Fabrication of AFM-Compatible Hydrogel Probe to Achieve Hydration Superlubrication in Salt Solutions

Hydrogels demonstrate effective lubricating properties, but the underlying mechanisms at the nanoscale remain elucidated. In this study, a novel strategy is proposed by fabricating the hydrogel probes compatible with atomic force microscopy (AFM) to establish a superlubrication system based on the hydration interactions. The probe is made of polyethylene glycol diacrylate (PEGDA)-based hydrogel microspheres, which can achieve an extremely low friction coefficient of 0.0014 when sliding on the mica surface in NaCl solution. The friction coefficients in salt solutions sequence μ(NaCl) < 0.01 < μ(KCl) < μ(LiCl), which contradicts the sequence based on the ion hydration capacity (K⁺ < Na⁺ < Li⁺), suggesting that the hydrogel-based lubrication systems are governed by the interplay between ion hydration lubrication and ions-polymer interactions simultaneously. The observed superlubrication in NaCl solution is ascribed to the superior hydration capacity of Na⁺ ions, which forms a stable and smooth hydration layer within the contact zone. This work provides new avenues for AFM-based hydrogel studies, and also significantly advances the comprehension of hydrogel lubrication mechanisms at the nanoscale.