Charge segregation depends on particle size in triboelectrically charged granular materials

Experiments are carried out to examine triboelectric charging in granular systems composed of particles that are chemically identical but differ in size. A methodology is developed so that only particle-particle interactions (but not particle-wall interactions) contribute to the charging. Since all particles are chemically identical, there is no apparent driving force for charge transfer, but charging occurs nonetheless, such that smaller particles tend to charge negatively while larger particles tend to charge positively. For bimodal systems, a model for the frequency of collisions of particles with different size predicts the concentrations for which the observed charge segregation is maximized.