(631e) Contact Charging Between Insulators: A Theoretical Study

Triboelectric charging – the electrostatic charge that develops on surfaces when they are rubbed together – is ubiquitous, and has important consequences in many industries and many natural phenomena.  However, the mechanism of the charging process remains very poorly understood. For example, it is not yet known what type of species are transferred during the charging process (electrons or ions), and there is no way to predict which surface charges positive and which charges negative. To give insight, we use a combined theory and experiment approach.  We address a simple system – well-defined crystallographic faces of single-crystal quartz (SiO2) and sapphire (Al2O3) – so that high level theory can be applied.  The simulations use first-principles molecular dynamics, where density functional theory is used to determine the electronic structure of the system and the interatomic forces.  Both dry and wet surfaces were investigated, where wet surfaces take into account the effects of ambient humidity. For dry situation, our results show that an electron transfer mechanism would cause sapphire to charge positively and quartz to charge negatively, in agreement with results from the experiments.  This direction of charge transfer is shown to correlate with electrostatic potential energy on both surfaces. We also carry out simulations to with water, to address an ion transfer mechanism.  Dissociation and interaction of water has been observed, and the trajectories of the hydroxyl ions (OH-) and hydrogen ions (H+) are examined. Additional simulations have addressed the interaction of quartz and sapphire with periclase (MgO).