Long-standing and unresolved issues in triboelectric charging

Static electrification is among the earliest of the sciences, well known to us all and with widespread and important consequences. Yet, its most basic foundations remain poorly understood. For example, after centuries of research, it is still not clear whether electrons, ions or even bulk material transfer is responsible for the observed charging. Recent work has leveraged the most advanced experimental and theoretical approaches, and has addressed the phenomenon from perspectives of quantum mechanics, surface chemistry, mechanochemistry and statistical physics. While the resulting findings have advanced many aspects of our understanding, they have also led to the discovery of new surprises that we are only beginning to appreciate. This Review addresses both recent advances and their accompanying surprises. Triboelectric charging is well known to us all and has widespread and important consequences. Nonetheless, its most basic foundations remain poorly understood, and progress is often countered by the emergence of baffling new observations. Recent work shows the difficulty may arise because charging is governed by competing and unstable dynamical processes.