Contact Electrification DOI: 10.1002/ange.201200057 Material Transfer and Polarity Reversal in Contact Charging** H. Tarik Baytekin, Bilge Baytekin, Jared T. Incorvati, and Bartosz A. Grzybowski* Since the times of Volta, [1] scientists have strived to construct the so-called triboelectric series (TES) that would rank the contact-charging properties of various materials (Fig- ure 1 a,b). While for contacting metals such ranking has been relatively straightforward, [1c,d, 2] construction of a robust TES for bulk dielectrics has proven surprisingly challenging. In fact, the literature on the subject has been marred with irreproducible results, including situations where different polarities of charge have been reported by different labo- ratories for supposedly the same contact-charged materials. [3] Such ambiguities have been noted and discussed in Refs. [3f, 7c,d,f]. Here, we show that contact charging of dielectrics cannot be predicted solely on the basis of their bulk properties—instead, it is necessary to account for the transfer of minute amounts of material between the contacting surfaces. Material transfer can lead to such counterintuitive phenomena as the reversal of charge polarity during contact charging, and is related to the differences in the mechanical properties of the contacting dielectrics. For decades, the study of contact electrification has been mostly phenomenological in nature, rarely reporting more than the measurements/trends in the charges developed on macroscopic objects—not surprisingly, these studies have provided limited insights into the microscale phenomena underlying charging. It has only been relatively recently that surface characterization techniques have become available that allow for studying the changes in surface composition of the contact-charged surfaces with nanoscopic resolution. In this context, an important recent result from our group [4a] and others [3h, 5] is that contact charging is not solely due to the transfer of charge carriers (be it electrons, [6a] ions, [3f,g] or both [6b] ) between the surfaces but entails spatially inhomoge- neous material transfer. [7] The patches of the transferred materials give rise to characteristic charge mosaics (i.e., regions of opposite polarities, (+) and (À)) on each of the contact-charged surfaces. Still, because the amounts of materials exchanged between the surfaces are very small (less than a microgram per square centimeter; see discussion later in the text), it remains unclear whether material transfer is just a secondary effect modifying only slightly the inherent charging properties of the bulk materials, or whether it can change the charging properties of the contacting materials Figure 1. Construction of a triboelectric series for dielectrics is one of the oldest pursuits of modern science. a) Volta demonstrating “electro- phorus”, an electrostatic machine, to Napoleon who first honoured him with a medal and then raised him to the position of a Count and a Senator of the Realm of Lombardy. [19] (Portrait by Nicola Cianfanelli) b) A traditional triboelectric series of various dielectric materials (data adapted from Ref. [3g]). In this particular picture, the propensity of the material to charge negatively upon contact increases to the right (e.g., PTFE is expected to charge negatively when contacted with PS.). c) Illustration of experiments in which polymer beads are shaken against a polymeric dish (agitated by a LinMot shaker) and the charges developed on the beads and on the dish are measured by a Faraday cup connected to a high-precision electrometer (for details, see the Supporting Information). d) A typical charging curve for PTFE beads (here, n = 50 beads, red markers show total charge on all beads) and the PS dish (blue markers) for different times of charging/ shaking, t. e) Total charges on different numbers of beads (n = 10 (red), 25 (green), 50 (blue)) plotted as a function of charging/shaking time, t. The markers correspond to experimental data, continuous lines serve to guide the eye. Arrows indicate times, t r , when the beads change polarity—as shown in the inset, these times decrease with n. In both (d) and (e), the error bars are based on the standard deviations of the bead charge distributions, averaged over five independent experiments for each condition. [*] Dr. H. T. Baytekin, Dr. B. Baytekin, J. T. Incorvati, Prof. Dr. B. A. Grzybowski Department of Chemistry and Department of Chemical and Biological Engineering, Northwestern University 2145 Sheridan Road, Evanston, IL 60208 (USA) E-mail: grzybor@northwestern.edu Homepage: http://dysa.northwestern.edu/ [**] 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. We thank Bruker for supplying PeakForce QNM software. XPS and Raman spectra were taken in KECK-II and BIF facilities at NU. H.T.B. and B.B. contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201200057. A ngewandte Chemi e 4927 Angew. Chem. 2012, 124, 4927 –4931  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim