Contact electrification and the interaction force between a micrometer-size polystyrene sphere and a graphite surface

Two independent techniques are used to measure the interaction force between a single 3 μm radius polystyrene sphere and an atomically flat, highly oriented pyrolytic graphite substrate. The variation of the interaction force with the surface-to-surface separation between the sphere and plane is determined using both a static and a dynamic atomic force technique. The measured interaction force is dominated at long range by an electrostatic force arising from localized charges triboelectrically produced on the sphere when it makes contact with the substrate. For small sphere-substrate separations, evidence for a van der Waals force is observed. The data provide consistent estimates for both the Hamaker coefficient and the triboelectrically produced charge which can be measured to an accuracy of ±10 electrons.