Relaxation and Recovery in Hydrogel Friction on Smooth Surfaces

Hydrogels are crosslinked polymer networks that can absorb and retain a large fraction of liquid. Near critical sliding velocity, hydrogels pressed against smooth surfaces exhibit time-dependent frictional behavior occurring over multiple timescales. The origin these dynamics is unresolved Here, we characterize this regime show it consistent with two distinct molecular processes: sliding-induced relaxation quiescent recovery. Our experiments use custom pin-on-disk tribometer to examine poly(acrylic acid) on poly(methyl methacrylate) variety conditions, from minutes hours. We at fixed the friction coefficient decays exponentially reaches steady-state value. time constant associated decay varies sensitive any precedent shearing interface. This process reversible; upon cessation sliding, recovers its original state. also initial direction shear be imprinted as an observable “memory”, visible after 24 hrs repeated shearing. attribute nanoscale extension near-surface network, leading model recovery parallel