Experimental observation of single-file diffusion of Brownian particles

The self-diffusion of particles restricted to one-dimensional channels is termed single-file diffusion if individual particles are unable to pass each other. We employ the interference pattern of two coherent laser beams to confine electrically charged polystyrene colloids in parallel ''optical channels" formed by light forces and study the particle diffusion within these channels by optical microscopy. For small channel spacings the particles can electrically couple between adjacent channels and their mean square displacement along these channels at long observation times t is proportional to t. For large channel spacings, however, where the coupling of particles is largely reduced, we find the mean square displacement to be proportional to t(0.5) which signifies the occurrence of single-file diffusion.