Acoustic tractor beams use the force of sound to stably suspend particles in the air. In contrast with magnetic levitation, these can suspend most solids, liquids or even small insects and food.
For the first time engineers from the University of Bristol have demonstrated that it is possible for objects that are larger than the wavelength of the sound to be held stably in an acoustic tractor beam. This discovery opens the door to the manipulation of capsules or micro surgery inside the human body. The transport of larger objects now also looks to be a possibility. The researchers first thought that the acoustic tractor beams were fundamentally limited to the levitation of small objects. Earlier attempts with particles larger than the wavelength were unstable, where the objects would rotate uncontrollably. This is because a rotating sound field will transfer some of its rotating motion tot the object, which causes it to rotate on an increasingly faster trajectory until it is ejected.
The new approach, which has been published in Physical Review Letters, uses fast fluctuating acoustic vortices. These are comparable to sound tornadoes with a twister-type structure (hard sound around the outside and a quiet centre).
The researchers discovered that the rotation speed can be finely controlled by changing the direction of rotation of the twister quickly, which makes the tractor beam stable. Subsequently they were able to increase the size of the quiet centre, which allowed larger objects to be held. Operating with ultrasonic waves of 40 kHz (bats can hear this), the researchers held a two centimetre polystyrene ball in the tractor beam. The size of is ball is more than two acoustic wavelengths and was 'caught' in the tractor beam. The research suggest that in the future much bigger objects can be levitated this way.
How you can build an acoustic tractor beam yourself using an Arduino, you can see in this video.