How humidity and contact history control charge transfer
In plain words
The same pair of materials can charge strongly in dry air and weakly, or with reversed sign, in humid air, and repeated contacts change the result. A quantitative account of these effects is missing.
Precise statement
Contact charge depends non-monotonically on relative humidity (often peaking at intermediate values) and evolves with repeated contacts and rubbing. Determine the roles of adsorbed water thickness (as ion source and as leakage path), surface ion mobility and mechanochemical surface change, and give a model predicting charge versus relative humidity and contact number for benchmark pairs (e.g. PTFE-glass, SiO2-SiO2). An answer is a model validated on controlled-atmosphere data.
What would settle it
Charge measured versus humidity and contact number with simultaneous water-layer thickness measurement, fitted by one model across pairs.