Streams of sound are now all you need to make objects dance in the air and combine. A levitation device is the first to use high-frequency sound waves to bring together floating particles and liquid droplets. In principle, the technique could even levitate a person or animal – although it’s not strong enough yet.
For now, such hands-free control could be used to study chemical reactions in extreme environments, to move hazardous materials and to simulate the low-gravity environment of space. At 24 kilohertz, the waves are too high-pitched to be audible to humans – but can be heard by some animals, including cats, bats and mice.
Other levitation methods use magnets or electrical fields, making mag-lev trains – and even levitating frogs – possible. But in these cases, the levitated objects must have particular magnetic or electrical properties.
Acoustic levitation imposes no such constraints. It can, in principle, float anything, says Dimos Poulikakos of the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland.
A sound wave is a pressure wave that produces a force and so has the potential to counteract gravity. To float things using sound, you need to ensure the force remains constant in a particular point in space.
This has been done previously, by using speakers or other resonators to fire pressure waves upwards and bounce them off a reflector. The original waves and their reflections then combine to create a standing wave, with a series of stationary "nodes" that stay put even as the wave oscillates.
If the standing wave has the right frequency, the force at these nodes exactly cancels gravity – and anything trapped there hovers in place.
"Scientists at Argonne National Laboratory have discovered a way to use sound waves to levitate individual droplets of solutions containing different pharmaceuticals. While the connection between levitation and drug development may not be immediately apparent, a special relationship emerges at the molecular level."
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