For those of us that have seen the original Star Wars, it’s hard to forget the famous scene where the crew aboard the Millennium Falcon comes out of a jump through hyperspace, arriving where Alderaan is supposed to be – only to find that it has been blown away. Then after quickly realizing, “That’s no moon... It's a space station!”, they quickly try to escape, but to no avail as they are trapped in the space stations tractor beam.
The sci-fi fantasy of tractor beams has been a reality for a while now. Up until now however, they’ve only been able to move microscopic particles over mere centimeters. That is all beginning to change however. A new report confirms that engineers at the University of Bristol have proven that it is possible to stably trap objects larger than the wavelength of sound in an acoustic tractor beam. This new breakthrough could lead to things like the manipulation of pharmaceutical drug capsules, micro-surgical implements within the body, container-less transportation of larger fragile samples and could even lead to levitating humans.
Acoustic tractor beams use sound to trap and hold particles in mid-air, and unlike magnetic levitation that only works on metals, acoustic tractor beams can grab most solids or liquids. Until recently, it was thought that acoustic tractor beams were limited to levitating only small objects. Experiments conducted in an effort to trap particles larger than the wavelength proved unstable and caused the objects to spin uncontrollably. This was determined to be because the rotating sound field would transfer some of its spinning motion to the objects, which would cause them to orbit faster and faster until finally, they were ejected.
University of Bristol engineers have a new approach however. It uses what they refer to as tornadoes of sound - rapidly fluctuating acoustic vortices. These vortices are made of a twister-like structure with loud sound that surrounds a silent core. Their research has also shown that the rate of rotation can be controlled by rapidly changing the twisting direction of the vortices, which stabilizes the tractor beam. They were also able to increase the size of the silent core, allowing it to hold larger objects. The report discusses how the researchers worked with ultrasonic waves at a pitch of 40kHz, which is the pitch of most dog whistles, to hold a two-centimeter 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, suggesting that indeed, the future definitely hold the possibility that much larger objects could be levitated in this way.
"Acoustic researchers had 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." Dr Asier Marzo, lead author on the paper from Bristol's Department of Mechanical Engineering explained.
For more on acoustics, check out Controlling Fluids with Acoustic Waves at the Nanoscale.