Shear heating, flow, and friction of confined molecular fluids at high pressure

Ewen, James P.; Gao, Hongyu; Müser, Martin H.; Dini, Daniele

doi:10.1039/c8cp07436d

Cited by 31 publications

(30 citation statements)

References 75 publications

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“…Similar behaviour has also been reported in previous NEMD simulations of thin C 60 films between polymer-like surfaces at low pressure (10 MPa) which showed boundary slip [12]. The LSS has been linked to a range of effects, including the glass transition [20] and changes in the flow behaviour [19,21]. As discussed above, non-linear flow persists into the steady state for some of the systems and conditions studied here (Figure 7).…”

Section: Accepted Manuscriptsupporting

confidence: 88%

“…Indeed, the Archard equation predicts a linear increase in temperature rise with film thickness [75]. The same trend has been observed in recent NEMD simulations of branched and cyclic alkanes [21]. The n-alkane films become more liquid-like at high shear rates, as shown by the reduction in R 2 ete ( Figure 3) and S xz 2 (Figure 4), and long-ranged order ( Figure 5).…”

Section: Accepted Manuscriptsupporting

confidence: 80%

“…The transition from Couette flow to CL has been observed for viscous polymers using in-contact photoluminescence experiments under EHL conditions [61]. It has also been observed in previous NEMD simulations of atomic fluids [16,17,18] as well as branched and cyclic alkanes [19,21] under EHL conditions. At high sliding velocity and low pressure (Figure 7b x Velocity m s −1 AMD g cm −3 z coordinate C16 C60 but there is a small amount of boundary slip at the fluidsurface interface (slip length ≤ 1 nm).…”

Section: Film Structure and Flowsupporting

confidence: 71%

“…Interestingly, C 60 reaches the LSS in Figure 10 at high pressure (1.5 GPa) while C 16 does not. Since all of the n-alkanes appear glassy (or even crystalline) in Figure 5, these results further support a link between the LSS and non-linear flow behaviour [19,21].…”

Section: Accepted Manuscriptsupporting

confidence: 64%

“…Further NEMD simulations showed that the nonequilibrium phase transitions observed at high pressure can lead to deviations from classical friction laws [18], which has also been observed for thinner n-alkane films at lower pressure [12]. Recently, unusual friction and nonequilibrium phase behaviour has also been observed in NEMD simulations of relatively thick films of branched and cyclic alkanes under EHL conditions [19,20,21]. To design new lubricant molecules to control EHL friction, a clear understanding of how molecular structure affects molecular-scale structure, flow and friction is required.…”

Section: Accepted Manuscriptmentioning

confidence: 72%

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Behaviour of n-alkanes confined between iron oxide surfaces at high pressure and shear rate: A nonequilibrium molecular dynamics study

Restrepo

Eijk

Ewen

2019

Tribology International

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The behaviour of n-alkanes confined and sheared between iron oxide surfaces has been studied using nonequilibrium molecular dynamics simulations. The molecular extension, orientation, film structure, flow, and friction have been investigated for a range of n-alkanes chain lengths under conditions representative of the elastohydrodynamic lubrication regime. At high pressure, the molecules show strong layering and long-range order, suggesting solid-like films. Conversely, high shear rates result in less elongated, layered, and ordered molecules; indicating more liquid-like films. Although Couette flow is usually observed for short n-alkanes, the flow is often non-linear for long n-alkanes. The friction coefficient increases logarithmically with shear rate, but the slope decreases with increasing pressure such that it becomes insensitive to shear rate for long n-alkanes.

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Section: Accepted Manuscriptsupporting

confidence: 88%

Section: Accepted Manuscriptsupporting

confidence: 80%

Section: Film Structure and Flowsupporting

confidence: 71%

Section: Accepted Manuscriptsupporting

confidence: 64%

Section: Accepted Manuscriptmentioning

confidence: 72%

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Behaviour of n-alkanes confined between iron oxide surfaces at high pressure and shear rate: A nonequilibrium molecular dynamics study

Restrepo

Eijk

Ewen

2019

Tribology International

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Synergistic lubrication effect of Al2O3 and MoS2 nanoparticles confined between iron surfaces: a molecular dynamics study

Sun

Meng

et al. 2021

J Mater Sci

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Nonequilibrium molecular dynamics (NEMD) simulations were performed to investigate the tribology behaviors of Al 2 O 3 and MoS 2 nanoparticles confined between iron (Fe) slabs. Results indicated that the combined use of these two nanoparticles yielded the lowest and most stable friction force, normal force, interface temperature and wear rate, which exhibited a significant synergistic lubrication effect. A novel parameter the rolling/sliding motion coefficient (K rs ) was proposed to evaluate the motion pattern of spherical Al 2 O 3 . There were 51% rolling ? 49% sliding motion when used alone and 91% rolling ? 9% sliding in the existence of MoS 2 . Similarly, about 72.3% of the friction was shared by interlayer sliding of MoS 2 monolayers in the presence of Al 2 O 3 , which was higher than used alone (54.8%). Then, the diffusion of atoms at the friction interface was explored to reveal the synergistic lubrication mechanism. The tribofilm formed by the diffusion of Fe and S atoms could protect the metal surfaces from further wear. The adsorption of S atoms to Al 2 O 3 nanoparticle could promote its rolling effect and prevent it from embedding into iron matrix. Besides, Al 2 O 3 could also facilitate the separation of MoS 2 monolayers to enhance their interlayer sliding effect.

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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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Shear heating, flow, and friction of confined molecular fluids at high pressure

Abstract: Understanding the molecular-scale behavior of fluids confined and sheared between solid surfaces is important for many applications, particularly tribology where this often governs the macroscopic frictional response.

Cited by 31 publications

References 75 publications

Behaviour of n-alkanes confined between iron oxide surfaces at high pressure and shear rate: A nonequilibrium molecular dynamics study

Behaviour of n-alkanes confined between iron oxide surfaces at high pressure and shear rate: A nonequilibrium molecular dynamics study

Synergistic lubrication effect of Al2O3 and MoS2 nanoparticles confined between iron surfaces: a molecular dynamics study

Contributions of Molecular Dynamics Simulations to Elastohydrodynamic Lubrication

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