Inhomogeneities in Sheared Ultrathin Lubricating Films

Nonequilibrium molecular dynamics computer simulations have been used to study nanoscopically confined oligomer films under shear. Beyond the well-known density layering across such films, other structural and dynamical inhomogeneities exist across such films and are discussed here. When these filmsare subjected to strong shear flows, slip appears at the confining surfaces or inside thepore, depending on the wall interactions. For strong wall affinities interlayer slip develops between the adsorbed layer and the free chains, resulting ina structural discontinuity; amolecularmechanism, involving shear induced conformational changes of the adsorbed chains, is associated with this interlayer slip. Moreover, the resistance to flow (quantified through an effective viscosity) changes considerably across the film, with a dramatic viscosity increase of the adsorbed layer near attractive surfaces. Shear thinning is mainly taking place inside this more viscous interfacial layer, whereas the dynamics in the middle of the film remain bulklike; thus, there also exists strong inhomogeneity in the dynamics of the system. A comparison with SFA experimental and theoretical studies is also made.