Experimental characterization of elastocapillary and osmocapillary effects on multi-scale gel surface topography

Surface topography significantly affects various surface properties of polymer gels. Unlike conventional materials where surface topography is largely a geometric property, the surface topography of a polymer gel is governed by the competition between capillary, elastic, and osmotic effects, which leads to complex stimuli-responsive effects. Elastocapillary deformation and osmocapillary phase separation are two phenomena that are known to flatten gel surface topography. Here we experimentally quantify how osmocapillary phase separation affects gel surface topography by fabricating ionogels with multi-scale topography and characterizing the swelling-dependent surface flattening. Our observation confirms the vital role of the osmocapillary length in governing the surface behavior of swollen ionogels. This study provides the first quantitative experimental verification of the osmocapillary phase separation and shows the insufficiency of the previous studies based on elastocapillary deformation alone. Osmocapillary phase separation can pull solvent out from a gel surface and lead to length-dependent surface flattening. The osmocapillary flattening can be orders of magnitude larger than the existing studies of elastocapillary flattening.