Probing the high-pressure viscosity of hydrocarbon mixtures using molecular dynamics simulations

Kondratyuk, Nikolay; Pisarev, V. V.; Ewen, James P.

doi:10.1063/5.0028393

Cited by 28 publications

(15 citation statements)

References 84 publications

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“…In the Green-Kubo method, errors accumulate at long times and as result, this is A c c e p t e d M a n u s c r i p t not something that can be remedied simply by using longer trajectories [65]. The time decomposition method [12] may help to some degree, as recently demonstrated for 2,2,4-trimethylhexane up to viscosities of 20 mPa s [66] and for a binary mixture of methylcyclohexane and 1-methylnaphthalene up to viscosities of 50 mPa s [20]. However, at higher viscosites, NEMD with extrapolation back to the Newtonian viscosity (using the Eyring equation) seems to be the only reliable method (this study and [27][28][29]).…”

Section: Accepted Manuscriptmentioning

confidence: 99%

“…Previous studies also include a variety of calculations for viscosity with EMD-GK combined with NEMD in most cases [16][17][18][19]. The systems in the literature include simple linear alkanes, cyclic [20] or long-chain hydrocarbons, and more complex structures such as squalane and 1-decene trimer (PAO-4) [21][22][23]. There are fewer research papers on viscosity index [24,25] and pressure-viscosity coefficient calculations, a fact that may indicate that these properties can be difficult to simulate.…”

Section: Introductionmentioning

confidence: 99%

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Evaluation of Methods for Viscosity Simulations of Lubricants at Different Temperatures and Pressures: A Case Study on PAO-2

Mathas

Holweger

Wolf

et al. 2021

Tribology Transactions

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The behavior of lubricants at operational conditions, such as at high pressures, is a topic of great industrial interest. In particular, viscosity and the viscosity-pressure relation are especially important for applications and their determination by computational simulations is very desirable. In this study we evaluate methods to compute these quantities based on fully atomistic molecular dynamics simulations which are computationally demanding but also have the potential to be most accurate. We used the 9,10dimethyloctadecane molecule, main component of PAO-2 base oil as the lubricant for our tests. The methods used for the viscosity simulations are the Green-Kubo equilibrium molecular dynamics (EMD-GK) and non-equilibrium molecular dynamics (NEMD), at pressures of up to 1.0 GPa and various temperatures (40-150 degrees Celsius). We present the theory behind these methods and investigate how the simulation parameters affect the results obtained, to ensure viscosity convergence with respect to the A c c e p t e d M a n u s c r i p t simulation intervals and all other parameters. We show that by using each method in its regime of applicability, we can achieve good agreement between simulated and measured values. NEMD simulations at high pressures captured zero shear viscosity successfully, while at 40 degrees Celsius EMD-GK is only applicable to pressures up to 0.3 GPa, where the viscosity is lower. In NEMD, longer and multiply repeated simulations improve the confidence interval of viscosity, which is essential at lower pressures. Another aspect of these methods is the choice of the utilized force field for the atomic interactions. This was investigated by selecting two different commonly used force fields.

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Section: Accepted Manuscriptmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Evaluation of Methods for Viscosity Simulations of Lubricants at Different Temperatures and Pressures: A Case Study on PAO-2

Mathas

Holweger

Wolf

et al. 2021

Tribology Transactions

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“…Commercial lubricants are complicated mixtures rather than single well-defined molecules. Kondratyuk et al [81] recently used the all-atom COMPASS [66] force field and Green-Kubo with the TDM [70] to study the high-pressure viscosity of mixtures. They studied the viscosity and density of methylcyclohexane, 1-methylnaphthalene, and their binary mixtures at 323 K and pressures of up to 500 MPa.…”

Section: High-pressure Newtonian Viscositymentioning

confidence: 99%

Contributions of Molecular Dynamics Simulations to Elastohydrodynamic Lubrication

2021

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The prediction of friction under elastohydrodynamic lubrication (EHL) conditions remains one of the most important and controversial areas of tribology. This is mostly because the pressure and shear rate conditions inside EHL contacts are particularly severe, which complicates experimental design. Over the last decade, molecular dynamics (MD) simulation has played an increasingly significant role in our fundamental understanding of molecular behaviour under EHL conditions. In recent years, MD simulation has shown quantitative agreement with friction and viscosity results obtained experimentally, meaning that they can, either in isolation or through the use of multiscale coupling methods, begin to be used to test and inform macroscale models for EHL problems. This is particularly useful under conditions that are relevant inside machine components, but are difficult to obtain experimentally without uncontrollable shear heating.

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“…51,52 The Green-Kubo method has become a widely employed tool in determining the viscosity of practically relevant liquids, for example, in tribology. 53 These new ways of obtaining the viscosity from the viscuit reformulation of GK derived here may find use in that field.…”

Section: Discussionmentioning

confidence: 88%

Viscuit and the fluctuation theorem investigation of shear viscosity by molecular dynamics simulations: The information and the noise

Heyes

Dini

Smith

2021

The Journal of Chemical Physics

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The shear viscosity, η, of model liquids and solids is investigated within the framework of the viscuit and Fluctuation Theory (FT) probability distribution function (PDF) theories, following on from Heyes et al. [J. Chem. Phys. 152, 194504 (2020).] using equilibrium molecular dynamics (MD) simulations on Lennard-Jones and Weeks-Chandler-Andersen model systems. The viscosity can be obtained in equilibrium MD simulation from the first moment of the viscuit PDF which is shown for finite simulation lengths to give a less noisy plateau region than the Green-Kubo method. Two other formulas for the shear viscosity in terms of the viscuit and PDF analysis are also derived. A separation of the time-dependent average negative and positive viscuits extrapolated from the noise dominated region to zero time provide another route to η. The third method involves the relative number of positive and negative viscuits, and their PDF standard deviations on the two sides for an equilibrium system. For the FT and finite shear rates, accurate analytic expressions for the relative number of positive to negative block average shear stresses is derived assuming a shifted gaussian PDF, which is shown to agree well with Non-equilibrium Molecular Dynamics simulations. A similar treatment of the positive and negative block average contributions to the viscosity is also shown to match the simulation data very well.

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Probing the high-pressure viscosity of hydrocarbon mixtures using molecular dynamics simulations

Cited by 28 publications

References 84 publications

Evaluation of Methods for Viscosity Simulations of Lubricants at Different Temperatures and Pressures: A Case Study on PAO-2

Evaluation of Methods for Viscosity Simulations of Lubricants at Different Temperatures and Pressures: A Case Study on PAO-2

Contributions of Molecular Dynamics Simulations to Elastohydrodynamic Lubrication

Viscuit and the fluctuation theorem investigation of shear viscosity by molecular dynamics simulations: The information and the noise

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