Lubrication Mechanism of Surface‐Attached Hydrogel Layers in Sliding Contact

Abstract

Swollen, surface‐attached hydrogel layers in sliding contact exhibit unusual friction properties. To study this phenomenon, hydrogel layers covalently bonded to solid substrates are generated by CH insertion crosslinking. Brief UV irradiation of thin layers of photoreactive prepolymers leads to simultaneous crosslinking of the polymer chains and attachment of the network to the substrate. As the surface‐attached hydrogel layers can only swell perpendicular to the surface, and the polymer subchains on the two sides of the friction pair cannot interpenetrate for entropic reasons, adhesion in such friction pairs is negligible. In this work, the tribological behavior of surface‐attached hydrogel pairs is investigated as a function of load and sliding velocity. It is observed that for very thin films, the indentation and accordingly the friction is a strong function of the film thickness and the role of the chain confinement on the lubrication of surface‐attached hydrogels is discussed. A model for the friction mechanism of such systems is developed.

Designing Superlubricious Hydrogels from Spontaneous Peroxidation Gradients

“…Tribological sliding experiments were also conducted at varying normal loads (Figure S6) and sliding speeds (Figure S7) to further investigate the tribological behavior of the hydrogels. For self-mated gel-on-gel sliding configurations, friction coefficients typically decrease with increasing load due to nonlinear contact radius scaling. , Friction coefficients also tend to increase with sliding velocity past a critical transition velocity, v *, due to the inability of the hydrogel to relax shear strain. , The 17.5 wt % polyacrylamide gels herein deviated from previously observed tribological behaviorfor the gels at 0.01 mol % O 2 , μ slightly increased with increasing load (Figure S6a); for both oxygen conditions, μ decreased with increasing sliding speed (Figure S7). Shoaib et al similarly observed decreasing μ with increasing v when below v *, indicating that faster sliding velocities may be required to surpass v * for these gels .…”

“…For self-mated gel-on-gel sliding configurations, friction coefficients typically decrease with increasing load due to nonlinear contact radius scaling. 79 , 80 Friction coefficients also tend to increase with sliding velocity past a critical transition velocity, v *, due to the inability of the hydrogel to relax shear strain. 21 , 80 The 17.5 wt % polyacrylamide gels herein deviated from previously observed tribological behavior—for the gels at 0.01 mol % O 2 , μ slightly increased with increasing load ( Figure S6a ); for both oxygen conditions, μ decreased with increasing sliding speed ( Figure S7 ).…”

“… 79 , 80 Friction coefficients also tend to increase with sliding velocity past a critical transition velocity, v *, due to the inability of the hydrogel to relax shear strain. 21 , 80 The 17.5 wt % polyacrylamide gels herein deviated from previously observed tribological behavior—for the gels at 0.01 mol % O 2 , μ slightly increased with increasing load ( Figure S6a ); for both oxygen conditions, μ decreased with increasing sliding speed ( Figure S7 ). Shoaib et al similarly observed decreasing μ with increasing v when below v *, indicating that faster sliding velocities may be required to surpass v * for these gels.…”

Janus hydrogels: merging boundaries in tissue engineering for enhanced biomaterials and regenerative therapies

“…cular or particle migration. [83][84][85][86] Therefore, investigating the surface and interfacial interactions of Janus hydrogels is crucial for a deeper understanding of their properties and applications, particularly in liquid-phase immobilization and multiphase reactions (Fig. 7).…”