Spontaneous Self-Organization of Identical Materials into a Triboelectric Series

Experiments have long suggested Contact Electrification (CE)—the exchange of electric charge through contact—is transitive, with different materials ordering into 'triboelectric series' based on the sign of charge acquired. At the same time, the effect is plagued by unpredictability, preventing consensus on the mechanism and casting doubt on the order series imply. Here we expose an unanticipated connection between the unpredictability and order in CE: nominally identical materials initially exchange charge randomly and intransitively, but over repeated experiments self-organize into triboelectric series. We find that this evolution is driven by the act of contact itself—samples with more contacts in their history charge negatively to ones with less. Capturing this 'contact bias' in a minimal model, we recreate both the initial randomness and ultimate order in numerical simulations, and leverage it experimentally to force the appearance of a triboelectric series of our choosing. With a battery of surface sensitive techniques to search for the underlying alterations contact creates, we only find evidence of nanoscale morphological changes, pointing to a mechanism strongly coupled with mechanics. Our results urge us to more carefully consider the essential role of contact history in CE and suggest that focusing on the unpredictability may hold the keys to understand it.