IN a colloidal suspension containing particles of two different sizes, there is an attractive force between the larger particles. This attraction is due to the extra volume that becomes available to the smaller particles when the larger particles approach one another, thus increasing the entropy of the system. Entropic 'excluded-volume' effects of this type have been studied previously in colloids and emulsions, in the context of phase-separation phenomena in the bulk and at fiat surfaces. Here we show how similar effects can be used to position the larger particles of a binary mixture on a substrate, or to move them in a predetermined way. Our experiments demonstrate the entropically driven repulsion of a colloidal sphere (in a suspension of smaller spheres) from the edge of a step; the magnitude of the entropic barrier felt by the sphere is approximately twice its mean thermal energy. These results indicate that passive structures etched into the walls of a container create localized entropic force fields which can trap, repel or induce the controlled drift of particles. Manipulation techniques based on these effects should be useful for making the highly ordered particle arrays required for structures with photonic band gaps, microelectronic mask materials, and materials for clinical assays.
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