= Colloidal probe technique
Attach a sphere to an <atomic force microscope> cantilever and approach a planar surface in a controlled fluid. Calibrated cantilever bending measures force as a function of corrected surface separation. The sphere's known curvature allows comparison with planar interaction models through the <Derjaguin approximation>. Approach speed, charge, roughness, hydration and jump-to-contact can limit interpretation.
A 1991 demonstration used a $3.5\,\mu\mathrm m$ radius silica sphere against a plane in sodium chloride solutions. The reported double-layer agreement and short-range deviations illustrate both the method and the need to distinguish electrostatic forces from a presumed purely dispersive interaction.<Direct measurement of colloidal forces using an atomic force microscope by William A. Ducker and others>[Nature 353, pp. 239–241, institutional abstract: attached silica sphere, planar geometry and NaCl solutions]{ref}
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