Does Flexoelectricity Drive Triboelectricity?*

Since the first reports of friction-induced static electricity in 600 B.C., the phenomenon of triboelectricity has fascinated, and perplexed, generations of scientists. While much progress has been made in the ensuing centuries regarding the nature and identification of charged species transferred during tribocharging, a universal thermodynamic driver for charge transfer has not been found. We identify flexoelectric potential differences induced by inhomogeneous strains at nanoscale asperities as the thermodynamic driver for tribocharge separation [PRL 123, 116103 (2019)]. Using single asperity elastic contact models, we show that nanoscale flexoelectric potential differences of 1–10 V or larger arise during indentation and pull-off. Importantly, we also demonstrate our model agrees with several experimental observations including bipolar charging during stick slip, inhomogeneous tribocharge patterns, charging between similar materials, and surface charge density measurements.