(347f) Structure and Crystallization of Bidisperse Repulsive Colloids In Two Dimensional (2D) Space

We will discuss the results from our study of the structure and phase behavior of binary mixtures of repulsive colloids under 2D confinement. For this study, we used mixed hydrophilic silica microspheres of two different sizes dispersed at the air-water interface as model 2D bidisperse colloids. The structures of the Langmuir monolayers of these bidisperse particles were investigated by direct optical microscopic examination at various particles densities. Repulsive pair interactions between like and unlike combinations of particles were confirmed from the corresponding pair correlation functions measured by microscopy at low particle densities. We demonstrate that at a size ratio of 0.580, the purely repulsive interactions can stabilize an LS2 binary crystal structure at high overall particle densities (> 0.824). At a size ratio of 0.375, we observed the formation of LS1 crystal domains at particle densities around 0.642. At other size ratios (e.g., 0.500), monolayer compression simply results in the macroscopic phase separation of the bidisperse colloid film into two close-packed phases, each consisting of only one type of particle. Full composition-composition phase diagrams for the above three size ratios and the detailed structural features of the identified phases will be presented. For a comparison of these experimental results, data from Monte Carlo simulations of these same systems, using effective two-body potentials extracted from inverse Monte Carlo simulation techniques [1], will be discussed. Through this comparison, we will elucidate the effects of higher-body interactions on the phase behavior of the 2D (bidisperse) colloid systems.

[1] Almarza, N.G. and E. Lomba. Phys. Rev. E 68, 011202 (2003)