Contact charging between surfaces of identical insulating materials in asymmetric geometries

The charging that occurs when a pair of insulating surfaces of identical chemical composition are rubbed (i.e. triboelectric charging) remains poorly understood. It is believed that asymmetry in contact plays an important role in this charging. To study this phenomenon, we have developed an experimental methodology that asymmetrically rubs two surfaces by contacting a rotating cylinder with a stationary cylinder – the rubbing is asymmetric in that the contacting area is much greater on the rotating cylinder than on the stationary cylinder. We find that the charge transfer occurs with a spatial distribution of charge, in terms of magnitude and polarity, on the contacted area. The direction of the average charge transfer is material dependent: for Teflon–Teflon contact, the surface with the larger contacting area charges positively, but for Nylon–Nylon contact the surface with the larger contacting area charges negatively. This difference is interpreted as being due to a negatively-charged species transferred in the case of Teflon (electrons or negative ions), but a positively-charged species transferred in the case of Nylon (positive ions).