Measuring particle diameter and particle-particle gap with nanometer precision using an optical microscope

A simple optical microscopy method has been developed for measuring particl e diameters and particle-particle gaps with a precision of a few nanometers . The method uses an inexpensive and straightforward image-processing techn ique to examine optical microscope images of the particles, which can be we t or dry. The technique is demonstrated on polystyrene latex and silica par ticles with diameters. greater than 450 nm suspended in KCl solutions, and the measured particle diameters compare to within a few percent with those measured by electron microscopy and light scattering. In addition, changes in the gap between two colloidal spheres were measured with nanometer preci sion, a result that will have important implications for measuring colloida l force profiles between two Brownian particles.