Nanorheology of strongly confined oligomeric lubricants

Lubricant films confined in nanometric slit pores and subjected to shear flow are studied by non-equilibrium molecular dynamics in a planar Couette flow geometry. An inhomogeneous, layered density profile is developed near the confining surfaces and the shape of the velocity profile across the pore depends on the wall energetics. A nonlinear velocity profile is developed and both slippage located at the wall and inside the film (interlayer slip) are observed. The local slope of the velocity profile and the location of the slippage plane are determined by the adsorption energy of the wall. Shear-induced changes of the adsorbed chain conformations are discussed in relation with the molecular mechanism of slippage and the magnitude of the interlayer slip.