Contact electrification between identical materials.

Contact electrification (CE), the transfer of charge between two surfaces that are brought into contact and then separated, plays a central role in several useful technologies, such as photocopying, laser printing, and electrostatic separation methods, but is also responsible for the build-up of charge leading to electrical shocks, explosions, mechanical jams, or damage of electronic equipment. Despite a long history of investigation that dates back to antiquity, the fundamental understanding of the phenomenon remains elusive, and the nature of charge carriers transferred during CE is a subject of ongoing scientific debate. On the other hand, existing theories generally accept that 1) CE requires a difference in material properties 16] and/or of the chemical potentials of the charge carriers on the contacting surfaces (the latter, in the case of grains of the same material but of different diameters), and 2) that the magnitude of charge separation is proportional to the effective/real area of contact. Herein, we demonstrate that neither of these long-held beliefs is necessarily true. Specifically, we show that atomically flat pieces of identical insulators can separate charge by contact electrification and can continue to charge when contacted multiple times (Figure 1). Remarkably, the magnitude of charge Q that develops scales not with the contact area A, but rather with its square root, Q/ ffiffiffiffi