Emergence and Growth Dynamics of Wetting-induced Phase Separation on Soft Solids

Abstract: Liquid droplets on soft solids, such as soft polymeric gels, can induce substantial surface deformations, leading to the formation of wetting ridges at contact points. While these contact ridges have been shown to govern the rich surface mechanics on compliant substrates, the inherently divergent characteristics of contact points and the multiphase nature of soft reticulated gels pose great challenges for continuum mechanical theories in modeling soft wetting phenomena. In this study, we report in-situ experimental characterizations of the emergence and growth dynamics of the wetting-induced phase separation. The measurements demonstrate how the migration of free chains prevents the stress singularities at contact points. Based on the Onsager variational principle, we present a phenomenological model that effectively captures the extraction process of free chains, including a crossover from a short-term diffusive state to a long-term equilibrium state. By comparing model predictions with experimental results for varied crosslinking densities, we reveal how the intrinsic material parameters of soft gels dictate phase separation dynamics.