Por Nuno Araújo (DF e CFTC, UL).

Abstract: Dust explosions in industrial processes and lightning events in volcanic plumes and sand storms call for a better understanding of granular charging. When one rubs two different insulating materials, they mutually charge in a process known as tribocharging. Several experimental and theoretical works have provided a strong analytic foundation for charging mechanisms due to geometric or material asymmetries. But, recent experimental results suggest that completely identical grains do also charge one another upon contact and the charge difference even increases with multiple contacts. In this case, the mechanism is not at all clear. What breaks the symmetry and sets the direction of charge transfer? How does charge transfer lead to the formation of a strong electromagnetic field in an agitated granular bed? Here, we investigate these questions using particle-based simulations, mathematical modeling, and experiments. We simulate a discrete-element model including electrical multipoles and find that infinitesimally small initial charges can grow exponentially fast. We confirm the predicted exponential growth in experiments using vibrated grains under microgravity, and we describe novel predicted spatiotemporal states. Finally, we discuss how strong fields might emerge even in the absence of charge transfer and how granular electrostatics can play a role in the formation of protoplanets.