Same-material tribocharging: single-collision statistics point to a global, surface-water driven mechanism

Models for same-material contact electrification often rely on a yet-to-be-identified local parameter whose stochastic fluctuations would drive charge exchange. Using acoustic levitation, we measure the charge gained by individual granular spheres after repeated contacts with substrates of the same material. By looking at the charge distributions, we find that the charge-driving parameter is global instead of local. Furthermore, we show that this parameter is not intrinsic to the samples, but acquired during sample preparation, as cleaning the samples fully resets their charging behavior. Changes in humidity affect charging irreversibly and in a random direction. Ultimately, our results point to a mechanism where adsorbates, likely water, recruited on the sample throughout its history play a driving role.