Sound Waves Hold Heavy Metal Aloft

Look, no hands! Reverberating sound waves suspend this bead of iridium.

Scientists have levitated globs of the heaviest solid and liquid so far--iridium and mercury, respectively--using only ultrasonic waves. Their novel apparatus is simpler and more powerful than previous sound-generating setups. The hands-off research is more than just a parlor trick: It could improve production techniques for everything from pharmaceuticals to superconductors.

Some materials can be too hard to handle, such as corrosive fluids that eat through nearly any container or superheated substances that would melt their crucibles. And crystals that grow on surfaces can end up contaminated or imperfectly formed. Acoustic levitation bids to solve such problems by using sonic pressure waves to float substances in thin air. But the technique has suffered from weakness and wobbliness, says materials physicist Wen-Jun Xie at China's Northwestern Polytechnical University in Xi'an, largely limiting it to lightweight organic materials or water.

Now Xie and his colleagues report they have used cushions of sound waves to float roughly gram-sized chunks of the heaviest solids and liquids using only a single emitter. Before this, it took 130 acoustic emitters to levitate a lighter gold bead the same size, Xie says. Instead of using brute force, the team relied on geometry, putting the flat emitter over a bowl-shaped cavity that acts as a reflector. Sound waves bouncing between the two reinforce each other, leading to a strong, stable sonic cushion, the team reports in an upcoming issue of Physical Review Letters.

Materials scientist Richard Weber of Containerless Research Inc. in Evanston, Illinois, finds the device elegant and probably portable. "It's remarkable that they levitated mercury ... with the sound pressure required to do that, you'd expect to atomize it," Weber said. The size of any levitated samples is currently limited to half the wavelength of sound used, however, so Xie says acoustically hovering trains are unlikely.

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