects could be levitated by acoustic tractor beams because prior attempts at trapping objects larger than the wavelength of sound failed. Those objects would spin uncontrollably when the rotating sound field transferred some of its spinning motion to the objects themselves, leading ultimately to their ejection.

However, the new approach uses rapidly fluctuating acoustic vortices, which is set up in a tornado-like structure, with loud sound surrounding a silent core.

The researchers discovered that the rate of rotation in this structure can be manipulated by rapidly changing the twisting direction of the vortices. This allows them to then increase the size of the silent core to hold much larger objects.

Using ultrasonic waves at a pitch of 40 kHz, which is similar to the pitch audible only to bats, the researchers were able to hold a two-centimetre polystyrene sphere, measuring over two acoustic wavelengths in size, in the tractor beam. This is the first time that an object this large has been trapped in a tractor beam.

“Now that we have gone beyond the wavelength limit, it is all a matter of power,” said lead author Dr. Asier Marzo. “More power, larger objects.”

Marzo is adamant though that for the team “the most interesting applications are in the small scale using high-frequency.”

Future Applications

The researchers believe their approach could pave the way for acoustic tractor beams to be used in many new applications.

“Acoustic tractor beams have huge potential in many applications,” Bruce Drinkwater, professor of ultrasonics, who supervised the work, said in a statement. “I’m particularly excited by the idea of contactless production lines where delicate objects are assembled without touching them.”

“I would be impressed if, within a decade, we go to the doctor and the same machine that is used for ultrasonic imaging can also be used to manipulate the particles that are inside our body,” said Marzo. “For instance, the doctor may check for small kidney stones and pull them towards the upper area where you will happily pee them before they get larger.”