Charge mosaics on contact-electrified dielectrics result from polarity-inverting discharges

Charging of dielectrics on contact and separation has puzzled scientists and engineers for centuries. In a conventional view, the charges emerging on the two surfaces derive from the properties of the contacting materials, are of opposite polarities and are distributed approximately uniformly. However, a body of evidence has been mounting that contact electrification can also produce heterogeneous charge distributions in the form of (+/-) charge mosaics on each of the surfaces—yet, despite many attempts, no predictive model explaining the formation of mosaics at different length scales has been proposed; the main line of thinking has been that they must reflect some spatial heterogeneity present in the contacting materials. Here we describe experiments and theoretical models that prove a fundamentally different origin of mosaic formation: namely, not due to the properties of the contacting materials but due to electrostatic discharges between the separating surfaces. In particular, as the gap between the contact-charging surfaces grows, the threshold of the electric-field magnitude required for electrostatic discharge by Paschen’s law decreases, and eventually becomes lower than the electric field created in the gap by surface charges. Once a discharge starts, it continues not only until neutralizing but also locally inverting the surface charges. It is then the cycles of such discharges along the delamination front that give rise to the bipolar charge mosaics.