Probing interactions between colloidal particles with oscillating optical tweezers

The potential of optical traps for the study of weak interactions in colloidal systems is well recognized. In this paper, a theoretical model is developed for two oscillating interacting hard spheres in a fluid in the low Reynolds number limit. The amplitude of oscillation of the particle and the phase lag between the motion of the particles and the optical traps are a function of both the hydrodynamic coupling between the particles and the interaction potential. The effect of the particle-particle interaction is most pronounced for particles oscillated asynchronously along their line of centers. The experimental realization of this scheme is demonstrated for the model system of polystyrene latices in water and the theoretical predictions are compared with the experimental results. The extension of this idea to oil-in-water emulsions is discussed.