Relaxation and Recovery in Hydrogel Friction on Smooth Surfaces

Hydrogels are cross-linked polymer networks that can absorb and retain a large fraction of water, often up to 90 percent by weight. They are widely used in many engineering applications as well as agriculture industries due to their ultralow friction, biocompatibility, and chemical transport capabilities. Previous research in our lab has identified and characterized three distinct regimes of friction in polyacrylamide (PAAm), polyacrylic acid (PAA), and agarose hydrogels on smooth surfaces. Most striking, near a critical velocity, the friction coefficient decreases by an order of magnitude and displays relaxation over multiple timescales. Here we examine this regime in closer detail for PAA hydrogels. We find that the frictional relaxation time decreases exponentially with sliding velocity. Additionally, any pre-shearing of the hydrogel will induce changes in the relaxation time prior to experiments, and if left to rest, the hydrogel friction will return to its original state. This behavior is qualitatively consistent with model of two superimposed relaxation processes: a velocity-dependent shearing and extension of entangled polymer chains, and a constant relaxation timescale from thermal fluctuations. However, we also show that the sliding interface can retain a “memory” of its initial sliding direction, a memory that persists over 24 hours of continuous experiments.