A Molecular Dynamics Analysis of the Traction Fluids

Non-equilibrium all-atom MD simulations are used to study the traction properties of hydrocarbon fluids. A fluid layer is confined between two solid Fe plates under the constant normal force of 1.0 GPa. Traction simulations are performed by applying a relative sliding motion to the Fe plates. Shear behaviors of nine hydrocarbon fluids are simulated on a sufficiently large film thickness of 6.7 nm, and succeeded in reproducing the order of the experimental traction coefficients. The dynamic mechanism of the momentum transfer on layers of fluid molecules are analyzed focusing on the intermolecular interactions (density profile, orientation factor, pair-correlation function) and intramolecular interactions (intramolecular interaction energy, conformation change of alicyclic ring). In contrast to the case of n-hexane, which shows low traction due to a fragile chain-like interaction, other mechanisms are obtained in the high traction molecules of cyclohexane, dicyclohexyl and santotrac 50. In cyclohexane, alicyclic rings face each other in a highly ordered molecular layer, and the motion of the conformational changes cooperates. In dicyclohexyl and santotrac 50, alicyclic rings, which distribute across the low ordered molecular layers, behave as a stiff bulky mass, and momentum transfers to the end of the molecular axis. The traction mechanisms of nine hydrocarbon fluids are also obtained during the course of the analysis.