Adsorption of Binary Mixtures of the Lennard-Jones Truncated and Shifted Fluid on Planar Walls

Heier, Michaela; Diewald, Felix; Müller, Ralf; Langenbach, Kai; Hasse, Hans

doi:10.1021/acs.jced.1c00350

Cited by 9 publications

(8 citation statements)

References 56 publications

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“…hardness and vibrations) play an important role for the formation of the tribofilm [1]. A large number of properties of solid-fluid interfaces have been individually comprehensively studied in the literature for different types of systems -both real substance [90,329,406] and model systems [26,56,92,161,162,219,325]. This comprises both equilibrium properties [26,56,56,162,219] and dynamic properties [90,92,325,329,406].…”

Section: Solid-fluid Interphasementioning

confidence: 99%

Influence of Lubrication on Tribological Nanoscopic Contact Processes

Stephan

2024

Preprint

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Lubricated tribological contact processes are important in both nature and in many technical applications. Fluid lubricants play an important role in contact processes, e.g. they reduce friction and cool the contact zone. The fundamentals of lubricated contact processes on the atomistic scale are, however, today not fully understood. A lubricated contact process is defined here as a process, where two solid bodies that are in close proximity and eventually in parts in direct contact, carry out a relative motion, whereat the remaining volume is submersed by a fluid lubricant. Such lubricated contact processes are difficult to examine experimentally. Atomistic simulations are an attractive alternative for investigating the fundamentals of such processes. In this work, molecular dynamics simulations were used for studying different elementary processes of lubricated tribological contacts. A simplified, yet realistic simulation setup was developed in this work for that purpose using classical force fields. In particular, the two solid bodies were fully submersed in the fluid lubricant such that the squeeze-out was realistically modeled. The velocity of the relative motion of the two solid bodies was imposed as a boundary condition. Two types of cases were considered in this work: i) a model system based on synthetic model substances, which enables a direct, but generic, investigation of molecular interaction features on the contact process; and ii) real substance systems, where the force fields describe specific real substances. Using the model system i), also the reproducibility of the findings obtained from the computer experiments was critically assessed. In most cases, also the dry reference case was studied. Both mechanical and thermodynamic properties were studied – focusing on the influence of lubrication. The following properties were studied: The contact forces, the coefficient of friction, the dislocation behavior in the solid, the chip formation and the formation of the groove, the squeeze-out behavior of the fluid in the contact zone, the local temperature and the energy balance of the system, the adsorption of fluid particles on the solid surfaces, as well as the formation of a tribofilm. Systematic studies were carried out for elucidating the influence of the wetting behavior, the influence of the molecular architecture of the lubricant, and the influence of the lubrication gap height on the contact process. As expected, the presence of a fluid lubricant reduces the temperature in the vicinity of the contact zone. The presence of the lubricant is, moreover, found to have a significant influence on the friction and on the energy balance of the process. The presence of a lubricant reduces the coefficient of friction compared to a dry case in the starting phase of a contact process, while lubricant molecules remain in the contact zone between the two solid bodies. This is a result of an increased normal and slightly decreased tangential force in the starting phase. When the fluid molecules are squeezed out with ongoing contact time and the contact zone is essentially dry, the coefficient of friction is increased by the presence of a fluid compared to a dry case. This is attributed to an imprinting of individual fluid particles into the solid surface, which is energetically unfavorable. By studying the contact process in a wide range of gap height, the entire range of the Stribeck curve is obtained from the molecular simulations. Thereby, the three main lubrication regimes of the Stribeck curve and their transition regions are covered, namely boundary lubrication (significant elastic and plastic deformation of the substrate), mixed lubrication (adsorbed fluid layers dominate the process), and hydrodynamic lubrication (shear flow is set up between the surface and the asperity). The atomistic effects in the different lubrication regimes are elucidated. Notably, the formation of a tribofilm is observed, in which lubricant molecules are immersed into the metal surface. The formation of a tribofilm is found to have important consequences for the contact process. The work done by the relative motion is found to mainly dissipate and thereby heat up the system. Only a minor part of the work causes plastic deformation. Finally, the assumptions, simplifications, and approximations applied in the simulations are critically discussed, which highlights possible future work.

