Load-Dependent Behavior and Underneath Mechanism of Hydrogel Friction

Hydrogel friction exhibits dynamical instability under an unstable load force, and comparatively less effort has been devoted to understanding the load-dependent behavior of hydrogel friction. In this work, the load-dependent behavior of hydrogel friction was observed, and the underlying mechanism was investigated using a glass substrate rotating against polyacrylamide-based hydrogels. As the load force (F-n) increases, the friction coefficients (mu) for hydrogels exhibit an initial rapid decrease followed by a gradual increase. In the mu-decrease regime, the dramatic decrease in mu can primarily be attributed to the deformation and desorption of polymer chains from the substrate surface under shearing. In the mu-increase regime, the mu-increase rate is determined by the hydrogel network and charge behavior at the gel-substrate interface. An underneath load-dependent mechanism is proposed to explain the unique friction behavior of hydrogels. During the whole process, increasing the load force can result in an increase in viscosity (eta) and a decrease in thickness (h) of the solution at the gel-substrate interface, further leading to a gradual increase in mu. In the mu-decrease regime, the terminal polymer chains adjacent to the substrate surface play dominant roles, and the desorption of the polymer chains from the substrate surface can lead to a sharp decrease in mu. In the mu-increase regime, h and eta of the solution at the interface play dominant roles, therefore leading to a gradual increase in mu. Overall, this work provides fundamental insights into understanding hydrogel friction and facilitates the development of water-based lubricants in biomedical applications.