Researchers at the University of Bristol have shown that the electrical charge accumulated through repeated collisions between two particles follows a clear and predictable pattern. Although each individual collision transfers a seemingly random amount of charge, the overall build-up behaves in a way similar to a charging capacitor, offering new insight into a long-standing question about how charge accumulates in clouds of colliding particles.
The study was led by Tom O’Hara and used a specially designed setup that levitates particles with sound. Triboelectric charging, the process that generates static electricity, is one of the most familiar effects in physics. Classroom demonstrations show that electrical charge can be exchanged whenever two objects touch and then separate, with the polarity and strength of the charging depending on the materials involved. Despite how common the effect is, physicists’ understanding of the underlying process remains incomplete.
“Triboelectric charging has been known since the time of Plato, but the fundamental mechanisms governing charge transfer remain incompletely understood,” O’Hara said. “This is especially important for insulating powders, where the large surface area allows for large amounts of surface charging.”
The phenomenon occurs in natural settings such as volcanic ash, dust storms and thunderstorm clouds, where charge build-ups eventually give way to lightning strikes. It also poses significant risks in industry, where triboelectric charge can ignite flammable dust or vapour clouds. Because these systems involve vast numbers of collisions under widely varying conditions, it has been extremely difficult to isolate what happens in any single collision.
To address this, the Bristol team built on an apparatus it developed in 2017. That earlier device, called TinyLev, uses acoustic levitation to control the motion of individual particles and can be built from low-cost, off-the-shelf components. It can trap objects including silica spheres, water droplets, and even small insects and electrical components.
The team has now unveiled MultiLev, a more controllable version of TinyLev. The new device traps two polystyrene particles and makes them collide repeatedly. The setup includes a pair of opposite-facing Faraday cups, which measure the charge on a particle placed inside them by detecting the current it induces, using a sensitive picoammeter. The cups are made from wire mesh with carefully chosen spacing, which makes them transparent to the sound waves used for levitation. Each particle can therefore be levitated inside its own cup, brought together with the other particle in the space between the cups by adjusting the acoustic field, and then returned to its own cup, where the charge transferred during the collision can be measured.
In the first stage of the experiments, the charge transferred in individual collisions appeared random, with values following a skewed normal distribution. As the researchers induced more and more collisions, however, a clear cumulative pattern emerged. Each particle’s charge rose steadily at first, then levelled off towards a maximum saturation value.
“This is consistent with the previously proposed condenser model, which treats the particles’ charging like that of a capacitor,” O’Hara said. The results show that while individual collisions can vary widely in the charge they transfer, many such events together produce predictable behaviour over time.
“By establishing a clear relationship between repeated particle collisions and charge saturation in a controlled system, we provide a useful foundation for investigating triboelectric charging in more complex environments,” O’Hara added.
The researchers suggest the approach could help scientists better understand charge build-up in real-world settings. That understanding could in turn contribute to better predictions of extreme weather events and to more informed safety regulations for manufacturing industries. The research is described in Physical Review E.
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