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 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.[ref]
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
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The colloidal probe technique is a powerful method used in surface science and materials characterization. It involves the use of a colloidal particle, typically a microsphere, which is functionalized to interact with a surface of interest. The primary objective of this technique is to measure the interaction forces between the colloidal particle and the surface, providing insights into the surface properties, such as roughness, chemistry, and mechanical behavior.