When granular materials flow, the particles rub against each other and can become electrostatically charged. The charging of particles causes several problems, such as disrupting the flow or even initiating explosions. While ?triboelectric charging? is one of the most well-known areas of science -- everyone notices it when they walk across a rug and get a shock as they touch a doorknob, or when they rub a balloon on their hair and see how the balloon and hair become charged -- its scientific basis is in essence completely unknown. For example, it is not clear whether the species transferred between surfaces are electrons, ions, or bits of material. This project will take a rational approach to understanding triboelectric charging in granular materials by developing simple model systems that focus on fundamental questions ? these model systems include granular materials with particular properties and special flow geometries, and bulk materials rubbed in controlled geometries to more simply address issues relevant to granular systems. Our investigation will consist of three thrusts: (1) particle flow experiments with specially chosen particles and flow patterns, (2) asymmetric rubbing experiments as a model for polydispersity effects in granular materials, and (3) experiments with well-defined material surfaces that can be compared with theory/simulation.
Our experiments will elucidate several key questions, including what type of species (e.g., electron, ion, or bits of material) are transferred during the charging process, what is the role of the particle size distribution in the charging of granular materials, and how does humidity (i.e. water adsorption on surfaces) influence charge transfer. By understanding triboelectric charging from a fundamental perspective, our work may help scientists and engineers working in industry address problems related to this pervasive issue.
