Wetting behaviour of colloid-polymer mixtures in confinement

• Main Author: Razak, Huzaifah Haritsah Abdul
• Other Authors: Aarts, Dirk G. A. L.
• Published: University of Oxford 2017
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.736137

We study the wetting behaviour of phase-separating colloid-polymer mixtures in confinement. The phase separation is due to the depletion attraction between the colloids mediated by the polymers and bears similarities to ordinary phase separation of oil and water. We use laser scanning confocal microscopy to investigate the behaviour of our colloidal mixture in the vicinity of different substrates, which serve as the confining walls. The glass walls used are chemically modified with two types of polymer, polyacrylamide and chitosan, and we also use clean glass as a reference. We find that the polymer grafting of the surfaces has a pronounced effect on the wetting behaviour. Here, we observe the formation of colloidal liquid droplets, which implies that the coated walls are partially wet. This geometry allows for the determination of the contact angle. The shape of the droplet is theoretically described by the interplay between gravity and interfacial curvature, with the contact angle entering as a boundary condition. The reconstructed interfacial drop profiles are then fitted to the theoretical model to quantitatively study its variation as a function of colloid-polymer state point, properties of the confining wall, and droplet size. We also visually observe Cahn's wetting transition from partial to complete wetting in three series of state points as we move closer to the binodal. This enables us to estimate the wetting transition line in our phase diagram. In addition, we disperse polyacrylamide-coated silica beads into our colloid-polymer mixtures to observe the wetting effects near curved surfaces. We generate theoretical plots based on the conditions of mechanical equilibrium, and compare to the experiments. Our studies shine light on partial wetting phenomena for systems where the interfacial roughness is comparable to the other relevant lengthscales in the system, such as the droplet size or the curvature of the surface.