NEMD Simulations and the Rheology of Liquid Hydrocarbons

Abstract

The study of non-Newtonian fluids undergoing planar Couette flow has been one of the most popular applications of Non-Equilibrium Molecular Dynamics (NEMD) simulations. The methodology employed in these studies is becoming well established¹, but limited applications of NEMD methods have been reported for realistic molecular fluids². Moreover, the correct rheological interpretation of NEMD results is still an area of dispute^3,4. For instance, there have been many parametrisations of a non-Newtonian flow curve, η = η(γ̇), suggested in the NEMD literature², none of which can be tested experimentally because the lowest NEMD shear rates achieved by present supercomputers are still two to three orders of magnitude higher than the maximum ones accessible by modern rheometers. On the other hand, there is a wealth of experimental information on the rheological properties of polymeric liquids^5,6 which cannot be directly compared with NEMD results because of the inordinate computing time required for accurate simulations of large molecules under realistic conditions. In this note we report on our simulations of liquid hydrocarbons in planar Couette flow and we attempt to bridge such a gap between simulations and experiments, by analysing the NEMD results using the same rheological functions used in polymer rheology and by suggesting possible scaling laws for their behaviour.