Identification of electrostatic and van der Waals interaction forces between a micrometer-size sphere and a flat substrate.

The interaction force gradient between a micron-size polystyrene sphere and an atomically flat highly oriented pyrolytic graphite substrate has been analyzed as a function of surface-to-surface separation distance z0 using an oscillating cantilever technique. The interaction force gradient was found to have two contributions. For z0>30 nm, an electrostatic force due to charges trapped on the polystyrene sphere dominates. For z0<30 nm, a van der Waals interaction, characteristic of a sphere near a flat plane, is observed. Fits to the data are in good agreement with theoretical expectations and allow estimates of the surface charge density triboelectrically produced on the sphere’s surface.