Granular charging at a distance

Contact charging of granular materials can play a dominant role in effects as varied as lightning in sandstorms, the formation of extraterrestrial planetesimals, and segregation of pharmaceutical powders. Yet the physics underlying this charging is exceptionally poorly understood. For example we have no clear theory for how insulating grains recruit enough charge carriers to deposit charge but not enough to discharge. In this talk, we note that charging and discharging kinetics may be distinct, and from this observation we develop a mathematical model. The model surprisingly predicts that charging can decrease as contact frequency increases. We confirm this prediction experimentally in a vibrated bed and propose future steps.