Structures, solvation forces and shear of molecular films in a rough nano-confinement

Investigations of surface roughness effects on the structure, dynamics and rheology of a molecular fluid (hexadecane) confined between solid (gold) surfaces, through the use of large-scale molecular dynamics simulations, reveal a remarkable sensitivity to the confining surface morphology. A most significant reduction of the ordering propensity is found in films confined by stationary rough surfaces with a consequent strong suppression of solvation forces and the development of liquid-like dynamic and response characteristics. When the rough-surface boundaries are set in motion at a high shear rate, the interfacial layers of the film stick to the adjacent solid boundaries, resulting in partial slip inside the film with the development of shear stress in the viscous molecular fluid, unlike the case of atomically flat crystalline boundaries where slip of the confined film at the boundaries is accompanied by vanishingly small shear stress in the film. These results are discussed in the context of the effect of roughness on the boundary conditions used in modeling fluid flow past surfaces, and they suggest that morphological patterning of surfaces could provide ways for controlled modifications of frictional processes in thin-film lubricated nanotribological systems.