MOLECULAR-DYNAMICS SIMULATIONS OF THE RHEOLOGY OF NORMAL-DECANE, HEXADECANE, AND TETRACOSANE

Extensive nonequilibrium molecular dynamics simulations have been carr ied out for liquid decane, hexadecane, and tetracosane at densities co rresponding to atmospheric pressure and near ambient temperatures. The strain-rate-dependent viscosity has been obtained for strain rates ra nging over several orders of magnitude. At high strain rate, the visco sities for all alkanes studied here have similar values and exhibit si milar power-law shear-thinning behavior with a slope between about -0. 40 and -0.33. Accompanying this shear thinning is the onset of orienta tional order and the alignment of the alkane molecules with the flow d irection. The alignment angle tends to 45 degrees at very low strain r ate and is significantly smaller at high strain rate, This suggests th at the chains substantially align in the flow direction and that the d ominant motion at high strain rate is the sliding of the chains parall el to the flow. At low strain rate, the shear viscosity shows a transi tion to Newtonian behavior. The Newtonian viscosity can be obtained fr om the plateau value of the shear viscosity at the lowest strain rates calculated from the nonequilibrium molecular dynamics simulation (NEM D). This is demonstrated by comparing the viscosity of decane obtained by extrapolating the NEMD simulation with an independent calculation using the standard Green-Kubo method. The transition from the non-Newt onian regime to the Newtonian regime is also correlated with the disap pearance of orientational order and with the longest relaxation time o f the liquid alkanes simulated. (C) 1996 American Institute of Physics .