How osmotic pressure governs sliding and surface structures of swollen crosslinked hydrogels

High-water-content hydrogels are being increasingly explored for applications in biomedicine, water filtration, and hybrid materials. As these materials slide against hard, impermeable countersurfaces, they exhibit time-dependent and history-dependent friction related to their response under stress. Here I present two vignettes of the role of osmotic pressure in understanding hydrogel lubrication. 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.