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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 z 0 using an oscillating cantilever technique. The interaction force gradient was found to have two contributions. For z 0 у30 nm, an electrostatic force due to charges trapped on the polystyrene sphere dominates. For z 0 р30 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.

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