Sheared colloidal crystals in confined geometry: a real space study on stationary structures under shear

We constructed an optical plate-plate shear cell suitable for the study of aqueous suspensions of charged colloidal spheres under low electrolyte concentrations (10(-6) M). The variable gap height was adjusted to 30 microm corresponding to 15-30 interparticle distances. The concentration of 300 microm diameter polystyrene spheres was chosen around 1 microm(-1) where previous studies had revealed the equilibrium structure to be fluid or body centred cubic. Under shear, layer structures of hexagonal symmetry form, often coexistent with a fluid phase. We used an adapted high resolution video microscope to perform a detailed study of the structure. The central ray of light was prevented from entering the objective and the images are constructed from higher order scattered light. The cover glass correction was adjusted in such a way that several layers of particles were visible simultaneously. The effective depth of sharpness was thus increased to several microm. From the 2D video frames we constructed time averaged 2D particle correlation diagrams (PCD) which correspond to the distribution functions for the projections of particle positions into the x y-plane. They allow us to discriminate different layers and to analyse their structure and their relative position. Layer structures showed approximately hexagonal symmetry bat with an elongation in the flow direction. They were always observed to be wall based. Melting in the z-direction occurred before in-plane melting and with a different mechanism. The resulting salt concentration-shear rate phase diagram is discussed in comparison with results from computer simulation.