Surface contact charging.

Experiments in several laboratories have demonstrated that identical materials brought into repeated contact generate unexplained and growing surface charge domains. Here we show that the growth of charge from these experiments can be fitted to a previously developed first-principles model for contact charging based on feedback of random surface polarizations. Surprisingly this mechanism, which leads to exponential growth in colliding granular beds, can also explain nonexponential growth of surface charging, as well as predicting spatiotemporal growth of charge domains and their dependencies on material parameters.