Nonequilibrium Molecular Dynamics Simulations of Shear Viscosity: Isoamyl Alcohol, n-Butyl Acetate and Their Mixtures

Nonequilibrium, NVT, molecular dynamics (NEMD) simulations were used to obtain the shear viscosity, η, of isoamyl alcohol, n-butyl acetate, and their binary mixtures at 35°C and 0.1 MPa. The fluids were modeled using rigid bonds, rigid bond angles, appropriate torsional potentials, pairwiseadditive Lennard-Jones dispersion interactions between united-atom sites, and partial point charges located at atomic centers. Simulations were performed at different shear rates, γ, and values obtained at γ = 0 are compared to experimental values. Two methods are commonly used to extrapolate purefluid simulated data to zero shear, η(0). The applicability of these two methods to mixtures of polar fluids was examined in this study. It was found that linear extrapolation with respect to γ can lead to ambiguous η(0) results for some mixtures because of a curvature in the data that shows no observably distinct change in rheology. On the other hand, a log-log plot of η(γ) versus γ is consistently very linear with a distinct change from shear-thinning to Newtonian rheology at lower shear rates. The latter method is recommended for consistency sake, even though agreement between experiment and η(0) values was better with the former method. This agreement was 12% and 21% for the two methods, respectively. A negative bias in the simulated values is attributable to the united-atom model.