Layering in colloidal fluids near a smooth repulsive wall

We have directly observed layering in three-dimensional colloidal fluids caused by the presence of a smooth repulsive glass wall. The system we study is a colloidal suspension of highly charged monodisperse latex spheres in water, with diameter 0.3 μm, charge ∼ 10⁴ electronic charges, and mean separation ∼ 0.8-2 μm. This allows direct atomic resolution of the colloid using ordinary optical microscopy and digital imaging techniques. The layering of the colloid is manifested as a density modulation perpendicular to the wall. For fluids with low bulk densities, we find the density profile perpendicular to the wall to be essentially identical to the pair distribution function in a thin slab of spheres parallel to the wall, with the exception of the spacing of the first peak. For a fluid with bulk density approaching that of the crystal, we find a smaller peak spacing than that of the in-plane pair distribution function, indicating incipient crystallization.