Stuctural and Dynamic Properties of Hexadecane Lubricants under Shear Flow in a Connned Geometry

Using shear dynamics simulations we investigated the structure and dynamics of hexadecane (n-C 16 H 34) lubricant lms connned between two solid surfaces (Fe 2 O 3) covered with a self-assembled monolayer of wear inhibitors dithiophosphate molecules DTP = S 2 P(OR) 2 with R = iPr, iBu, and Ph]. We found signiicant density oscillations in the lubricant lms, especially near the top and bottom boundaries. From the density oscillations we can deene 9-10 layers for a lm of 44 A thickness, and 5 layers for a 20 A thick lm. In the connned geometry, the motions of individual lubricant molecules are rather restricted, within only 1-2 layers. We also found the stick-slip motion of the lubricant molecules near the bottom boundary 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 type of organic R-group of the wear inhibitor molecules has signiicant eeects upon the characteristics of the stick-slip motion of the lubricant molecules. Rigid R-group with strong interactions with the lubricant makes the lubricant stiier than exible R-group does. Rough surfaces with longer R-group cause the stick-slip motion of the lubricant molecules more profound than smooth surfaces.