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. These factors limit predictive capability of surface performance, and as such that aspect of their design for new applications is often omitted. Here I present new paradigms for understanding this friction. In the first, we consider the competitive rates of surface slip and pressure-driven dehydration due to the 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. In the second, we consider the surface of the swollen hydrogel to have contributions from both the fluid and polymeric phases, and thus it is acts as a complex fluid. Frictional hysteresis as measured using tribo-rheometry is described by a complex fluid model. Finally, we describe the inherent generation of less-dense surface layers (~single microns) that arise due to the discontinuity of osmotic pressure from the bulk to the boundary of a crosslinked hydrogel. We postulate that understanding and controlling this layer is necessary for controlled friction.

Alison C. Dunn is an assistant professor in Mechanical Science & Engineering at the University of Illinois at Urbana-Champaign. Her laboratory is focused on how material properties and structures drive tribological processes using an experimental and theoretical approach. She works on soft materials like hydrogels and silicone, as well as insect cuticle and hardened steel. She won the NSF CAREER award in 2018 and the ASME Burt L. Newkirk Award in 2019.