How osmotic pressure governs surface structures and sliding friction on swollen crosslinked hydrogels

High-water-content hydrogels are being increasingly explored for applications in biomedicine, water filtration, and hybrid materials. They are generally known to be lubricious, or very low friction, but their friction exhibits time dependence and history dependence related to their response under stress. The response to surface stress for a dilute or semi-dilute hydrogel arises in part from how water assists with supporting and distributing the load; this is inherently linked to the osmotic pressure holding the water in the bulk. Here I present two vignettes illustrating this connection. The first considers the competitive rates of surface slip and pressure-driven dehydration due to applied loads. Given initial measurements of friction at very low and very high speeds, the ratio of the timescales of these effects can predict friction along the intermediate spectrum. The second vignette describes the inherent generation of less-dense surface layers (~single microns) that arise from the bulk due to the discontinuity of osmotic pressure between the bulk and the open bath submerging a crosslinked hydrogel. These layers are confirmed with multiscale indentation techniques and material creep localized to the near-surface region.