Sound-Levitated Particles Reveal a Predictable Pattern in Static Charge
When two particles bounce off one another again and again, each collision moves an unpredictable amount of electric charge between them, yet the total follows a clear pattern. That is the finding of physicists at the University of Bristol, led by Tom O'Hara, who studied repeated collisions of two particles held in the air by sound. The result was published on October 23 in Physical Review E under the title "It takes two to tribo: Stochastic charge evolution in repeated binary collisions of acoustically levitated particles".
An old effect, still not fully understood
Triboelectric charging, the process that generates static electricity, happens whenever two objects touch and separate: charge is exchanged, and its sign and size depend on the materials involved. "Triboelectric charging has been known since the time of Plato, but the fundamental mechanisms governing charge transfer remain incompletely understood," O'Hara explains. "This is especially important for insulating powders, where the large surface area allows for large amounts of surface charging."
The effect appears in volcanic ash, dust storms and thunderstorm clouds, where charge build-up eventually ends in lightning. It is also a risk in industry, where charge can ignite flammable dust or vapour clouds. Because such systems involve huge numbers of collisions under varying conditions, it is very hard to isolate what happens in a single collision.
How they did it
In 2017 a Bristol team built TinyLev, a low-cost acoustic levitator made from off-the-shelf parts that can trap objects such as silica spheres, water droplets and small insects. The new device, MultiLev, is a more controllable version. It traps two polystyrene particles and makes them collide repeatedly.
The set-up includes two opposite-facing Faraday cups, which measure the charge on a particle placed inside by detecting the current it induces with a sensitive picoammeter. The cups are made of a wire mesh with carefully chosen spacing, so the sound waves used for levitation pass through them. This let the researchers levitate each particle inside its own cup, adjust the acoustic field to bring the particles together between the cups, and then return each particle to its cup to measure the charge that was transferred in the collision.
Random steps, a predictable total
At first, the charge exchanged in individual collisions looked random, with values following a skewed normal distribution. But as the team induced more collisions, a clear cumulative pattern emerged: each particle's charge rose steadily at first and then levelled off towards a maximum, a saturation value. "This is consistent with the previously proposed condenser model, which treats the particles' charging like that of a capacitor," O'Hara says.
"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," he adds. According to the report in Physics World, the approach could ultimately help researchers understand charge build-up in real-world settings, which could lead to better predictions of extreme weather and more informed safety rules for manufacturing industries. See more in our Science section.