Probing large viscosities in glass-formers with nonequilibrium simulations

Jadhao, Vikram; Robbins, Mark O.

doi:10.1073/pnas.1705978114

Cited by 69 publications

(92 citation statements)

References 44 publications

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“…As shown in Ref. [18], this approach successfully reproduces published experimental data for η N over a wide range of T , P , and η N . The only significant difference between Eyring fits and nonequilibrium experiments in Fig.…”

Section: B Shear Thinning and Time-temperature Superposition In Squasupporting

confidence: 78%

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Rheological Properties of Liquids Under Conditions of Elastohydrodynamic Lubrication

Jadhao

Robbins

2019

Tribol Lett

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There is an ongoing debate concerning the best rheological model for liquid flows in elastohydrodynamic lubrication (EHL). Due to the small contact area and high relative velocities of bounding solids, the lubricant experiences pressures in excess of 500 MPa and strain rates that are typically 10 5 − 10 7 s −1 . The high pressures lead to a dramatic rise in Newtonian viscosity ηN and the high rates lead to large shear stresses and pronounced shear-thinning. This paper presents detailed simulations of a model EHL fluid, squalane, using nonequilibrium molecular dynamics methods to extract the scaling of its viscosity with shear rate (10 5 − 10 10 s −1 ) over a wide range of pressure P (0.1 MPa to 1.2 GPa), and temperature T (150 − 373 K). Simulation results are consistent with a broad range of equilibrium and nonequilibrium experiments. At high T and low P , where ηN is low, the response can be fit to a power-law, as in the common Carreau model. Shear-thinning becomes steeper as ηN increases, and for ηN 1 Pa-s, shear-thinning is consistent with the thermally activated flow assumed by another common model, Eyring theory. Simulations for a bi-disperse Lennard-Jones (LJ) system show that the transition from Carreau to Eyring is generic. For both squalane and the LJ system, the viscosity decreases by only about a decade in the Carreau regime, but may fall by many orders of magnitude in the Eyring regime. Shear thinning is often assumed to reflect changing molecular alignment, but the alignment of squalane molecules saturates after the viscosity has dropped by only about a factor of three. In contrast, thermal activation describes shear thinning by six or more decades in viscosity. Changes in the diagonal elements of the stress tensor with rate and shear stress are also studied. * vjadhao@iu.edu † mr@jhu.edu [6,7]. A second reason is that EHL conditions present fundamental theoretical challenges. They are in the regime where fluids exhibit poorly understood glassy rheology, with rapid rises in η N with pressure, strongly nonlinear viscoelastic behavior, rapid shear thinning, and changes in molecular order. While there is a general agreement on the qualitative changes in shear stress σ, the underlying mechanisms of shear thinning and the functional form of the rheological response are hotly debated [4,6,7].Two phenomenological models at the center of this active debate are based on work by Eyring [8,9] and Carreau [10], respectively. The Eyring model assumes that flow occurs by thermal activation over a single characteristic energy barrier height and predicts that σ rises as log(γ) at highγ. In the Carreau model, σ increases as a power law, σ ∝γ n , with n typically around 0.5. As discussed below, this power law behavior can arise from several different physical pictures of flow, including thermal activation over a broad range of energy barrier heights.Both of these rheological models can be fit to existing rheometer data on simple fluids that extends up to rates of about 10 4 s −1 . The two models predict dramatically d...

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Section: B Shear Thinning and Time-temperature Superposition In Squasupporting

confidence: 78%

“…As noted in Ref. [18], η N is often fit to an Arrhenius form assuming a single energy barrier at high T where η N is low. Our results for the nonlinear response show that a single barrier is not appropriate in this limit and should be more appropriate at low T .…”

Section: Discussionmentioning

confidence: 99%

Rheological Properties of Liquids Under Conditions of Elastohydrodynamic Lubrication

Jadhao

Robbins

2019

Tribol Lett

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“…Such simulations confirm the logarithmic dependence of shear stress on shear rate predicted by Eyring at high shear stress for most of the liquids studied and also, in some cases, the whole sinh −1 transition from linear Newtonian dependence at low shear rate to logarithmic dependence at high shear rate [68−70]. Figure 6 shows the predicted stress-strain rate behaviour of squalane, which matches the Eyring model over an extremely wide shear rate range [70]. 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].…”

Section: Ehd Frictionsupporting

confidence: 77%

Stress-augmented thermal activation: Tribology feels the force

2018

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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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“…Recently, Jadhao and Robbins [131] used NEMD simulations to determine the nonequilibrium shear response of squalane over a wide range of pressures and temperatures using a UA force-field [117]. The viscosity-shear rate behaviour from the NEMD simulations agreed well with experimental data conducted at lower shear rates (10 2 −10 4 s −1 ).…”

Section: Shear Thinningmentioning

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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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Probing large viscosities in glass-formers with nonequilibrium simulations

Cited by 69 publications

References 44 publications

Rheological Properties of Liquids Under Conditions of Elastohydrodynamic Lubrication

Rheological Properties of Liquids Under Conditions of Elastohydrodynamic Lubrication

Stress-augmented thermal activation: Tribology feels the force

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

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