Adsorption-induced transient friction of hydrogels on hydrophilic countersurfaces

Soft hydrated permeable surfaces of hydrogels exhibit unique lubrication behaviors, including frictional hysteresis found in tribo-rheometry measurements. A hydrogel lubrication model that describes the transient behavior was previously developed using the structure kinetics model in the field of rheology and rate-and-state friction model, where the friction change is described as a competition between buildup and breakdown rates. In this study, the model is further modified to include the effect of hydrophilicity of a countersurface. Ultraviolet (UV)/ozone treatment on an aluminum surface significantly removes organic materials, resulting in extremely hydrophilic surface. Friction response of a polyacrylamide hydrogel against untreated and UV/ozone-treated aluminum exhibited noteworthy difference in the trajectory of hysteresis. Model fits were conducted using the modified lubrication model on both hystereses, and the fitting parameters of both hystereses are compared with each other to identify a parameter addressing hydrophilicity. Based on the model fits, we suggest that the hydrophilicity of the countersurface initially prevents the adsorption on the hydrogel surface because it holds water better. However, once water goes out of the contact due to contact pressure, a stronger adsorption occurs, which increases friction and decreases the speed dependence of friction.