Through-Thickness Velocity Profile Measurements in an Elastohydrodynamic Contact

Abstract

7594 8991This work presents for the first time through-thickness velocity profiles obtained in an EHL contact by photobleached imaging. The velocity profile was inferred by following the evolution of the shape of a photobleached plug formed through the thickness of the fluorescently-doped lubricant, oligomer polybutene (PB), in the contact when shear was applied. The proposed methodology was validated by successfully obtaining the expected linear profile with PB experiencing Couette flow. The methodology was then applied to PB in an EHL contact. The variation of the profiles within the contact area was also investigated.The velocity profile of PB in an EHL contact severely deviates from the common linear assumption and exhibits inhomogeneous shear: three regions of varying shear rate have been observed. The phenomenon is shown to be neither due to thermal nor diffusion effects. PB also shows significant slip at the glass-liquid interface. The amount of slip varies with position in the contact. Possible causes, such as pressure induced viscosity enhancement, as well as the significance of the findings and the benefits of the technique are discussed.The linear velocity profile in an EHL contact is usually assumed for both the film thickness and friction predictions. The profile has however never been measured experimentally until now. This work enables the validation of conventional assumptions and the study of flow heterogeneity of lubricants in a contact. This facilitates an improved understanding of the rheology of confined lubricant and hence more accurate predictions of tribological properties.

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

“…There have been previous NEMD, 74 and experimental investigations, 75 ('shear banding') in liquid systems composed of more structured molecules are known to occur for more normal lubricant liquid like systems, [34][35][36]85,86 so this feature appears to be generic to the liquid state under these conditions.…”

“…27,31,32 Some of the previously observed NEMD steady states have been detected experimentally. For example, Plug-Slip, 34,35 and Central Localization, 36 steady states have been detected using a novel methodology based on phosphorescence imaging. Martinie and Vergne, 37 also gave evidence that the LJ shear localised states are present in real lubricants under traction experiment conditions despite the simplicity of the computer model.…”

Wall slip in an EHL contact lubricated with 1-dodecanol

“…Some evidence to support was provided by Bair et al [23], albeit for considerably thickener films, which showed oil samples within a high pressure rheometer that fractured under high pressure forming slip bands [23]. Recently, important work by Ponjavic et al [24], using fluorescence microscopy to obtain through-thickness velocity profiles, has shown that oligomer polybutene exhibits wall-slip under EHL conditions [24]. In this case, no dimple was formed.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…In the field of tribology, shear localisation has only been directly observed experimentally in viscous model lubricants, such as polybutadiene and polyphenyl ethers (e.g., 5P4E) [196][197][198][199][200][201]. However, it has also been inferred from film thickness measurements in realistic mineral oils [202,203] and there is currently a growing consensus that this phenomenon may also be important for more realistic lubricants under extreme EHL conditions [185].…”