Fig. 1: Identical materials spontaneously order into a triboelectric series.

a, We prepare identical samples of PDMS by curing a single parent slab and then cutting eight smaller (1 cm × 1 cm × 0.3 cm) pieces, labelled A–H. Scale bar, 1 cm. b, We mount sample pairs on PTFE rods and use a linear actuator to push them together. Inline force feedback allows us to reach a set pressure (P ≈ 45 kPa, approximately 1% macroscopic strain). We measure charge exchange with an electrometer connected to a Faraday cup enclosing one sample. Before all measurements, samples are discharged to <0.5 pC (see Methods). The chamber/sample storage area is held at 30 ± 2% RH and 22 ± 1 °C. c, Example of charge versus contacts with two pristine samples, one charging persistently positively and the other negatively. d, Charge exchange between two pristine samples that exhibits an unpredictable sign change. To average over such variability, we define the average exchange after five contacts as ΔQ₅, which is always measured after both samples have been completely discharged. e, To test for a triboelectric series, we measure ΔQ₅ for all pair combinations with a staggered contact sequence (see Methods), creating a matrix in which the colours indicate charge given to the column sample. f, The inability to sort the matrix (see Methods) in e such that the upper-right (lower-left) corner is purely positive (negative) indicates that the samples charge randomly, that is, do not form a series. g, As highlighted in f, the defects in a series are indicative of cycles; here A charges positively to G, E charges positively to A, but E charges negatively to G. h, Repeating tests over several days with the same set of samples, randomness gives way to order and, by the fifth day, the samples form a perfect triboelectric series.