A triboelectric series of identical materials.

A triboelectric series is an ordering of materials according to their tendency to acquire positive or negative charge when in contact with each other. Despite the fact that the first triboelectric series was experimentally established over 250 years ago, the underlying principle that orders the materials is still unresolved. Considerable work has been done to try to solve this mystery, with hypotheses atributting it to diverse properties such as water content, acidity/basicity, among others. However, without a way to isolate and change a single potential driving parameter, work with different materials so far has not led to any definitive answer.

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 exchanged with a Faraday cup. Testing all possible combinations of all samples, we consistently find nearly perfectly ordered series, with samples in fixed positions on the positive/negative end. Unlike series with different materials, our series has the advantage that any extraneous differences in material properties are not present. Consequently, the existence of a triboelectric series of identical materials represents a unique opportunity to identify the underlying parameter(s) that order the series and drive the exchange of charge.