Abstract
To develop precision mechanical systems, a molecular level understanding of nanometer-thick polar liquid lubricant films under confined shear between two solid surfaces is essential. To obtain such an understanding, we first developed a coarse-grained (CG) molecular model of polar perfluoropolyether (PFPE). This model accurately reproduced distribution functions of molecular structure calculated from all-atom simulations. Using this model, we then conducted molecular dynamics simulations of nanometer-thick polar PFPE films subjected to confined shear between carbon surfaces with random roughness. We found that a short correlation length of the surface roughness prevented solid-liquid interfacial slip and leaded to large shear stress, compared with a long correlation length. Additionally, in the case of the short correlation length, more PFPE molecules formed a polar interaction with the upper solid surface owing to shear-induced molecular motion and orientation, which may increase the risk of lubricant depletion as the upper solid surface is withdrawn.
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