Abstract
Lubricant viscosity is a key driver in both the tribological performance and energy efficiency of a lubricated contact. Elastohydrodynamic (EHD) lubrication produces very high pressures and shear rates, conditions hard to replicate using conventional rheometry. In situ rheological measurements within a typical contact are therefore important to investigate how a fluid behaves under such conditions. Molecular rotors provide such an opportunity to extract the local viscosity of a fluid under EHD lubrication. The validity of such an application is shown by comparing local viscosity measurements obtained using molecular rotors and fluorescence lifetime measurements, in a model EHD lubricant, with reference measurements using conventional rheometry techniques. The appropriateness of standard methods used in tribology for high-pressure rheometry (combining friction and film thickness measurements) has been verified when the flow of EHD lubricant is homogeneous and linear. A simple procedure for calibrating the fluorescence lifetime of molecular rotors at elevated pressure for viscosity measurements is proposed.
Highlights
Friction in fluid film lubricated contacts is heavily influenced by the viscosity of the lubricant
3.1 Viscosity from Fluorescence Lifetime Measurements Compared with High-Pressure Rheology
The relationship between local IGEPAL viscosity and local pressure can be described by a single exponential fit within the range studied
Summary
Friction in fluid film lubricated contacts is heavily influenced by the viscosity of the lubricant. The lubrication regime is characterised by nearly parallel, elastically deformed surfaces and is dominated by the pressure–viscosity response of the lubricant This lubrication regime is known as elastohydrodynamic (EHD) lubrication. Conventional techniques that estimate lubricant viscosity under conditions representative of EHD lubrication conditions include friction measurements [2] and highpressure rheometry [3, 4]. When using friction measurements to determine the lubricant viscosity in a ball on flat contact, the lubricant is entrained and sheared to produce conditions close to those typically found, in engineering contacts. During these tests, the ball is halfsubmerged in an oil bath [5].
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