Oil-in-water emulsions are widely used as lubricants in metal forming, machining and some machinery elements where non-flammable lubricants are required. Their lubricating mechanisms have been extensively investigated based on the measurements of film thickness and/or traction in the past few decades and a number of physical explanations for their performance have been forwarded. However, direct observation of the emulsion flow, as a direct method of evaluating suggested theoretical explanations, has been greatly restricted by the available instruments. In this paper, a newly devised digital video camera and microscope were used to directly observe the emulsion flow in EHL contacts at industrially relevant speeds for both line and point contacts. Previous low-speed results for line contact were confirmed and extended into high-speed cases. That is, some droplets were rejected from the inlet, others penetrated to the contact zone, and others remained fixed in position a certain distance from the edge of contact. For point contact, side flow behavior was also observed, and the number of droplets that remained stationary were limited to a single streamline. To clarify the oil droplet behavior and investigate the effect of particle size on entrainment, three tight emulsions with different mean droplet sizes were examined on an EHD rig at speeds from 12 mm/s up to 1.5 m/s.

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