Experimental characterization of osmocapillary flattening on gel surfaces with multiscale topogrophy

When the surface stress tries to deform a polymer gel, it can either deform the polymer network with the solvent, known as elastocapillary deformation or pull the solvent out from the polymer network, known as osmocapillary phase separation. While elastocapillary deformation on gel surfaces has been widely studied, osmocapillary phase separation remains largely a theoretical hypothesis. Here we synthesized non-volatile ionogels cast on sandblasted glass with known surface topography and characterized the swelling-dependent surface topography. Our experiments show that the flattening of the surface features over different length scales can be accurately described by established linear elastocapillary theory when the gel has a low swelling ratio. However, as the gel swells, the elastocapillary theory gradually underestimates the surface flattening. The additional flattening must be explained with an osmocapillary model. This work highlights the importance of osmocapillary phase separation when studying highly swollen gels where existing works often assume that elastocapillary deformation is the only surface deforming mechanism.