Soft Wetting and Phase Separation on Swollen Polymer Networks

When a water drop is deposited on a soft adhesive substrate, the surface tension of water drives deformation of the substrate to increase contact area; this is commonly known as soft wetting. In recent soft wetting theories, elastic forces and surface stress oppose substrate deformation. However, very soft, crosslinked materials often contain a liquid phase (i.e. solvent) and is can be considered a swollen network. For a covalently crosslinked network, solvent may separate from network near the contact line to minimize the elastic energy. Although phase separation near contact lines have been considered, the phase distribution at the contact zone is not well understood, and the condition required for phase separation is unclear. We implement confocal microscopy to visualize the crosslinked polymer and solvent phase separately during soft wetting. By controlling the degree of crosslinking and swelling ratio, we investigate both the solvent leeching and the deformation of the network. We find that phase separation does not clearly occur when the swelling ratio is small. We expect that examining the microscale contact zone will help in developing a theory for wetting of soft gels.