Dynamic rheology of boundary ultra-lubricated hydrogels in frictional oscillation study

The sliding behavior of hydrogels, crucial for biomaterial development, is traditionally studied under steady shear. However, biological motions are typically dynamic and oscillatory. This work characterizes the lubrication performance of boundary ultra-lubricated polyvinyl alcohol (PVA) hydrogels based on their nonlinear viscoelastic response under oscillatory shear testing. Physically crosslinked PVA hydrogels, both fixed and slidable (interface-lubricated), were prepared. Their rheological behavior was investigated through large-amplitude oscillatory shear (LAOS) tests, with a focus on the effect of the surfactant Tween 80 (T80). Compared to fixed hydrogels, slidable hydrogels exhibited pronounced strain softening. The addition of T80 to the hydrogel bulk further enhanced this effect, including strong strain softening even at small strains and reducing the transition oscillation amplitude. Analysis of Lissajous-Bowditch plots using the parameters strain-hardening ratio ( S ) and shear-thickening ratio ( T ) revealed an intracycle strain stiffening and shear-thinning tendency in slidably lubricated hydrogels with increasing amplitude. It is proposed that T80 weakens the gel’s physical network via hydrogen bonding with surfactant micelles and prevents direct PVA-chain contact with the opposing surface. Furthermore, a proportional relationship between the steady coefficient of friction (CoF) and the energy dissipation ratio ( κ ), CoF ∝ 10 −2 . 0 κ , was established. This study demonstrates that the nonlinear viscoelastic parameter κ serves as an effective metric for evaluating hydrogel lubrication performance in dynamic rheological studies, providing deeper insight into frictional energy dissipation mechanisms.