Nonequilibrium Molecular Dynamics Simulation of Steady Shear Flows of Fluids Consisting of Model Linear Molecules ; Shear Flow of n-Butane

Abstract

Steady shear flows of liquid n-butane were simulated at T=150K and T=300K by an isothermal nonequilibrium molecular dynamics. In the model molecule of n-butane, the bond lengths are fixed by SHAKE algorithm, and the bond angle and dihedral angle are controlled by the corresponding potentials respectively. The liquids exhibit shear-thinning in viscosity at high shear rates and the shear-thinning becomes more remarkable with lowering the temperature. The shear flow also affects the molecular conformation; at the high shear rates, the distribution of the bond angle is more concentrated around θ₀ where the bond angle bending potential is minimum, and the trans configuration becomes increasingly populated. It is suggested that the non-Newtonian viscosity is caused by both the flow-induced orientation and the change of molecular conformation with shear rate. Comparison of our results with those obtained by Morriss, Davies, and Evans [J. Chem. Phys. 94, 7420 (1991)] indicates that the shear-thickening observed in the latter work is caused by the increase in the gauche population at high shear rates.