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 hat highly oriented pyrolytic graphite substrate has been analyzed as a function of surface-to-surface separation distance z(0) using an oscillating cantilever technique. The interaction force gradient was found to have two contributions. For z(0) greater than or equal to 30 nm, an electrostatic force due to charges trapped on the polystyrene sphere dominates. For z(0) less than or equal to 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.