In elastohydrodynamic lubrication (EHL), one generally imposes force balance, i.e., the contact force resulting from the pressure in the contact is equal to the applied load. When studying the effect of structural vibrations, this force balance equation obviously does not hold and the more general equation of motion is required. In Wijnant and Venner (1996), an EHL contact model was introduced that incorporates both squeeze and entraining motion as well as the equation of motion. It was shown numerically that due to a small initial deviation or initial velocity, the rolling element starts an oscillatory motion around the equilibrium position. This motion is slightly damped because of the viscous losses in the lubricant. Moreover, it was shown that these oscillations cause film thickness modulations with a wavelength, directly related to the dimensionless frequency Ω. This paper compares results from experiments that were carried out on a ball and disk apparatus with results obtained with the EHL contact model. In this experiment, the applied load was rapidly increased by impacting a wedge between the base and the ball holder. This results in an increase of the contact area and, as a result of inertia forces of the ball, disk and supports, and oscillatory motion of the contacting bodies. Modulations in the film thickness which result from these oscillations, are clearly visible. The contact model was tailored to this experiment and a qualitatively close agreement has been found.

Using optical interferometry in a standard ball-and-disc apparatus and a high-speed video camera with light enhancer, thickener particles entering a grease-lubricated elastohydrodynamic (EHD) contact were traced as they passed through the contact in pure rolling. In previous studies, it has not been possible to track specific particles but, due to the short exposure time and high recording rates used here, sharp images of a passing particle could be captured. Two greases, based on the same synthetic poly(á-olefin) but thickened with Li-12-OH and lithium complex respectively were studied and for a freshly lubricated track it could be documented that the lithium complex thickener causes fewer, but larger, particles to pass the contact. The influence of the base oil bleeding on film formation is also discussed in the paper.

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The sign and magnitude of the ζ potential of mineral oxides in allegedly pure organic solvents (high-quality chemical reagents) depends on the concentration of acidic and basic impurities, chiefly organic acids and amines. "Zetametric titration" with HClO(4) makes it possible to estimate the concentration of basic impurities and offers an attractive alternative to other analytical methods. Titanium(IV) oxide has been chosen as the probe powder in view of its very low solubility and nearly neutral point of zero charge in aqueous systems. Some systems have a high buffer capacity and addition of acid and base has a minor effect on the ζ potential of titania. Specific adsorption of alkali metal cations on anatase can be also a significant problem.

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