The first-ever sonic tractor beam that can levitate objects larger than half the wavelength of the ultrasound used has been created by researchers at the University of Bristol, in the UK. The new technique involves creating a “virtual vortex” of ultrasound that can be adjusted, while maintaining the trapping force. This reduces the destabilizing forces that perturb larger objects, enabling such particles to be held. The researchers say that the new technique also allows smaller objects to be moved using a much wider range of ultrasound frequencies, opening up medical applications such as moving kidney stones.
In 2015, Bristol’s Asier Marzo and Bruce Drinkwater developed a sonic tractor beam that used ultrasound to levitate, rotate and move objects in multiple directions. That device used a grid of 64 off-the-shelf, miniature loudspeakers controlled by a programmable array of transducers to create acoustic holograms that could trap and manipulate objects in mid-air. The researchers created three different acoustic shapes – tweezers, a vortex that traps objects at its core, and a cage. Using these they were able to levitate and control polystyrene particles ranging with diameters of 0.6-3.1 mm.
Most sonic tractor beams have a fundamental limitation that they can only levitate particles smaller than half the wavelength of the ultrasound used. This is because the particles sit in the areas of low intensity – or low amplitude – in the acoustic field, which are half-a-wavelength long. Larger particles occupy areas of high and low intensity and become unstable.
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