Physical origin of the triboelectric series
In plain words
Materials can be ranked in a list, the triboelectric series, in which each material charges negative against those above it. The list is only partly reproducible and no material property is known that predicts it.
Precise statement
Find a measurable material property (effective work function, acid-base surface chemistry, water affinity, Lewis basicity, elastic or fracture properties) that predicts sign and magnitude of contact charge for arbitrary insulator pairs, and determine whether the series is transitive under clean controlled contact or contains cyclic violations. An answer is a predictor with quantified success rate on a benchmark of pairs plus a transitivity test.
What would settle it
Controlled-atmosphere pairwise charging of a large set of materials, with transitivity tested and a single property predicting the order.