Structural and dynamic properties of hexadecane lubricants under shear flow in a confined geometry

Using shear dynamics simulations we investigated the structure and dynamics of hexadecane (n-C₁₆H₃₄) lubricant films of a nanoscale thickness, confined between two solid surfaces (Fe₂O₃) covered with a self-assembled monolayer of wear inhibitors [i.e., dithiophosphate molecules DTP = S₂P(OR)₂ with R = iPr, iBu, and Ph]. We found significant density oscillations in the lubricant films, especially near the top and bottom boundaries. From the density oscillations we can define 9-10 layers for a film of 44 Å thickness, and 5 layers for a 20 Å thick film. The motions of individual lubricant molecules in the direction perpendicular to the surfaces are rather restricted, spanning only 1-2 layers during the entire 200 ps. We also observed the stick-slip motion of the lubricant molecules near the bottom and top boundaries in the direction of shear. However, the change from stick to slip state (or vice versa) for a lubricant molecule does not correlate with the change in its radius of gyration or end-to-end distance. The characteristics of the stick-slip motion of the lubricant molecules are strongly influenced by the type of organic R-group in the wear inhibitor molecules.