On the Velocity Profile of Couette Flow of Lubricant Within a Micro/Submicro Gap

Abstract

The hydrodynamic lubrication properties of sliding bearings are influenced by the velocity profile under shear. However, at present, there are very few reports on the velocity profile distribution of the fluid film lubrication in conformal contacts. In this article, an apparatus was built for in situ measurement of the velocity profile of thin oil film under a micro/submicro gap at ambient pressure using a photobleached imaging technique and imaging the shape evolution of the bleached area under shear. The film thickness is calibrated by interferometry. The results show that the velocity profile of oligomer PB450 doped with a fluorescent agent is a typical linear distribution under higher film thickness, which is in accordance with the classical lubrication theory. When the gap of the disk and slider is less than 2 μm, there are obvious partial inhomogeneous shear flows at ambient pressure, and the slip length increases with the decreasing film thickness. Lubricants of different viscosities and molecular structures show an inhomogeneous flow transition at different confinements. Correspondingly, abnormal velocity profile results combined with generalized Reynolds equation could explain the phenomenon that the convergence ratio of the maximum load-carrying capacity is larger than 1.2 experiencing hydrodynamic lubrication. Hence, this work contributes to an improved understanding of rheology as well as more accurate predictions of tribological properties.