Abstract
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.