The short version
- Physicists in Bristol used shaped sound waves to hold two plastic beads in midair and bump them together repeatedly while measuring their electric charge.
- Each bump moved a random amount of charge, but over many bumps the two beads built up opposite charges that levelled off at a maximum.
- The work could help explain sparks in dust storms and volcanic ash, and in factories that handle flammable powders.
Physicists at the University of Bristol have used sound to watch static electricity build up one collision at a time. Their “MultiLev” platform uses carefully shaped sound waves to hold two tiny polystyrene beads in midair, knock them together, and pull them apart again. A pair of Faraday cups on either side, designed to let the sound pass through, measures each bead’s charge after every bump. The paper, led by Tom O’Hara, is published in Physical Review E.
Each collision moved a fairly random amount of charge. Added up, though, the two beads built steadily growing opposite charges that levelled off toward a maximum, the way a capacitor charges. That model fit the data closely, with R² above 0.99. Beads coated in conductive graphite charged too, but built up much less.
Charge from colliding grains builds up in windblown dust, dust storms and volcanic ash clouds, and a static spark can set off an explosion where flammable powders are handled. The team says the mechanism of the charge exchange is still unclear, its traps cannot yet lift denser materials, and it cannot yet vary collision speed.
