Fluid separation and network deformation in wetting of soft and swollen elastomers

When a water drop is deposited on a soft substrate, the surface tension of water drives an out-of-plane deformation of the substrate; this is commonly known as soft wetting. However, a very soft, crosslinked elastomer often contains a fluid (i.e. solvent) and can be considered a swollen network. During wetting, solvent may separate from network near the contact line to minimize the elastic energy of the substrate deformation. Although phase separation near contact lines has been considered, the amount of solvent separation and network deformation in the wetting ridge is not well understood, and the conditions for phase separation are unclear. We use confocal microscopy to visualize the crosslinked polymer and solvent separately at the wetting ridge. By controlling the degrees of crosslinking and swelling, we investigate how both control the solvent leeching and the network pull-up. With increasing swelling, the solvent separation increases while the network pull-up height decreases, demonstrating a synergy between network deformation and solvent separation. Our results reveal that a swelling fluid, commonly found inside soft networks or slippery surfaces, can play a critical role at the tip of a wetting ridge