The development in acoustic tractor beams comes from the University of Bristol. Acoustic tractor beams work on the basis of controlling sound in order to hold particles in mid-air. In a sense the beam can be tuned to hold onto most solids or liquids. What is special about the new study is that scientists have shown that it is now possible to trap objects larger than the wavelength of sound in an acoustic tractor beam, and hold them in a stable lock.
Over the short term this discovery can enable the capture and manipulation of micro-surgical implements or slow-release medicine capsules within the human body. Over the longer-term, the experiment could pave the way for the levitation of humans.
By a creating a “virtual vortex” with an orbital angular momentum that can be tuned independently of the trapping force, the researchers succeeded in devising a method that can adjust the rotational speed of particles inside a vortex beam and, for the first time, create three-dimensional acoustics traps for particles of wavelength order (that is, Mie particles). Mie particles related to the Mie solution to the scattering problem on a sphere, allowing predictions to be made about subvisible particles.
The process is shown in the following video:
In one study the researchers used ultrasonic waves at a pitch of 40kHz (a similar pitch to that which only bats can hear) to hold a two-centimeter polystyrene sphere in the tractor beam. In a statement, Dr Asier Marzo considers the potential: “Acoustic researchers had been frustrated by the size limit for years, so its satisfying to find a way to overcome it. I think it opens the door to many new applications… [like the] idea of contactless production lines where delicate objects are assembled without touching them.”
The research is published in the journal Physical Review Letters, with the paper titled “Acoustic virtual vortices with tunable orbital angular momentum for trapping of Mie particles.”
