An improved potential model for n-hexadecane molecular dynamics simulations under extreme conditions

Chynoweth, S.; Michopoulos, Yanos

doi:10.1080/00268979400100091

Cited by 37 publications

(35 citation statements)

References 14 publications

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“…In analogy with the pressure case, a generalized shear-dependent expression has been obtained for the viscosity-temperature coefficient by combining equations (3) and (9): 'to. 2 = v o , 1 exP{-~ATz -T 1 ) I Equation (10) has been applied to n-hexadecane at TI = 100.1 "C and T2 = 204.4"C using the qxj) values of Table 2 and the curves of Fig. 2 to obtain the aA.3) plot of Fig.…”

Section: Temperature Effectsmentioning

confidence: 99%

Simulated Non-Newtonian Lubricant Behaviour under Extreme Conditions

Chynoweth

Coy

Michopoulos

1995

Proceedings of the Institution of Mechanical Engineers, Part J:

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Equilibrium and non-equilibrium molecular dynamics techniques are used to simulate a molecular liquid (n-hexadecane) at extreme physical conditions (high temperatures, pressures and shear rates), typical of those encountered in many elastohydrodynamic lubrication applications, and to study the effects of pressure and temperature on the calculated rheological properties. Pressure and temperature effects are found to be more pronounced at low shear rates than at high shear rates, where the non-Newtonian behaviour is dominated by the strong external kinematic effects. The observed non-Newtonian behaviour of the pressure and temperature coeficients of viscosity and the variation of certain molecular geometric functions calculated to assist the microscopic understanding are combined to provide a possible explanation for the phenomenon of limiting shear stress under elastohydrodynamic lubrication conditions. Further implications for the behaoiour of lubricant-like molecules in practical applications are also discussed.

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Section: Temperature Effectsmentioning

confidence: 99%

Simulated Non-Newtonian Lubricant Behaviour under Extreme Conditions

Chynoweth

Coy

Michopoulos

1995

Proceedings of the Institution of Mechanical Engineers, Part J:

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“…In the all-atom ͑AA͒ intermolecular potential model, 15 more realistic asymmetry is provided to the overall intermolecular interactions by placing LJ interaction sites at each atomic center rather than at each carbon. The interaction parameters for this model were regressed from equilibrium properties, 14 but we know of no systematic study of the use of this model to obtain viscosities from simulations. The objectives of this work were to systematically study the efficacy of different intermolecular potential models in obtaining accurate viscosity information from NEMD simulations, to investigate the cause of any model inadequacies, and to improve on the models where possible.…”

Section: Introductionmentioning

confidence: 99%

Predicting the viscosity of alkanes using nonequilibrium molecular dynamics: Evaluation of intermolecular potential models

Allen

Rowley

1997

The Journal of Chemical Physics

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Nonequilibrium molecular dynamics ͑NEMD͒ viscosity simulations of branched and linear alkanes at liquid densities were performed using both united-atom ͑UA͒ and all-atom ͑AA͒ intermolecular potential models in order to study the relative efficacy of the models in predicting fluid viscosity. Both models were used in conjunction with fixed bond lengths and bond angles, but different torsional potentials were investigated. The commonly used Ryckaert-Bellemans intermolecular potential model, which accurately predicts viscosities for short straight-chain alkanes, produced values for branched and long-chain alkanes that were significantly below experimental values. Likewise, a more complex UA model that uses transferrable site potentials and is commonly used to simulate thermodynamic properties also under predicted viscosities for branched and long-chain molecules. The UA models were also found to be density dependent, substantially under predicting viscosity at high liquid densities for all model fluids tested. Predicted viscosities using AA intermolecular potential models were generally substantially too large compared to experiment when using model parameters from the literature, even though thermodynamic properties were adequately predicted. However, evidence suggests that accurately modeling the hydrogen interactions and the rotation potential of methyl groups is essential for accurate viscosity simulations. Therefore, a new set of parameters for the hydrogen interactions was regressed using viscosity simulations of 2-methylpropane and n-pentane. Like the UA model, the AA model with the new parameters is still somewhat density dependent, but gives reasonably accurate predictions of viscosity for most fluids.

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“…[5][6][7] In chain systems without entanglements, on one hand, shear thinning occurs at sufficiently high (but sometimes unrealistically large) shear rates due to chain elongation. [8][9][10][11][12] In entangled polymers, on the other hand, shear thinning occurs at very small shear larger than τ −1 rep , where disentanglements are induced by shear. 3 Thus supercooled chain systems are most easily driven into a nonlinear response regime even by extremely small shear, though the crossover shear stress from linear to nonlinear regimes may not be very small.…”

Section: G(t) = G G (T) + G R (T) (12)mentioning

confidence: 99%

“…In another application, nonequilibrium molecular dynamics (NEMD) simulations have been useful to investigate chain deformations and rheology in flow. [8][9][10][11][12]17,31 In particular, Kröger et al 17 studied the molecular mechanisms of the violations of stress-optical behavior for a melt consisting M = 260 chains with bead number N = 30 after application of elongational flow.…”

Section: G(t) = G G (T) + G R (T) (12)mentioning

confidence: 99%

Dynamics and rheology of a supercooled polymer melt in shear flow

Yamamoto

Ōnuki

2002

The Journal of Chemical Physics

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Using molecular dynamics simulations, we study dynamics of a model polymer melt composed of short chains with bead number N = 10 in supercooled states. In quiescent conditions, the stress relaxation function G(t) is calculated, which exhibits a stretched exponential relaxation on the time scale of the α relaxation time τα and ultimately follows the Rouse dynamics characterized by the time τR ∼ N 2 τα. After application of shearγ, transient stress growth σxy(t)/γ first obeys the linear growth t 0 dt ′ G(t ′ ) for strain less than 0.1 but saturates into a non-Newtonian viscosity for larger strain. In steady states, shear-thinning and elongation of chains into ellipsoidal shapes take place for shearγ larger than τ −1 R . In such strong shear, we find that the chains undergo random tumbling motion taking stretched and compact shapes alternatively. We examine the validity of the stressoptical relation between the anisotropic parts of the stress tensor and the dielectric tensor, which are violated in transient states due to the presence of a large glassy component of the stress. We furthermore introduce time-correlation functions in shear to calculate the shear-dependent relaxation times, τα(T,γ) and τR(T,γ), which decrease nonlinearly as functions ofγ in the shear-thinning regime.

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An improved potential model for n-hexadecane molecular dynamics simulations under extreme conditions

Cited by 37 publications

References 14 publications

Simulated Non-Newtonian Lubricant Behaviour under Extreme Conditions

Simulated Non-Newtonian Lubricant Behaviour under Extreme Conditions

Predicting the viscosity of alkanes using nonequilibrium molecular dynamics: Evaluation of intermolecular potential models

Dynamics and rheology of a supercooled polymer melt in shear flow

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