Spontaneous tribocharging of similar materials

We investigate the spontaneous triboelectrification of similar materials. This effect, first reported in 1927, has been little studied but is easily reproduced. We find in two separate experimental systems, where materials are prepared in the same way and rubbed symmetrically, that symmetry breaking occurs so that one material becomes positive and the other negative. Curiously, the distribution of charges on the materials appears to be self-similar, with different charge patterns on the positive and the negative surface. We propose a mechanism in which an initial localized charge may spawn the production of smaller localized charges of the same polarity. Copyright c © EPLA, 2008 Introduction. – It has been recognized since the 16th century [1,2] that insulators (originally termed “electrics”) can readily be charged by rubbing or contact, while conductors (“non-electrics”) are more difficult to charge. Careful experiments have shown this effect to be distinct from the ability of conductors to carry acquired charges to ground [3]. Thus PTFE —one of the best insulators known— is the material of choice to produce megaVolt potentials in Van de Graaf generators. This is paradoxical since insulators lack free charge carriers, and it has remained unexplained to this day how they recruit these charges. The paradox has been compounded by experiments first performed in the 1920s [4] and later rigorously repeated [5,6] that demonstrate that even identical insulators can transfer charges during rubbing. In one sense, this should come as little surprise, since charge is also generated through contact between identical materials in the best known electrification system: atmospheric lightning. Similarly, large charges are acquired by aeolian transport of desert sands, which have little to rub against beyond other sand [7–9]. It is troublesomely unclear how insulators in general acquire the free charges that they convey during tribocharging, and how identical insulators in particular can charge one another at all. In this letter, we investigate the mechanism by which charges can be acquired and amplified by contact between (a)E-mail: [email protected] identical insulators in two independent experiments. Unexpectedly, we find that self-similar charge distributions seem to appear spontaneously on contacting surfaces, and we find evidence to suggest that this may occur through an instability by which localized charged regions beget further charges of the same sign. We find, furthermore, that insulators appear to charge at least in part by recruiting ions from the surrounding atmosphere. Results. – To tribocharge similar insulators, we study a first model system that is easily reproduced: symmetric rubbing of common latex balloons as shown in fig. 1(a), and a second, more carefully controlled, laboratory system consisting of rolling of insulating disks in an instrumented apparatus sketched in fig. 2(a). In the first system, balloons are obtained from the same lot, are the same color, are inflated to the same size, are initially washed with alcohol to remove surface oils or dirt, and are rubbed together symmetrically. As shown in fig. 1(a), the balloons spontaneously charge sufficiently to suspend one balloon from the other. A simpler experiment could scarcely be conceived, yet measurements (see fig. 1 caption) reveal that the balloons are initially chargeneutral, but following rubbing break symmetry to generate net charges, with one becoming highly positive (> 3 kV at 5 cm distance) the other equally negative. The choice of balloon polarity appears to be random: if we charge a pair of balloons and neutralize them by cleaning with alcohol, we find that, once dried and rubbed together again, the