When two objects touch and then separate, they can swap tiny amounts of electric charge, even if they are made from exactly the same material. This ‘triboelectric effect’ is the source of everyday static electricity, but it is still poorly understood in large samples of powder, where vast numbers of particles are constantly colliding with each other.

Through new research published in Physical Review E, Tom O’Hara and colleagues at the University of Bristol have investigated the effect using acoustic levitation: using sound waves to trigger repeated collisions between pairs of particles. Their experiment shows for the first time how charge builds up on repeatedly colliding particles, before leveling off at a set limit.