The presence of a liquid can prevent moving surfaces from coming into intimate contact. With smooth-surfaced rubber, it is possible to examine by optical interferometry the shape and thickness of liquid films between the surfaces. Furthermore, in water lubrication, if the acidity or alkalinity of the water is altered, laboratory friction measurements reveal the impact on its lubricity. Extensive testing shows that some rubber materials are more sensitive to water pH than others, and air aging has an influence on the results. In some circumstances the level of wet friction can decrease by two orders of magnitude-a surprising result by normal lubrication experience. These water pH results are described in some detail and mechanistic explanations are offered. The overall conclusion is that pH and the state of oxidation of a rubber surface can have a marked effect on its water-lubricated friction.
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