On the nature of shear thinning in nanoscopically confined films

¹ Materials Science & Engineering, Cornell University, Bard Hall, Ithaca NY 14583, USA
² Chemical Engineering, University of Florida - Gainesville, FL 32611, USA
³ Polymer Science Laboratory and Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Received: 19 September 1995
Accepted: 21 December 1995

Abstract

Non-Equilibrium Molecular Dynamics (NEMD) computer simulations were employed to study films in nanometer confinements under shear. Focusing on the response of the viscosity, we found that nearly all the shear thinning takes place inside the solid-oligomer interface and that the adsorbed layers are more viscous than the middle part of the films. Moreover, the shear thinning inside the interfacial area is determined by the wall affinity and is largely insensitive to changes of the film thickness and the molecular architecture. The rheological response of the whole film is the weighted average of these two regions — "viscous" interfacial layer and bulk-like middle part — resulting in an absence of a universal response in the shear thinning regime, in agreement with recent SFA experiments of fluid lubricants.