SHEAR BEHAVIOR OF SQUALANE AND TETRACOSANE UNDER EXTREME CONFINEMENT .2. CONFINED FILM STRUCTURE

This paper focuses on the structural characteristics of confined squal ane and tetracosane under shear flow conditions. Nonequilibrium molecu lar dynamics simulation is used to explore the rheology of these model lubricants. A preceding paper describes the molecular model and the s imulation method, and examines interfacial slip. The lubricants are co nfined between model walls that have short chains tethered to them, th us screening the wall details. In this paper we examine the density pr ofiles and chain conformations of the alkanes under shear flow conditi ons. Our results indicate a profound influence of the walls on the flu id structure. In particular, when the wall spacing is close to an inte gral multiple of the molecular diameter, tetracosane shows the formati on of distinct layers with the molecules being in a fully extended sta te. This behavior is not observed for squalane. Under shear flow condi tions the molecules tend to orient parallel to the walls, as would be expected, with a greater degree of orientation (a) close to the walls, (b) at the positions of local density maxima, and(c) at higher strain rates. (C) 1997 American Institute of Physics. [S0021-9606(97)50947-0 ].