NANORHEOLOGY OF MOLECULARLY THIN-FILMS OF N-HEXADECANE IN COUETTE SHEAR-FLOW BY MOLECULAR-DYNAMICS SIMULATION

In this work the rheological and structural properties of n-hexadecane have been studied by molecular dynamics simulation. The model consist s of two structured atomic walls between which the fluid is sheared by moving the walls in opposite directions. The fluid consists of chains of n-hexadecane molecules. Each molecule has 16 interaction sites whe re each site on the molecule represents a CH, or CH, group. The Lennar d-Jones potential governs the intermolecular interactions. Stretching, angular and torsional potentials are used for the intramolecular inte ractions to preserve the integrity of the molecules. An isothermal sim ulation of the Couette shear flow is conducted to reveal the rheologic al properties of n-hexadecane at high Weissenberg numbers in films as thin as 1 nm. The results obtained show an increase in the average vis cosity of hexadecane as the film thickness is decreased to scales comp arable to the molecular diameter of the chain segments. These results agree with recent experimental findings for very thin films, revealing shear thinning and normal stress difference effects which are an indi cation of non-Newtonian behaviour. Structural properties such as the d ensity profiles, bond angle and dihedral angle distribution functions and average end-to-end distance of the molecules are obtained for alms of different thickness and at different shear rates. The effects of t he wall-fluid interaction strength on the fluid properties are also in vestigated in different adsorption limits. It seems that adsorption is a determining factor in the properties of these ultrathin films. The results indicate different shear responses depending on the adsorption limit of the surface. (C) 1998 Elsevier Science B.V. All rights reser ved.