Probing the intriguing frictional behavior of hydrogels during alternative sliding velocity cycles

Understanding the friction behavior of hydrogels is critical for the long-term stability of hydrogel-related bioengineering applications. Instead of maintaining a constant sliding velocity, the actual motion of bio-components (e.g., articular cartilage and cornea) often changes abruptly. Therefore, it is important to study the frictional properties of hydrogels serving under various sliding velocities. In this work, an unexpected low friction regime (friction coefficient μ < 10^-4 at 1.05×10^-3 rad/s) was observed when the polyacrylamide hydrogel was rotated against a glass substrate under alternative sliding velocity cycles. Interestingly, compared with the friction coefficients under constant sliding velocities, the measured μ decreased significantly when the sliding velocity changed abruptly from high speeds (e.g., 105 rad/s) to low speeds (e.g., 1.05×10^-3 rad/s). In addition, μ exhibited a downswing trend at low speeds after experiencing more alternative sliding velocity cycles: the measured μ at 1.05 rad/s decreased from 2×10^-2 to 3×10^-3 after 10 friction cycles. It is found that the combined effect of hydration film and polymer network deformation determines the lubrication and drag reduction of hydrogels when the sliding velocity changes abruptly. The observed extremely low friction during alternative sliding velocity cycles can be applied to reduce friction at contacted interfaces. This work provides new insights into the fundamental understanding of the lubrication behaviors and mechanisms of hydrogels, with useful implications for the hydration lubrication related engineering applications such as artificial cartilage.