Osmocapillary phase separation in the surface roughness and adhesion of gels

Gel is a polymer network infiltrated with a solvent. On the gel surface, the surface tension can deform the gel surface, which is known as elastocapillary deformation, or pull the solvent out from the polymer network, which is known as osmocapillary phase separation. Here we develop a linear-elasticity-based model to study how osmocapillary phase separation affects the surface roughness and surface energy of the gel. It shows that the surface behavior of a gel can be characterzed by two dimensionless groups. With the variation of these two dimensionless groups, the gel surface can be either rigid like a hard solid, flat like a liquid, deformed without phase separation like a rubber, or deformed with siginificant osmocapillary phase separation on the surface. The last type of behavior has been overlooked in many previous studies. The analysis shows that while a highly swollen hydrogel is still a solid, its surface can be mostly covered by the liquid solvent. The implication of osmocapillary phase separation on our understanding of the adhesion and friction of gels will be discusssed.