A comparative study on the phase behaviour of highly charged colloidal spheres in a confining wedge geometry

We studied the structures formed in aqueous dispersions of charged colloidal spheres under a constant low salt concentration of c = 6 x 10(-6) Mol l(-1). Particles of diameter 2a = 1000 nm were confined to a low angle wedge geometry with plate separation 0 < S < 50 mu m and observed with video microscopy. Irrespective of the initial particle density n we reproducibly observe the particles to migrate to the narrow wedge side on the timescale of a few days. Thereby an interface between a crystalline structure and a near particle free region is formed, which propagates slowly until the dilute region is exhausted of particles. While the origin of this separation is still unclear, the final extension of the crystalline region is stable on the timescale of months. Within the crystal phase we observe a characteristic sequence of structures with increasing plate separation similar to that seen in previous experiments on hard sphere-like systems but here with non-touching particles. Moreover, we find that mechanical equilibrium is a necessary prerequisite for observing the full richness of different phases. A detailed comparison to recent theoretical calculations for bilayers was performed and semi-quantitative agreement with the predictions observed.