A triboelectric series of identical materials.*

A triboelectric series is an empirical ordering of materials according to their tendency to charge positive or negative when in contact with each other. Despite the fact that the first triboelectric series was reported over 250 years ago, the underlying principle that orders the materials is still unresolved. In this talk, I will show that even identical materials can be ordered in a triboelectric series. Our experiments consist of bringing pairs of soft polymer samples (PDMS) cut from the same batch into uniform contact and measuring the charge exchange with a Faraday cup. Testing all possible combinations of the samples, we find traces of an ordering. Repetition of the same experiment enhances the order until eventually a perfect series is achieved, with samples in fixed positions on the positive/negative end. Exploring the possible properties that could explain the ordering, we discover that contact themselves are influencing the charging and causing the evolution of the series. We propose a simple model to explain these findings and we reproduce our experimental results with simulations. These results could be the key to find the parameter behind the ordering and perhaps the mechanism of tribocharging itself. Consequently, the existence of a triboelectric series of identical materials represents a unique opportunity to shed some light on these mysterious phenomena.