High-resolution particle sizing by optical tracking of single colloidal particles

The motion of individual Brownian particles is observed using the confocal Tracking Microscope recently introduced by Schätzel (K. Schätzel, W. G. Neumann, J. Müller and B. Materzok, App. Opt. 31 (1992) 770–778). Particles are laterally trapped in a strongly focused laser beam. By evaluating the light-pressure-induced drift velocity and the backscattered intensity we are able to detemine particle size histograms with a resolution better than 2%. This is demonstrated on a mixture of seven species of polystyrene latex spheres in the diameter range between 300 and 450 nm, where six classes of diameters are identified. We discuss the scope of the method and potential applications.