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. 2017 Dec 15;3(12):eaap8576. doi: 10.1126/sciadv.aap8576

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Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

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Fig. 1 — Two materials with different work functions φ_w exchange electrons when brought into contact, generating surface charges. Rubbing is not essential as long as intimate contact is made. When the two materials separate, an electrostatic voltage is produced, which establishes a force to move the charges back if there is any conducting path. When at least one of the materials is not conductive at the surface, surface charges remain after the separation.