Load-Dependent Behavior and Underneath Mechanism of Hydrogel Friction
ACS APPLIED POLYMER MATERIALS(2024)
Abstract
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.
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Key words
hydrogel,load-dependence,friction,lubrication,soft surface
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