Rheology of n-alkanes by nonequilibrium molecular dynamics

Abstract

We have simulated sheared liquid n-butane and n-decane by isothermal nonequilibrium molecular dynamics. Both alkanes exhibit nonlinear rheological behavior. Shear thinning at small shear rates, second Newtonian viscosities at moderate shear rates, normal stresses, dilatancy, and birefringence are observed for both fluids. Zero shear rate viscosities were calculated for both fluids: η(γ=0)=0.24 cp for butane at T=292 K, η(γ=0)=0.159 cp for decane at 481 K. These viscosities agree with experimental data for both fluids. The zero shear rate butane viscosity agrees with the Green–Kubo result of Marechal and Ryckaert, Chem. Phys. Lett. 101, 548 (1983). Shear induced molecular rotation, deformation, and alignment are quantitatively described to give a complete picture of molecular behavior in a sheared fluid. Equilibrium and shear simulations of butane with modified dihedral potentials were performed to investigate the effect of molecular conformation on the rheological properties of butane.