Electrostatic discharges are the cause of bipolar charge mosaics at scales between 0.1 mm and 50 mm*

The phenomena of dielectrics becoming charged upon contact and separation have perplexed scientists and engineers over centuries. In traditional understanding, the charges that appear on the two surfaces are due to the qualities of the materials in contact, they are of opposing polarities and are generally evenly distributed. In recent decade or so, it was found that contact electrification can lead to uneven charge distributions -- occasionally even resulting in positively-charged and negatively-charged regions coexisting as neighbours on each of the two surfaces. It's commonly believed these patterns signify some omnipresent spatial irregularities within the contact materials.

We have investigated these charge inhomogeneities in experiments where PMMA and PDMS surfaces were contacted and then disjoined to produce contact charges on them. By observing very weak flashes of light accompanying the electrostatic discharge (ESD) events during detachment of the two surfaces, and comparing their spatial map with the maps of charge density measured by scanning Kelvin probe, we provide direct evidence showing that bipolar charge patterns at 0.1-50 mm scales are created by sequences of spontaneous ESD events between dielectric surfaces during detachment. We show how elementary electrostatics permits that a discharge process (dielectric breakdown of gas) can produce charge inversion, not just neutralization to zero charge density. We implement a simple numerical model that simulates the sequence of ESDs accompanying the peeling process and generates the resulting charge density map, which can turn out bipolar even if the initial (primary) charge density is uniform.

Most of these results were reported in Sobolev, Y.I., Adamkiewicz, W., Siek, M. et al., Nature Physics 18, 1347–1355 (2022). In the present talk, I discuss the relation of these results to recent studies in contact electrification by other authors, outline the possible future directions, talk about the experimental obstacles we have encountered, and the drawbacks of our theoretical model.