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 spher e and an atomically hat highly oriented pyrolytic graphite substrate h as been analyzed as a function of surface-to-surface separation distan ce z(0) using an oscillating cantilever technique. The interaction for ce 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 t he 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 p lane, is observed. Fits to the data are in good agreement with theoret ical expectations and allow estimates of the surface charge density tr iboelectrically produced on the sphere's surface.