Thixotropic Mechanics in Soft Hydrated Sliding Interfaces

Soft hydrated permeable surfaces exhibit unique lubrication behaviors, including recently discovered frictional hysteresis. This duration-dependent frictional effect can be analogous to the thixotropic fluid response under shear-driven Couette flow. We illustrate torque-speed hysteresis loops using tribo-rheometry measurements between an aluminum annulus and polyacrylamide surface. Frictional torque response was measured under stepwise sliding speed increments at five different step durations. The torque-sliding speed curves exhibit hysteresis loops and the shape of the hysteresis loops depends on step durations. Longer duration shows greater hysteresis with higher average friction. Torque curves at highest speeds converge to one line with a power law exponent of α = 0.7. Based on the experimental data, a hydrogel lubrication model was developed using a thixotropic fluid model, where viscosity change is described as a competition between structural buildup and breakdown. Simulation using the model correlates well with the experimental results, indicating the existence of effective structural change on the hydrogel surface.