On the nature of shear thinning in nanoscopically

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 partresulting in an absence of a universal response in the shear thinning regime, in agreement with recent SFA experiments of fluid lubricants. Recent experimental studies of ultra-thin films by the Surface Forces Apparatus (SFA) reveal striking behaviour in the rheological response of lubricating films when confined in dimensions comparable to the molecular size [l]-[lo]. Such films become strongly inhomogeneous ]:S] and their effective viscosity increases dramatically when reducing the film thickness 1121. Moreover, they exhibit shear thinning for very moderate shear rates and the onset of this non-Newtonian behaviour shifts to lower shear rates in narrower confinements [6], [7]. Molecular-Dynamics (MD) computer simulations have proven to be effective in interpreting this counter-intuitive behaviour of nanoscopically confined films [11]-[l51 and unvealed the origin of the ‘$lassy” dynamics in ultra-thin confinements [ll], [12]. On the other hand, the phenomenon of shear thinning in nano-confinements is not well understood. Pioneering shear-SFA studies of oligomers demonstrated that, in the nonNewtonian regime, the eflective viscosity seemed to follow a universal behaviour: ~ ~ e ~ + ~ / ~ [ 6 ] . Subsequent non-equilibrium MD simulations verified this power law, but at the same time showed the possibility of a richer response to shear [15]. Since then, a variety of shear thinning power laws have been reported in the literature [6] ,[7], [4]. In all these cases a common behaviour is observed for wide enough films: linear Newtonian-like response for small shear rates, followed by extensive, power law, shear thinning. In order to gain more insight in the mechanisms of shear thinning in films of nanometer thickness, we carried out MD simulations of confined oligomer fluids under Couette flow [12]-[14]. A well-studied bead-spring model chain [11]-[l61 is confined between two atomically @ Les Editions de Physique 372 EUROPHYSICS LETTERS TABLE I. Power law fits (qee N q-“) to the shear thinning regions for various systems. Nearly all the shear thinning takes place inside the solid-oligomer interface and the power law -describing the response of viscosity in this regionis determined by the wall afinity, whereas it is rather insensitive to the oligomer molecule architectures. I n the middle of the film we only fit a power law for comparison reasons, inspired by the experimental standard procedures (within parenthenses the fit-uncertainty of the last decimal). Local effective viscosities wall affinity pore width type of oligomer a a EW (€1 h(a) middle part first layer