Coarse-Grained Molecular Dynamic Simulations of Nanometer-Thick Polar Lubricant Films Sheared Between Solid Surfaces With Random Roughness

To increase the recording density of hard disk drives, a fundamental understanding of the molecular structure and motion of nanometer-thick liquid polar perfluoropolyether (PFPE) films under shear between the head and disk is crucial. With the aim of achieving such an understanding, we developed a coarse-grained (CG) bead-spring model for polar PFPE Z DOL films coated on carbon surfaces. This model reproduces the structural properties and isothermal compressibility derived from the parent all-atom simulations. Using this model, we performed CG molecular dynamic simulations to investigate the dynamic behavior of nanometer-thick PFPE Z DOL films sheared between solid surfaces with random roughness. We used two types of surface roughness that had identical average arithmetic roughness, but different correlation lengths. We found that a short correlation length prevented slip at the liquid-solid interface and gave rise to large shear stresses, compared with a long correlation length. The strong shear drove some PFPE polar end beads to replace nonpolar beads in the vicinity of surfaces with a short roughness correlation length. This replacement increased the number of PFPE Z DOL molecules that contacted with the top solid surface through both of their polar ends.