On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction

Ewen, James P.; Gattinoni, Chiara; Zhang, J.; Heyes, D. M.; Spikes, H. A.; Dini, Daniele

doi:10.1039/c7cp01895a

Cited by 54 publications

(108 citation statements)

References 76 publications

(203 reference statements)

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“…This is not definite confirmation of the validity of the Eyring model however since other models show equally good fit within the limits of accuracy of the simulations [71,72]. Interestingly some liquids do not follow this type of Eyring dependence, instead showing a levelling out of shear stress with increasing strain rate to reach a limiting shear stress [69].…”

Section: Ehd Frictionmentioning

confidence: 76%

“…Without such information it is not possible to test Eyring's shear thinning model fully, or indeed other rheological models. This issue may be addressed via molecular simulation [69] but a recent, complementary experimental approach is phosphorescence imaging, where the velocity of the fluid across a thin EHD film can be profiled from the convection of a small, laser-excited column of fluid within the film [147].…”

Section: Future Directionsmentioning

confidence: 99%

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Stress-augmented thermal activation: Tribology feels the force

Spikes

2018

Friction

122

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Abstract:In stress-augmented thermal activation, the activation energy barrier that controls the rate of atomic and molecular processes is reduced by the application of stress, with the result that the rate of these processes increases exponentially with applied stress. This concept has particular relevance to Tribology, and since its development in the early twentieth century, it has been applied to develop important models of plastic flow, sliding friction, rheology, wear, and tribochemistry. This paper reviews the development of stress-augmented thermal activation and its application to all of these areas of Tribology. The strengths and limitations of the approach are then discussed and future directions considered. From the scientific point of view, the concept of stress-augmented thermal activation is important since it enables the development of models that describe macroscale tribological performance, such as friction coefficient or tribofilm formation, in terms of the structure and behaviour of individual atoms and molecules. This both helps us understand these processes at a fundamental level and also provides tools for the informed design of lubricants and surfaces.

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Section: Ehd Frictionmentioning

confidence: 76%

Section: Future Directionsmentioning

confidence: 99%

Stress-augmented thermal activation: Tribology feels the force

Spikes

2018

Friction

122

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“…Conversely, traction fluids, which are based on inflexible cycloaliphatic groups, gave high friction coefficients that were less sensitive to shear rate and pressure. The observed differences in friction behaviour was rationalised through the stronger shear localisation which was observed for the traction fluids (CL and PS) compared to the lubricants (Couette and CL) in the NEMD simulations [29]. Washizu et al [206] also reported shear localisation (CL) in NEMD simulations on n-hexane confined between gold slabs at high pressure (0.1−8.0 GPa).…”

Section: Shear Localisationmentioning

confidence: 82%

“…Recently, a combination of tribological experiments and confined NEMD simulations has been used to investigate the effect of base fluid molecular structure on nonequilibrium phase behaviour and friction by Ewen et al [29]. An extensive parameter study, including several lubricant and traction fluid molecules subjected to pressures (0.5−2.0 GPa) and shear rates (10 4 −10 10 s −1 ) typical of the EHL regime, revealed clear relationships between the friction and flow behaviour.…”

Section: Shear Localisationmentioning

confidence: 99%

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

2018

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Nonequilibrium molecular dynamics (NEMD) simulations have provided unique insights into the nanoscale behaviour of lubricants under shear. This review discusses the early history of NEMD and its progression from a tool to corroborate theories of the liquid state, to an instrument that can directly evaluate important fluid properties, towards a potential design tool in tribology. The key methodological advances which have allowed this evolution are also highlighted. This is followed by a summary of bulk and confined NEMD simulations of liquid lubricants and lubricant additives, as they have progressed from simple atomic fluids to ever more complex, realistic molecules. The future outlook of NEMD in tribology, including the inclusion of chemical reactivity for additives, and coupling to continuum methods for large systems, is also briefly discussed.

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“…This gives rise to shear thinning, which has also been represented by semi-empirical analytic expressions. The molecular level factors that govern the shear rate dependent viscosity are still poorly understood, 1,2 and while the distortion of the local structure of a liquid under shear is now well characterized for small molecules, 3,4 theoretical prediction of the shear rate dependence of the viscosity 5 has proved to be more problematic. Phenomenological models of shear thinning are still prevalent in part for this reason.…”

Section: Introductionmentioning

confidence: 99%

Incremental viscosity by non-equilibrium molecular dynamics and the Eyring model

Heyes

Dini

Smith

2018

The Journal of Chemical Physics

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The viscoelastic behavior of sheared fluids is calculated by Non-Equilibrium Molecular Dynamics (NEMD) simulation, and complementary analytic solutions of a time-dependent extension of Eyring's model (EM) for shear thinning are derived. It is argued that an "incremental viscosity," η i , or IV which is the derivative of the steady state stress with respect to the shear rate is a better measure of the physical state of the system than the conventional definition of the shear rate dependent viscosity (i.e., the shear stress divided by the strain rate). The stress relaxation function, C i (t), associated with η i is consistent with Boltzmann's superposition principle and is computed by NEMD and the EM. The IV of the Eyring model is shown to be a special case of the Carreau formula for shear thinning. An analytic solution for the transient time correlation function for the EM is derived. An extension of the EM to allow for significant local shear stress fluctuations on a molecular level, represented by a gaussian distribution, is shown to have the same analytic form as the original EM but with the EM stress replaced by its time and spatial average. Even at high shear rates and on small scales, the probability distribution function is almost gaussian (apart from in the wings) with the peak shifted by the shear. The Eyring formula approximately satisfies the Fluctuation Theorem, which may in part explain its success in representing the shear thinning curves of a wide range of different types of chemical systems.

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On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction

Cited by 54 publications

References 76 publications

Stress-augmented thermal activation: Tribology feels the force

Stress-augmented thermal activation: Tribology feels the force

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

Incremental viscosity by non-equilibrium molecular dynamics and the Eyring model

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