Triboelectric charging, also known as contact charging, is a process that occurs when two initially neutral surfaces are in contact, transfer charge, and upon separation, remain electrically charged. This charging occurs in a wide range of contexts, including industrial applications such as fluidized beds, pharmaceuticals, dust explosions and pneumatic conveying, and natural phenomena such as sand storms, dust devils, lightning and volcanic plumes. The mechanism of triboelectrification of granular materials is poorly understood, especially in single component systems.
Triboelectric charging of chemically uniform, insulating granular materials with a broad size distribution is generally known to exhibit bipolar charge segregation, characterized by smaller negatively-charged particle and larger positively-charge particles. This study focuses on the development of experiments to validate previously accepted theory and experimental observations of bipolar charging in these systems. Samples of soda lime glass beads with bimodal size distributions are initially charged in a flow apparatus that induces only particle-particle interactions in a controlled environment. Particles are separated and characterized by their charge polarity and sized using optical methods. Charging behavior is found to be intimately coupled to the relative size and mass concentration of the particle size distribution with smaller particles tending to charge negatively and larger particles charging positively. Several single component systems are examined where this phenomenon occurs including polyethylene resin particle, glass beads, and Mars simulant (JSC-1 Mars simulant). These results are explained by a population balance model, in which the charge segregation occurs from an asymmetrical transfer of a negative species (i.e. electrons or hydroxide ions) between small and large particles.