Confocal scanning laser microscopy on fluid-fluid demixing colloid-polymer mixtures

We study gas-liquid phase separating colloid-polymer mixtures using a horizontally placed confocal scanning laser microscope. The phase separation proceeds via spinodal decomposition; first images immediately show sharp interfaces, which is explained in terms of the colloid diffusion time. The diffusion in both the liquid and gas phase is measured in a real space fluorescence recovery after a photo-bleaching experiment. The coarsening rate of the characteristic length in the system can be understood in terms of the capillary velocity. We observe that the spinodal structure collapses due to gravity at a typical size of the order of the capillary length, which is obtained from the static gas-liquid profile near a single wall and is accurately described by the interplay between hydrostatic and Laplace pressure. The present technique allows for precise contact angle measurements and the system shows complete wetting for all statepoints measured. Finally, we study the possibility of capillary condensation in colloid-polymer mixtures and show first indicative experimental results. The observed Kelvin length is surprisingly large, possibly because the system is not yet in complete equilibrium.