In situ confocal Raman spectroscopy of lubricants in a soft elastohydrodynamic tribological contact

Abstract

This paper describes an experimental setup that combines friction measurements with in situ confocal Raman microscopy on a lubricating fluid film. The setup allows the determination of film thickness and shear-rate profiles, the position-dependent lubricant composition within the rubbing contact, as well as optical imaging of the contact and its surroundings. The tribological contact can be investigated at continuous rubbing conditions over a range of sliding speeds from 1×10−3to2m∕s. The setup’s capabilities are demonstrated for a soft poly(dimethylsiloxane) sphere pressed and rotated against a quartz flat. Friction forces are presented in the boundary, mixed, and elastohydrodynamic lubrication (EHL) regimes for both a hydrophilic and a hydrophobic flat surface. Optical images of the contact show that, for single-phase aqueous Newtonian lubricants in the EHL regime, starvation of the contact occurs above a critical value of the product of the entrainment speed and the lubricant viscosity. Starvation results in a lower film thickness and lower friction values than predicted by numerical calculations that do not take starvation into account. The capability to determine, using Raman spectroscopy, the composition of the lubricant within the contact is demonstrated for oil-in-water emulsions, stabilized by a nonionic surfactant (Tween 60). The oil concentration was found to depend on both the sliding speed and the position within the rubbing contact. For the investigated tribological conditions, the oil content within the contact was lower than that of the bulk emulsion. This is attributed to the fact that the aqueous matrix phase preferentially wets the hydrophilic quartz slide, which prevents oil droplets from entering the contact if the film thickness is of the order of the droplet size. The ability to determine friction forces, lubricant composition and film thickness, and shear-rate profiles will provide invaluable information in the pursuit of a better understanding of lubrication of soft and hard contacts by multiphase complex fluids.

Laser-induced fluorescence for film thickness mapping in pure sliding lubricated, compliant, contacts

“…However using a value of η = 0.15 Pas a good agreement is achieved. Similar work carried out by the authors using optical interferometry [28] and work carried out by Bongaerts et al [29] using Raman spectroscopy to measure film thickness in compliant contacts, Bongaerts et al [29] suggested that the discrepancy between the measured and lower operating viscosity might be due to either an increase in lubricant temperature or to the glycerol adsorbing excess water from the atmosphere. One possibility is heating by the illuminating laser light.…”

“…Bongaerts et al [29] successfully demonstrated the use of confocal Raman spectroscopy to measure lubricant film thickness within a compliant, tribological contact lubricated with simple Newtonian fluids and stabilized oil/water emulsions.…”

“…Far fewer are suited to measure film thickness in contacts where one or both surfaces are an elastomer or tissue material. However, optical interferometry [25][26][27][28], magnetic resistance [5] and Raman spectroscopy [29] have been used successfully to measure lubricant film thickness within such contacts.…”

Optical Characterization of Elastohydrodynamic Lubrication Pressure with Surface Plasmon Resonance

“…In addition to optical interferometry, other optical imaging techniques have been reported for the studies of EHL contacts in recent years. Bongaerts et al [35] demonstrated the application of in situ confocal Raman microscopy for friction measurements in EHL contacts. Optical imaging on the EHL contact and the determination of film thickness has been demonstrated.…”

Effects of Surface Chemical Properties on the Frictional Properties of Self-Assembled Monolayers Lubricated with Oleic Acid

“…Infrared (IR) absorption spectroscopy [2][3][4][5][6][7][8][9][10][11][12][13][14][15] and Raman spectroscopy [16][17][18] are particularly attractive tools for studying buried interfaces nondestructively. There are many practical and interpretational problems associated with the application of Raman and IR spectroscopy to in-contact studies.…”