Dynamic wetting phenomena and contact angle hysteresis of drops on polymer brushes and gels

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Droplets deposited on polymer brushes or gels are surrounded by a wetting ridge. The reason is that the vertical component of the involved interfacial forces pulls the brush/gel up. The shape and height of the ridge greatly influence the contact angles and friction of drops moving on the surface. We aim to understand the dynamic contact angles, the reorganization of the ridge caused by the moving drops and the resulting mechanisms of viscous dissipation. To gain detailed information on the physics involved in these dynamic wetting phenomena, we intend to combine laser scanning confocal microscopy and coarse grained molecular dynamic simulations. We intend to focus on the following model systems: (i) Polymer gels. Here we will study PDMS gels infiltrated by silicone oil. Hydrogels will be used for comparison. We plan to vary the thickness and the crosslinking density of the gel, the viscosity of the infiltrated liquid and the velocity of the drop moving on the surfaces. (ii) PDMS brushes with and without immersed free polymer chains. Preliminary experimental studies have shown that the presence of free chains in the brush has a significant impact on the dynamic properties of deposited droplets, and, in particular, on the contact angle hysteresis. Depending on the type of brush and amount of free chains the contact angle varies greatly. This opens new avenues for controlling the wetting properties of polymer brush surfaces by simple physical mechanisms.