The Rheology of Liquid Pentane Isomers by Non-Equilibrium Molecular Dynamics Simulations

Abstract

In this paper, non-equilibrium molecular dynamics (NEMD) simulations of planar Couette flow are reported for an expanded collapsed atom model for liquid pentane isomers at 273.15 K. The strain rate dependent viscosity for liquid pentane isomers exhibits shear-thinning and a linear dependence on γ1/2. Newtonian viscosities for liquid pentane isomers obtained by a linear extrapolation to zero strain rate are: 0.256cP for normal pentane, 0.219cP for isopentane, and 0.168cP for neopentane. The strain rate dependent pressure difference and normal stress difference vary nearly linearly with the γ3/2 law and the γ law, respectively, for all three liquid pentane isomers. The overall trend of the square of radius of gyration and end-to-end distance for normal pentane is a linear increase with strain rate. For isopentane, the trend hardly changes for the range of shear rate in this study. The alignment angle decreases with increasing strain rate and the alignment angle of the straight chain alkane is less than that of the branched chain alkane. The average percentage of C‒C‒C‒C trans for normal pentane as a function of strain rate is in excellent correlation with the square of the radius of gyration and the average end-to-end distance. Applying the strain rate in the x-direction, the alignment angle is forced to decrease and the percentage of C‒C‒C‒C trans increases with increasing strain rate.