This paper presents a set of experimental results of the EHL film thickness with oil-in-water (O/W) emulsions in a wide range of rolling speed for different oil concentrations and pH values. The O/W emulsions have wide applications in metal-forming and machining processes as well as hydraulic systems. However, their lubrication mechanisms are very complex and have not been fully understood. A newly developed high speed optical EHL rig was used to measure the film thickness and observe the two-phase flow around the EHL point and line contacts. Experimental observations indicate that phase inversion/oil pool formation mechanism around the inlet zone takes place only at very low speeds, which are most likely far below practical speed ranges for major industrial applications. When the speed is low, the lubricant film thickness is dominated by the bulk properties of oil phase, and can be estimated by the conventional EHL theory together with the consideration of starvation effect. After the speed exceeds a certain limit, called first critical speed, there is a transition region, where no stable oil pool is observed and the film thickness starts to decrease, or increases slightly then decreases. It is believed that in this transition region there is still a considerable amount of oil concentrated in the inlet zone, and this local oil concentration decreases as the speed increases. The film thickness appears to be dominated by the entrainment of oil-enriched two-phase lubricant in the inlet zone. The increase of film thickness is due to entraining effect and the decrease due to the increased oil phase starvation. If the speed is further increased exceeding a second critical speed, the film thickness will stop decreasing and start to increase again. In this high speed region the local oil concentration of entrained lubricant in the inlet zone is believed to become quite constant and close to that of the bulk lubricant supply. The film thickness, therefore, continuously increases for all of the tested line and point contact cases as the speed goes up, and is always significantly smaller than that of neat oil but larger than that of pure water. The destabilized emulsions with lower pH values can form more stable oil pools and considerably thicker films. This is because the oil droplets in these low pH emulsions can be more easily trapped and brought into the contact by the solid surfaces. However, for the tested emulsions, the oil pools still cannot survive reasonably high speeds.

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