Spontaneous ordering of identical materials into a triboelectric series.

When two insulating, neutral materials are contacted and separated, they exchange electrical charge. Experiments have long suggested that this 'contact electrification' 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 rhyme and reason that series imply.