As granular materials flow, the particle contacts can cause electrostatic charges to develop on the particles. This electrostatic charging is often significant in industrial processes such as fluidized beds, pneumatic conveying, and spray deposition, and pharmaceutical dispersal processes such as dry powder inhalers. Despite the widespread significance of this phenomenon, the origins of the electrostatic charging are not well known. In particular, it is not clear why charging occurs when all particles are composed of the same material, which would seem to preclue any driving force for charge transfer during particle contact.

In our work, particle dynamics simulations are carried out to address the origins of this electrostatic charging. The simulations test our hypothesis that the electrostatic charging in single-component granular materials can be explained in terms of simple population balance ideas. The simulations are based on hard-sphere particle dynamics coupled with an ad hoc model for the collision-induced transfer of trapped electrical charges. In this model, trapped electrons are randomly placed on the surface of the particles, and these electrons are transferred by a collision if the trapped electron is within a cutoff distance from the point of collision. The simulation results show that these simple ideas alone lead to electrostatic charging that increases in magnitude with the breadth of the particle size distribution, which has also been observed in experimental studies. Furthermore, the simulations show that smaller particles tend to charge negatively and larger particles tend to charge positively, as found in experimental and field studies.