Ion-Dependent Frictional Modulation in Hydrogels Originating from Hydration and Mechanics via the Hofmeister Effect

Hydrogels have attracted extensive frictional exploration owing to their excellent hydration lubrication, which closely resembles that observed in biological systems. Their exceptional performance arises from the surface-bound water layer and the inherent softness of the polymer matrix. However, the multiphase structure raises substantial complexity to friction, making effective modulation particularly challenging. In this work, poly(vinyl alcohol) (PVA) hydrogels were soaked in various salt solutions to exploit the Hofmeister effect, broadly tuning hydration and mechanics via ion-specific alteration of polymer chain aggregation. Hydrodynamic pressure in the mixed lubrication regime drove fluid into the contact gap and produced rehydration to overcome the extrusion. At certain velocities within the boundary lubrication regime, friction was controlled by the porous network, while in the hydrodynamic regime, friction was dominated by the fluid film. Importantly, variation in ion types enabled elucidation of interplay between interfacial water and bulk modulus under applied loading. These findings highlight ionic tailoring as a promising strategy to modulate and maintain lubrication in hydrogels.