(338e) Electrostatic Charging in Granular Flow

When granular materials flow, the particles rub against one another and become electrostatically charged.  The electrostatic charge on the particles can greatly alter how the material flows; this can have important consequences, such as disrupting industrial operations.  We use a combined experimental-theoretical approach to address the electrostatic charging in granular materials with a polydispersity in particle sizes.  We have developed a methodology to investigate the electrostatic charging due strictly to particle-particle interaction, such that the particles being examined have no contact with any surface besides those of other particles.  Our experiments show that under these conditions smaller particles tend to charge negatively and larger particles tend to charge positively. Results on a range of granular materials give the same result regarding the particle size dependence of electrostatic charging.  We believe that this size dependence is universal (i.e., independent of material properties) in single component granular systems, and we describe a simple theory based on statistical mechanics to explain why this universality occurs. The implications of this bipolar charging on the flow of the material are discussed.