Nanorheology of molecularly thin films of \(n\)-hexadecane in Couette shear flow by molecular dynamics simulation (Q1303027)

The authors adopt a model incorporating two structured atomic walls between which the fluid is sheared by the moving of walls in opposite directions. The fluid has chains of \(n\)-hexadecane molecules, and each molecule has 16 interaction sites each of which represents a \(\text{CH}_2\) or \(\text{CH}_3\) group. The Lennard-Jones potential comprising stretching, angular and torsional potentials governs the intramolecular interactions and maintains the integrity of the molecules. An isothermal simulation of Couette shear flow reveals the rheological properties of \(n\)-hexadecane at high Weissenberg numbers in thin films. There is an increase in the average viscosity of hexadecane as the film thickness decreases to scales close to the molecular diameter of the chain segments. Shear thinning and normal stress difference show power-law non-Newtonian behaviour. Density profiles, bond angle and dihedral angle distribution functions, and average end-to-end distance of molecules are obtained for film of different thickness and for different shear rates. The authors demonstrate that the adsorption is a determining factor in the properties of ultrathin films, and shear responses depend on the adsorption limit of the surface.