Floating on sound: how sound waves are cracking a static mystery

Did you know sound can hold things in the air? It's called acoustic levitation. With carefully shaped sound waves, you can trap tiny objects in mid-air, with nothing touching them. I knew about the party trick. I didn't know scientists were using it to answer a real question until I read about this one.

The question is about static electricity. When two things touch and come apart, they can swap a tiny bit of electric charge, even when they're made of exactly the same stuff. That's the "triboelectric effect", and it's behind everyday static. It turns out it's still not well understood, especially in powders, where huge numbers of particles keep crashing into each other. Researchers have long argued about whether the charge on colliding particles keeps growing forever or tops out somewhere.

Tom O'Hara and his colleagues at the University of Bristol built a sound-levitation setup they call MultiLev. It let them float two tiny polystyrene balls, make them collide, and pull them apart again. Each ball then went back into its own "Faraday cup", a device that measures charge. The cups were designed to let sound pass straight through, so the levitation kept working.

Here's what they found. Each single bump swapped a fairly random amount of charge. But over many bumps a pattern showed up. The two balls built up opposite charges that grew steadily and then levelled off at a limit, a lot like a capacitor charging up. The study is published in Physical Review E.

What I love most is that they still don't know exactly how the charge gets swapped. The team hopes stronger sound traps will let them float heavier materials and control the collisions even more closely. They say this could help explain static in dust storms and volcanic plumes, and make factories that handle flammable powders safer.

As a sound designer, I get excited when sound becomes a tool and not just something we listen to. Sound can push, hold and move things. We're still learning what else it can do, and I think that's the fun part.

Source: Phys.org, "Sound-levitated particles reveal how static charge builds up" (28 Oct 2026), https://phys.org/news/2026-10-levitated-particles-reveal-static.html. Paper: O'Hara et al., Physical Review E (2026), https://doi.org/10.1103/p9zp-hf26

Takeaway: Sound doesn't just fill a room. It can hold things in the air and help us answer questions we're still asking.