Nonequilibrium molecular dynamics simulation of the rheology of linear and branched alkanes

Journal Article · Sun Mar 01 04:00:00 EST 1998 · International Journal of Thermophysics

DOI:https://doi.org/10.1023/A:1022565427881· OSTI ID:675551

Cui, S T; Cummings, P T; Cochran, H D; Moore, J D; Gupta, S A ^[1]

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1. Univ. of Tennessee, Knoxville, TN (United States). Dept. of Chemical Engineering

Liquid alkanes in the molecular weight range of C{sub 20}-C{sub 40} are the main constituents of lubricant basestocks, and their rheological properties are therefore of great concern in industrial lubricant applications. Using massively parallel supercomputers and an efficient parallel algorithm, the authors have carried out systematic studies of the rheological properties of a variety of model liquid alkanes ranging from linear to singly branched and multiply branched alkanes. They aim to elucidate the relationship between the molecular architecture and the viscous behavior. Nonequilibrium molecular dynamics simulations have been carried out for n-decane (C{sub 10}H{sub 22}), n-hexadecane (C{sub 16}H{sub 34}), n-tetracosane (C{sub 24}H{sub 50}), 10-n-hexylnonadecane (C{sub 25}H{sub 52}), and squalane (2, 6, 10, 15, 19, 23-hexamethyltetracosane, C{sub 30}H{sub 62}). At a high strain rate, the viscosity shows a power-law shear thinning behavior over several orders of magnitude in strain rate, with exponents ranging from {minus}0.33 to {minus}0.59. The power-law shear thinning is shown to be closely related to the ordering of the molecules. The molecular architecture is shown to have a significant influence on the power-law exponent. At a low strain rate, the viscosity behavior changes to a Newtonian plateau, whose accurate determination has been elusive in previous studies. The molecular order in this regime is essentially that of the equilibrium system, a signature of the linear response. The Newtonian plateau is verified by independent equilibrium molecular dynamics simulations using the Green-Kubo method. The reliable determination of the Newtonian viscosity from nonequilibrium molecular simulation permits us to calculate the viscosity index for squalane. The viscosity index is a widely used property to characterize the lubricant`s temperature performance, and the studies represent the first approach toward its determination by molecular simulation.

• Sponsoring Organization: Oak Ridge National Lab., TN (United States); USDOE, Washington, DC (United States)
• DOE Contract Number: AC05-96OR22464
• OSTI ID: 675551
• Report Number(s): CONF-970629--
• Journal Information: International Journal of Thermophysics, Journal Name: International Journal of Thermophysics Journal Issue: 2 Vol. 19; ISSN 0195-928X; ISSN IJTHDY
• Country of Publication: United States
• Language: English

Related Subjects

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATION
ALKANES
LUBRICANTS
MOLECULAR DYNAMICS METHOD
MOLECULAR STRUCTURE
PERFORMANCE
RHEOLOGY
VISCOSITY