Putting a New Spin on Sound Waves

It’s already possible to do some really extraordinary things with sound waves, like levitating small particles and manipulating them in-air (useful for caustic chemistry reactions) but we’re about to see another tool added to the sonic utility belt: spin. Scientists from Nanjing University in China have recently created a passive device that, for the first time, easily allows planar sound waves to be converted into corkscrew-shaped spiral waves without requiring elaborate geometric arrangements of sound sources.

An illustration of the new passive angular filter 

The new device will allow researchers and engineers to easily adapt systems to impart angular momentum—spin—onto objects from afar. The device should find applications in the same kind of sonic levitation experiments in the video linked above, providing an extra dimension of control over the motion of particles.

While circularly-polarized sound waves like this aren’t new, this one appears to set records for simplicity of design and ease of use, according to the authors of a paper slated for publication tomorrow in Physical Review Letters which describes the device’s development.

Angular momentum is one of the more surprising and counterintuitive aspects of physics—even after hundreds of times doing it, the classic “bike wheel” demonstration of angular momentum’s conservation still amazes me. When I first started working in a lab as an undergraduate, I was amazed to learn that part of our experiment involved using spiraling light from a circularly-polarized laser to spin the nuclei of helium atoms—essentially the same process enabled by this new device, but at a much smaller scale, with photons acting on atoms suspended in a magnetic field to get all the atoms in a gas spinning the same direction. In some respects, the new device described in the forthcoming article does the same thing for sound waves that the “corkscrew filter”—technically called a quarter wave plate—does for light.

You can learn more about the wizardry of acoustic levitation here, and check out the full paper when it becomes available here.

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