Fluid-solid transitions on walls in binary hard-sphere mixtures

We present measurements and theory of a fluid-solid phase transition at a hard, flat wall in a two-sized hard-sphere mixture. With particle concentrations well below the level for bulk phase separation, a fluid monolayer (enriched in the larger spheres) forms at the wall. At slightly higher concentrations, the wall-fluid freezes into a wall-solid phase which coexists with the bulk fluid. Measurements of large-sphere area fraction vs. small-sphere bulk concentration in the fluid and solid phases are in reasonable agreement with the predictions of a model that accounts for entropic excluded-volume effects at a hat wall.