The interface in demixed colloid-polymer systems: wetting, waves and droplets

Phase transitions in colloid–polymer mixtures have attracted a large amount of attention over the last 20 years (W. C. K. Poon, J. Phys.: Condens. Matter, 2002, 14, R859; R. Tuinier, J. Rieger and C. G. de Kruif, Adv. Colloid Interface Sci., 2003, 103, 1). By comparison, the interfacial tension between the coexisting phases has received little attention. Here, we show that the ultralow interfacial tension in fluid–fluid demixed colloid–polymer systems, which is roughly one million times smaller than in ordinary liquids, manifests itself in a wide variety of interface characteristics and processes. Discussed are the interfacial wetting behaviour close to a hard wall, the thermal capillary waves at the free interface and the process of droplet coalescence and breakup. These subjects can be studied in a single experiment by combining modern soft matter chemistry and laser scanning confocal microscopy This combination allows a further exploration of a broad range of interface issues.