Structure and phase transitions in confined binary colloid mixtures

We report the results of a study of crystallization in quasi-two-dimensional binary mixtures of large and small colloids. The experiments sample the parameter spaces of colloid particle diameter ratio, large particle density, and small particle packing fraction. The depletion potential between the large particles, induced by the presence of the small particles in the system, affects the density at which the large particles undergo a liquid-to-solid freezing transition. For systems with a large to small particle diameter ratio of 4.6, the addition of small particles increases the large particle liquidus transition density, a seemingly counterintuitive result given that the depletion potential is purely attractive when the small particle packing fraction is low. When the large to small particle diameter ratio is 8.8, the same trend in the large particle liquidus transition density is seen, but to a lesser extent. The other system properties for the system with diameter ratio 8.8 show the same trends as for the system with diameter ratio 4.6. Liquid-liquid phase separation is observed for binary mixtures with diameter ratios of 20 and 40. Although the particles used in our experiment can be well modeled as hard spheres, our results cannot be readily explained by extant descriptions of the depletion interaction developed for three-dimensional binary hard sphere mixtures. Inversion of the pair correlation functions obtained from our measurements yields a depletion interaction that is much stronger than predicted for the same densities and diameter ratio in a three-dimensional hard sphere mixture. Our results imply that the depletion interaction is strongly dependent on the degree of confinement of the system. (C) 2003 American Institute of Physics.