Researchers previously thought that acoustic tractor beams, which use the power of sound to suspend particles in mid-air, were limited to levitating small objects. Previous efforts to trap particles larger than the wavelength resulted in objects spinning uncontrollably because rotating sound field-transfers some of its spinning motion to the objects and causes them to orbit faster and faster until they are ejected.
A new approach developed by University of Bristol engineers demonstrates the ability to stably trap objects larger than the wavelength of sound in an acoustic tractor beam. The method uses rapidly fluctuating acoustic vortices, akin to tornadoes of sound, made of a twister-like structure with loud sound surrounding a silent core.
Improved control of the rate of rotation is effected by rapidly changing the twisting direction of the vortices, which stabilizes the tractor beam. This enabled an increase in the size of the silent core, which allowed it to hold larger objects. Working with ultrasonic waves at a pitch of 40 kilohertz, a pitch similar to that audible only to bats, the researchers held a 2-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.
The research suggests that in the future, much larger objects could be levitated in this way, and provides potential for the manipulation of drug capsules or microsurgical implements within the human body. Containerless transportation of delicate, larger samples is now also a possibility.
This uplifting research is published in the journal Physical Review Letters.