New EHL Modeling Data for the Reference Liquids Squalane and Squalane Plus Polyisoprene

Bair, Scott; Andersson, Ove; Qureshi, Farrukh; Schirru, Michele

doi:10.1080/10402004.2017.1310339

Cited by 26 publications

(31 citation statements)

References 48 publications

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“…The transition viscosity varies from 2.30 × 10 7 Pa·s to 15.6 × 10 7 Pa·s with the arithmetic average being 6.2 × 10 7 Pa·s and the geometric mean being 4.7 × 10 7 Pa·s. This is in the same range as for the EHL reference liquid squalane [10]. The glass transition viscosity is much less than the "universal value" of 10 12 Pa·s.…”

Section: Viscosity At the Glass Transitionsupporting

confidence: 51%

“…However, the situation for organic liquids is less clear [7,8]. Here, for one liquid lubricant, the glass transition by dilatometry combined with falling cylinder viscometry places the viscosity at the transition at between 10 7 Pa·s and 10 8 Pa·s, in the same range as squalane [10].…”

Section: Resultsmentioning

confidence: 99%

“…The glass transition is a phenomenon occurring at a specific value of viscosity for a given liquid. It seems well-established that for the much studied inorganic liquids (mineral melts) the glass transition viscosity is of the order of 10 12 Pa·s, as in Ref. [22] for example.…”

Section: Resultsmentioning

confidence: 99%

“…Schweyer [8] found  g = 10 8 Pa·s for asphalts under pressure. Measurements at ambient pressure of viscosity up to 3.7 × 10 8 Pa·s from the University of Chicago [9] compared with glass transition measurements by transient hot-wire at Umea University [10] give the viscosity at the transition to be  g = 1.23 × 10 7 Pa·s for squalane when the observation time is 0.3 seconds.…”

Section: Introductionmentioning

confidence: 99%

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The viscosity at the glass transition of a liquid lubricant

Bair

2018

Friction

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Abstract:In the classical study of elastohydrodynamic lubrication (EHL) which does not employ real, measurable viscosity in analysis, the possibility of a glass transition has not been considered in many years. Indeed, the two rheological assumptions of classical EHL, the Newtonian inlet and the equivalence of a traction curve to a flow curve, would not have persisted so long had the pressure dependence of the viscosity been accurately stated. With the recent appearance of viscosity obtained from viscometers in EHL analysis, the possibility of a glass transition in the contact should be reexamined, especially for the fragile traction fluids. This article employs published data for a synthetic cycloaliphatic hydrocarbon to estimate the glass transition viscosity so that, when using real viscosities in EHL simulations, the state of the liquid may be assessed. Far into the glassy state the liquid should be treated as an elastic solid with a yield stress.

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Section: Viscosity At the Glass Transitionsupporting

confidence: 51%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The viscosity at the glass transition of a liquid lubricant

Bair

2018

Friction

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“…If such a crossover existed in the isobaric data, it should also occur in the isothermal, pressure-dependent data (9) for squalane at the same viscosity of 1,000 Pa•s. It clearly does not (10).…”

mentioning

confidence: 95%

Purported fragile-to-Arrhenius crossover in squalane

Bair¹

2017

Proc. Natl. Acad. Sci. U.S.A.

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Jadhao and Robbins (1) claim to have discovered a crossover from Vogel-Fulcher-Tammann (VFT) to Arrhenius in squalane occurring at a viscosity of μ = 1,000 Pa·s using extrapolation of the simple Eyring equation to the limiting low-shear viscosity from nonequilibrium molecular dynamic simulation at high shear rates. Stickel et al. (2) introduced a non-model-specific derivative analysis of relaxation time and viscosity data. The Stickel function iswhere x is a dynamic property of the liquid, low-shear viscosity, μ, or relaxation time, λ. A reference was offered by Jadhao and Robbins (1) as support for a VFT-to-Arrhenius crossover. This is the article by Mallamace et al. (5), which has been refuted (3, 6, 7). Mallamace et al. (8) responded to the criticism voiced by Elmatad (6). If such a crossover existed in the isobaric data, it should also occur in the isothermal, pressure-dependent data (9) for squalane at the same viscosity of 1,000 Pa·s. It clearly does not (10).

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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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New EHL Modeling Data for the Reference Liquids Squalane and Squalane Plus Polyisoprene

Cited by 26 publications

References 48 publications

The viscosity at the glass transition of a liquid lubricant

The viscosity at the glass transition of a liquid lubricant

Purported fragile-to-Arrhenius crossover in squalane

Rheological Properties of Liquids Under Conditions of Elastohydrodynamic Lubrication

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