Contact electrification of insulating materials

TLDR

Contact electrification, the generation of electrostatic charge when two materials are brought into contact and then separated, has been observed for over 2500 years and occurs in natural phenomena such as dust storms and technological processes like electrophotography, yet its underlying mechanism remains poorly understood. This review addresses three open questions: identifying the species transferred during contact electrification, explaining charge transfer between identical insulating materials, and understanding particle‑size dependence in granular systems. We examine recent theoretical and experimental studies that explore non‑equilibrium states and other mechanisms to shed light on these questions.

Abstract

The electrostatic charge that is generated when two materials are contacted or rubbed and then separated is a well-known physical process that has been studied for more than 2500 years. Contact electrification occurs in many contexts, both natural and technological. For example, in dust storms the collisions between particles lead to electrostatic charging and in extreme cases, extraordinary lightning displays. In electrophotography, toner particles are intentionally charged to guide their deposition in well-defined patterns. Despite such a long history and so many important consequences, a fundamental understanding of the mechanism behind contact electrification remains elusive. An open question is what type of species are transferred between the surfaces to generate charge—experiments suggest various species ranging from electrons to ions to nanoscopic bits of material, and theoretical work suggests that non-equilibrium states may play an important role. Another open question is the contact electrification that occurs when two insulating materials with identical physical properties touch—since there is no apparent driving force, it is not clear why charge transfer occurs. A third open question involves granular systems—models and experiments have shown that a particle-size dependence for the charging often exists. In this review, we discuss the fundamental aspects of contact electrification and highlight recent research efforts aimed at understanding these open questions.