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Hydrogels composed of a solvent-saturated, cross-linked polymer network exhibit unique interfacial rheology and ultralow friction, making them valuable in biomedical applications. Despite their widespread use in open-air environments, the lubrication behaviors of hydrogels under these conditions remain poorly understood. Here, the microscopic mechanisms underlying the friction characteristics of hydrogels through a combination of experiments, theoretical analyses, and molecular dynamics simulations is explored. It is found that water evaporation from the hydrogel surface reduces the hydrodynamic layer thickness and increases surface viscosity, leading to a gradual rise in friction. On the other hand, optimizing pore size and water mobility within the hydrogel enhances water transport from the interior to the surface, mitigating evaporation and enabling consistently low friction. It is also explored how soaking time, water affinity, and applied normal load influence hydrogel lubrication. The findings elucidate the microscopic mechanisms governing the friction behaviors of hydrogels and provide guidelines for designing hydrogel systems with sustained exceptional lubrication properties in open-air applications.
Zhu et al. (Mon,) studied this question.