Electrostatics DOI: 10.1002/anie.200905281 Contact Electrification between Identical Materials** Mario M. Apodaca, Paul J. Wesson, Kyle J. M. Bishop, Mark A. Ratner, and Bartosz A. Grzybowski* Contact electrification (CE), [1–3] 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, [4] laser printing, [5] and electrostatic separation methods, [6] but is also responsible for the build-up of charge leading to electrical shocks, explosions, mechanical jams, or damage of electronic equipment. [7, 8] Despite a long history of investigation that dates back to antiquity, [9] the fundamental understanding of the phenomenon remains elusive, and the nature of charge carriers transferred during CE is a subject of ongoing scientific debate. [1, 10–15] On the other hand, existing theories generally accept that 1) CE requires a difference in material properties [1, 6, 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), [17] and 2) that the magnitude of charge separation is proportional to the effective/real area of contact. [1, 15, 18] 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 A p (Figure 2). These observations—supported by theoretical considerations—suggest that CE between identical materials is driven by the inherent, molecular-scale fluctuations in the surface composition or structure of the material. Charge separation was observed when pieces of the same material were contacted: in our experiments, we confirmed this phenomenon for poly(propylene), poly(styrene), Teflon, poly(vinyl chloride), and poly(dimethylsiloxane) (PDMS). Whilst the qualitative features of CE were similar for all these systems (see the Supporting Information, Section 4), most quantitative studies were performed using PDMS, because it can be cast and cured against atomically flat masters and when cured, it is known to come into conformal contact with many types of surfaces, including that of PDMS itself. [19, 20] To prepare PDMS blocks for CE experiments (Figure 1 a), a degassed PDMS/crosslinker (Sylgard 184, Dow) was cast against disjoint regions of an atomically flat [100] silicon wafer (Montco Silicon Technologies, Inc.), either silanized with 1H,1H,2H,2H-perfluorooctyltrichlorosilane or unsilan- ized, and was cured at 65 8C for times between 24 and 96 h. Once cured, the PDMS pieces were gently peeled off the wafer. PDMS casting, curing, peeling, and all subsequent manipulations were performed in a glove-box under an inert atmosphere (nitrogen or argon) and in the presence of Drierite desiccants (W. A. Hammond Drierite Co. Ltd). Prior to CE experiments, any adventitious charge that might have Figure 1. a) Experimental procedure. b) Typical raw data of the charges Q developed on two contacting PDMS pieces as a function of the number of touches n. Q is measured by placing the pieces in a Faraday cup (twice for each piece and condition, hence the splitting of the individual peaks). Note that one piece continues to charge positively whilst the other charges negatively. [*] M. M. Apodaca, Prof. M. A. Ratner, Prof. B. A. Grzybowski Department of Chemistry, Northwestern University 2145 Sheridan Rd., Evanston, IL 60208 (USA) E-mail: grzybor@northwestern.edu Homepage: http://dysa.northwestern.edu P. J. Wesson, Dr. K. J. M. Bishop, Prof. B. A. Grzybowski Department of Chemical and Biological Engineering, Northwestern University 2145 Sheridan Rd., Evanston, IL 60208 (USA) [**] We thank Professors Abraham Nitzan and Alexander Z. Patashinski, and Maksymilian A. Grzybowski for many helpful discussions. This work was supported by the Non-equilibrium Energy Research Center (NERC) which is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0000989. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200905281. Communications 946  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2010, 49, 946 –949