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The field has “huge potential,” they said, after having successfully levitated a small ball using just sound waves. The object was the largest ever trapped in a so-called acoustic tractor beam, named after a term first used almost 100 years ago.
The team created “tornadoes of sound” from an ultrasonic generator, with vortices of 40kHz vibrations surrounding a silent core. Previous attempts to trap large objects failed as items spun uncontrollably and were ejected, but the Bristol engineers rapidly changed the vortices’ direction to stabilise the beam, successfully levitating a 2cm polystyrene ball.
The sphere was larger than two wavelengths of the sound, potentially leading the way for levitating much larger objects in future.
“Acoustic researchers had been frustrated by the size limit for years, so it's satisfying to find a way to overcome it,” said lead author and mechanical engineer Asier Marzo. “I think it opens the door to many new applications.”
Beams could manipulate drug capsules or micro-surgical implements within the body, the team said. They could also transport delicate samples without containers, and even one day levitate humans with high-frequency – and therefore inaudible and safe – sound vibrations.
The technology could also potentially transform manufacturing, said supervisor Bruce Drinkwater, who was “particularly excited” by the idea of contactless production lines, where delicate objects would be assembled in mid-air.
The research appeared in Physical Review Letters.
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