Sound-Levitated Plastic Beads Show Static Charge Builds Up, Then Hits a Ceiling
Bristol physicists used sound waves to collide two tiny polystyrene balls over and over, finding the charge grows like a capacitor filling up and then levels off.
Rub a balloon on your hair and it sticks to the wall. The everyday spark behind that trick, and behind dust-storm lightning and factory fires, has resisted a complete explanation for centuries. Physicists at the University of Bristol have now watched how static charge builds on a single pair of particles, one collision at a time.
When two objects touch and then separate, they can swap tiny amounts of electric charge, even if they are made of exactly the same material. This is the triboelectric effect, the source of everyday static electricity. It remains poorly understood in large samples of powder, where vast numbers of particles collide constantly and no single particle's history can be followed.
In research published in Physical Review E, Tom O'Hara and colleagues tackled the problem with acoustic levitation, using carefully shaped sound waves to hold objects in midair. Their platform, called MultiLev, gives far more control than earlier setups. It lets the team bring two tiny polystyrene balls together, make them collide, then pull them apart again.
To measure the charge on each ball, the researchers placed a pair of Faraday cups on either side of the collision point. Those devices detect the charge on an object placed inside them. The cups were designed to let sound waves pass straight through, so each particle could be returned to its own cup after every collision without disrupting the levitation.
Each individual collision moved a fairly random amount of charge between the balls. But as collisions piled up, a clear pattern emerged. The two particles gained steadily growing opposite charges, and that growth leveled off toward a maximum rather than continuing indefinitely. The behavior closely matches the way a capacitor charges. That speaks to a long-running debate over whether charge on colliding particles keeps growing or approaches a maximum.
The precise process that moves the charge remains unclear. In future studies, the team hopes stronger acoustic traps will let them levitate denser materials and move particles farther apart between collisions for cleaner measurements. They also want to vary how fast the particles collide and track exactly where on their surfaces they touch.
The payoff is practical as well as fundamental. Charge exchange through collisions plays a role in windblown dust, clouds of volcanic ash and the powders handled in many industries. A better understanding of how static builds up in dust storms and volcanic plumes could follow, and the work could help improve safety standards in industries that handle flammable powders, where a single static spark can be enough to trigger an explosion.
