Measurement of Forces in Liquids Using a Force Microscope

A force microscope has been used to measure surface forces between a colloidal sphere (3.5 μm radius) and a flat surface in aqueous solution. The force between silica surfaces was measured as a function of surface separation, salt concentration, and pH, and the results agree well both with earlier measurements between macroscopic surfaces and, at separations greater than 3 nm, with the force predicted by DLVO theory. The technique is simple and reproducible and could be used to measure the forces acting on other colloid particles and fibers of a variety of compositions. This is demonstrated by measurement of the force on a gold-coated sphere.