How Contact Electrification Depends on Particle Size
When microparticles of the same insulating material collide, how do pairs of particles become oppositely charged? Although the question remains unresolved, Nicolás Mujica of the University of Chile and his collaborators have now uncovered an important clue [1]. By observing uniformly sized collections of particles in free fall, the researchers discovered that for each particle size, the range of surface charge densities found in the collection is the same. The finding could lead to a better understanding of a host of phenomena, including the generation of lightning in volcanic plumes and the pneumatic transport of coal dust in pipes.
In their experiment, Mujica and his collaborators used batches of composite particles made of zirconium dioxide and silicon dioxide. In each of the six batches, the particles had a uniform diameter, which ranged from 172 to 545 µm. A run began by releasing particles from a hopper to fall under gravity through an evacuated transparent tube. As they made their way out of the hopper, the particles rubbed against each other, becoming either positively or negatively charged. The release triggered the dropping of a camera, which recorded video of the particles as they fell. Electrodes on either side of the tube generated a static electric field, which produced a sideways acceleration of the charged particles. Combining the particles’ known masses with their measured accelerations, the researchers calculated the particles’ charges.
For each particle size, the researchers plotted the charge probability distribution—the likelihood for each charge value to be found on any given particle. Since the charges were both positive and negative, the distributions were peaks centered at zero but with different widths. Mujica and his collaborators found that those differences disappeared when they plotted surface charge density instead. Although the physical origin of the distribution is unclear, Mujica says that microscopic models of charge exchange now have a new and clear constraint to satisfy.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- M. Lara et al., “Particle size scaling of non-Gaussian granular charge distributions,” Phys. Rev. Mater. 10, 045604 (2026).