(111e) Nonequilibrium Dynamics and the Electrostatic Charging of Granular Systems

As granular materials flow, the particle contacts can cause electrostatic charges to develop on the particles. Despite the widespread significance of this phenomenon, the origins of the electrostatic charging are unclear: e.g., when all particles are chemically identical, this would seem to preclude any driving force for charge transfer. We suggest that this electrostatic charging is a special case of a more general class of non-equilibrium phenomena. For species that cannot equilibrate between states on a particle surface (e.g., electrons in insulators), only particle collisions can facilitate transitions to different states. The non-equilibrium dynamics that follow from collision-induced transitions generate species accumulation on a particle-size-dependent subset of the system. These ideas rationalize experiments on granular systems which show that smaller particles charge negatively, and that the charging magnitude increases with the breadth of the particle size distribution.