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Section: Solid-fluid Interphasementioning

confidence: 99%

Influence of Lubrication on Tribological Nanoscopic Contact Processes

Stephan

2024

Preprint

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“…Also, mixtures were studied by some authors. 3,8,17,23 Yet, mostly, mixtures of real substances were considered, and specific systems were studied, such as argon + krypton, 26 water + alcohol, 11 brine + CO 2 , 7 and brine + ndecane. 15 In some mixtures, competitive adsorption and/or wetting were also addressed, i.e., systems where the fluid components of a mixture have different affinities to the substrate surface.…”

Section: Introductionmentioning

confidence: 99%

“…27−29 Alternatives are studies of model systems, by which the influence of different molecular interactions on macroscopic observables can be investigated in a systematic manner. [1][2][3]23,24,30 Model systems have been used for studying adsorption at solid−fluid interfaces 23,31−36 and vapor−liquid interfaces. 37−40 Yet, in most cases, only two-phase contacts were considered.…”

Section: Introductionmentioning

confidence: 99%

“…Molecular dynamics simulations have been used numerous times in the literature for studying wetting and adsorption. − In many cases, pure components were studied, e.g., in refs , , , and . Also, mixtures were studied by some authors. ,,, Yet, mostly, mixtures of real substances were considered, and specific systems were studied, such as argon + krypton, water + alcohol, brine + CO 2 , and brine + n -decane .…”

Section: Introductionmentioning

confidence: 99%

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Molecular Dynamics Study of Adsorption and Wetting of Mixtures of Simple Fluids on Planar Walls

Fertig,

Hasse,

Stephan

2024

J. Phys. Chem. C

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The influence of molecular interactions on the adsorption and wetting of binary mixtures was studied by molecular dynamics simulations of sessile droplets on planar walls. The results provide insights into the competing effects between the fluid− fluid interactions and the solid−fluid interactions on the adsorption and wetting of mixtures. All interactions were modeled by the Lennard-Jones truncated−shifted (LJTS) potential. The influence of the different dispersive interactions in the fluid mixture as well as those between the fluid components and the wall on the gas and liquid adsorption, the three-phase contact, and the contact angle was determined. For the fluid mixtures studied in the present work, the phase behavior and the vapor−liquid interfacial properties are known from recent studies. Together with the present results, this provides a comprehensive picture of the influence of different molecular interactions on the closely related phenomena of wetting and adsorption in mixtures.

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“…34 For non-bonded interaction, coulombic interaction is not considered, as hexadecane is non-polar; thus, the electronic force between the substrate and lubricant does not exist, and there is no need to calculate coulombic interaction in slabs for atoms that are constrained by harmonic potential. The solid−liquid interaction affects the wetting, 35,36 boundary slip, 37 heat conduction, 38 adsorption, 39 and solidification 13 behavior of oil molecules on solid surfaces, which will make a great difference in tribological systems. Especially, the Kapitza resistance, 40 which is a measure of resistance to thermal flow at the interface between two materials, greatly affects the temperature rise of the oil film 41 and may affect the mechanochemical reactions during the friction process.…”

Section: ■ Introductionmentioning

confidence: 99%

Investigating the Behavior of Various Lubrication Regimes under Dynamic Conditions Using Nonequilibrium Molecular Dynamics

Wei,

Gao,

Yang

et al. 2023

Langmuir

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It is crucial to comprehend how the oil film varies under dynamic operating conditions and the accompanying friction properties to better grasp the friction mechanism and control friction behavior. To model the friction characteristics under boundary lubrication (BL) and elastohydrodynamic lubrication (EHL), nonequilibrium molecular dynamics simulations with various numbers of hexadecane molecules as lubricating oil were conducted in this research under the conditions of dynamic speed and dynamic load. All the dynamic operating conditions have the form of sine waves, with various frequencies and amplitudes. According to the findings, the friction force is strongly connected with interfaces where relative sliding takes place, whose number, velocity difference, and the degree of solidification have significant influences. The variation of amplitude under dynamic load can cause a regular change in the density of the lubricating layer, while the variation of frequency can cause a change in molecular layer's range of motion. Both effects are crucial for friction. The structure of the lubricating layer with lower friction varies with various frequencies for dynamic velocity. Both high and small amplitudes of velocity offer advantages to form a stable film structure at low frequencies in the BL and EHL regions, while the amplitude in the BL area has minimal association with friction at high frequencies. At high frequencies in the EHL region, the friction rises as the amplitude of velocity grows and the lubricating layer becomes more unstable.

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Adsorption of Binary Mixtures of the Lennard-Jones Truncated and Shifted Fluid on Planar Walls

Cited by 9 publications

References 56 publications

Influence of Lubrication on Tribological Nanoscopic Contact Processes

Influence of Lubrication on Tribological Nanoscopic Contact Processes

Molecular Dynamics Study of Adsorption and Wetting of Mixtures of Simple Fluids on Planar Walls

Investigating the Behavior of Various Lubrication Regimes under Dynamic Conditions Using Nonequilibrium Molecular Dynamics

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