Rheological and structural studies of liquid decane, hexadecane, and tetracosane under planar elongational flow using nonequilibrium molecular-dynamics simulations

Abstract

Material functions of liquid n -hexadecane under steady shear via nonequilibrium molecular dynamics simulations: Temperature, pressure, and density effects J. Chem. Phys. 130, 084904 (2009) We report for the first time rheological and structural properties of liquid decane, hexadecane, and tetracosane using nonequilibrium molecular-dynamics ͑NEMD͒ simulations under planar elongational flow ͑PEF͒. The underlying NEMD algorithm employed is the so-called p-SLLOD algorithm ͓C. Baig, B. J. Edwards, D. J. Keffer, and H. D. Cochran, J. Chem. Phys. 122, 114103 ͑2005͔͒. Two elongational viscosities are measured, and they are shown not to be equal to each other, indicating two independent viscometric functions in PEF. With an appropriate definition, it is observed that the two elongational viscosities converge to each other at very low elongation rates, i.e., in the Newtonian regime. For all three alkanes, tension-thinning behavior is observed. At high elongation rates, chains appear to be fully stretched. This is supported by the result of the mean-square end-to-end distance of chains ͗R ete 2 ͘ and the mean-square radius of gyration of chains ͗R g 2 ͘, and further supported by the result of the intramolecular Lennard-Jones ͑LJ͒ potential energy. It is also observed that ͗R ete 2 ͘ and ͗R g 2 ͘ show a different trend as a function of strain rate from those in shear flow: after reaching a plateau value, ͗R ete 2 ͘ and ͗R g 2 ͘ are found to increase further as elongation rate increases. A minimum in the hydrostatic pressure is observed for hexadecane and tetracosane at about ˙͑m 2 / ͒ 1/2 = 0.02. This phenomenon is shown to be associated with the intermolecular LJ potential energy. The bond-bending and torsional energies display similar trends, but a different behavior is observed for the bond-stretching energy. An important observation common in these three bonded-intramolecular interactions is that all three modes are suppressed to a small value at high elongation rates. We conjecture that a liquid-crystal-like, nematic structure is present in these systems at high elongation rates, which is characterized by a strong chain alignment with a fully stretched conformation.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…PEF simulations have been widely used to study polymers flow [96,97] but more rarely for lubricant-sized molecules. Baig et al [98] studied the rheological and structural properties of linear liquid n-alkanes (C 10 , C 12 , C 20 ) using NEMD simulations under PEF. They showed that these molecules behave very differently to the way they do under shear flow, with strong alignment of fully stretched chains at high elongation rates suggesting the formation of a liquidcrystal-like, nematic structure [98].…”

“…Baig et al [98] studied the rheological and structural properties of linear liquid n-alkanes (C 10 , C 12 , C 20 ) using NEMD simulations under PEF. They showed that these molecules behave very differently to the way they do under shear flow, with strong alignment of fully stretched chains at high elongation rates suggesting the formation of a liquidcrystal-like, nematic structure [98]. However, it should be noted that concerns have raised with the validity of the p-SLLOD algorithm used in these simulations [99].…”

Communication: Role of short chain branching in polymer structure and dynamics

“…1(a), each system exhibits a typical shear-thinning behavior, which is generally attributed to collective chain alignment in the flow direction. 6,22,23 However, it should be noted that the degree of shear thinning is significantly affected by chain branching; i.e., shear-thinning weakens for the SCB and H-shaped polymers. These tendencies of branched polymers are supposed to arise from a lesser degree of chain stretch and alignment to the flow direction at a given flow strength (as compared to the linear analogues), due to a more compact molecular structure associated with their intrinsic nonlinear molecular architecture.…”

“…The observed minimum behavior of the hydrostatic pressure is ascribed to the competing effect between chain alignment/stretching (decreasing pressure) and molecular collisions (increasing pressure). 22 Another useful dynamical measure to appreciate the effect of chain branching is the stress ratio between the dissipative shear stress (σ x y ) and the restoring elastic stress (σ x x − σ y y ) [see the inset of Fig. 1(b)].…”

An examination of the validity of nonequilibrium molecular-dynamics simulation algorithms for arbitrary steady-state flows

“…[4][5][6][7][8][9] Two of these sets have assumed that the SLLOD algorithm, used successfully in shear flows for so long, would naturally carry over to PEF as well. [4][5][6][7] However, problems arose in these simulations in that aphysical phase transformations occurred at low strain rates due to an unavoidable numerical round-off error.…”