INHOMOGENEITIES IN SHEARED ULTRATHIN LUBRICATING FILMS

Nonequilibrium molecular dynamics computer simulations have been used to study nanoscopically confined oligomer films under shear. Beyond th e well-known density layering across such films, other structural and dynamical inhomogeneities exist across such films and are discussed he re. When these films are subjected to strong shear flows, slip appears at the confining surfaces or inside the pore, depending on the wall i nteractions. For strong wall affinities interlayer slip develops betwe en the adsorbed layer and the free chains, resulting in a structural d iscontinuity; a molecular mechanism, involving shear induced conformat ional changes of the adsorbed chains, is associated with this interlay er slip. Moreover, the resistance to flow (quantified through an effec tive viscosity) changes considerably across the film, with a dramatic viscosity increase of the adsorbed layer near attractive surfaces. She ar thinning is mainly taking place inside this more viscous interfacia l layer, whereas the dynamics in the middle of the film remain bulklik e; thus, there also exists strong inhomogeneity in the dynamics of the system. A comparison with SFA experimental and theoretical studies is also made.