Scientists Levitate Large Sphere Using Ultrasound

Acoustic levitation of a polystyrene sphere 50 millimeters in diameter. Marco Andrade et al/AIP

Scientists spending their days in a lab trying to levitate a large sphere using acoustic waves may not initially impress at a dinner party, but the feat is actually more difficult than it sounds. This is because an object doesn’t typically levitate if it is larger than the wavelength trying to lift it. 

Now, a trio of researchers has demonstrated that this difficulty can be surmounted – even if it all looks rather unimpressive on film. They achieved this feat with a polystyrene sphere that was 3.6 times larger than the acoustic wave levitating it. The results of their study have been published in Applied Physics Letters.

"Acoustic levitation of small particles at the acoustic pressure nodes of a standing wave is well-known, but the maximum particle size that can be levitated at the pressure nodes is around one-quarter of the acoustic wavelength,” said Marco Andrade, lead author from the University of São Paulo in Brazil, in a statement. "This means that, for a transducer operating at the ultrasonic range, the maximum particle size that can be levitated is around 4 millimeters.” In this case, that size was upped to 50 millimeters. 

The desire to harness the power of sound is not a new phenomenon. However, researchers typically achieve acoustic levitation via a standing wave, where the object is buffered by one sound wave from above and one from below. Instead, this team made a standing wave between three ultrasonic transducers arranged in a tripod formation.

Previous attempts to lift such a large object have only been achieved with droplets of water and wire-like and planar objects. Impressive as this new success is, the team's research is still a work in progress. ”At the moment, we can only levitate the object at a fixed position in space," said Andrade. "In future work, we would like to develop new devices capable of levitating and manipulating large objects in air." 

While this method can’t be used on anything mobile yet (hoverboards, anyone?), it could be used to manipulate materials too hot to touch, such as molten metals, in space. That, in turn, sounds like some pretty interesting dinnertime conversation to us.

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