This paper describes a study of the elastohydrodynamic and boundary film-forming properties of solutions of simple surfactants in water. Such solutions are finding increasing use both as highly fire-resistant and environmentally-friendly hydraulic fluids. They also provide a vehicle to help clarify the origins of the film-forming behavior of oil-in-water emulsions. <br/>Film formation was found to be dependent upon the type and concentration of surfactant and also on the pH of the solution. Two different types of behavior were seen. At slow speeds, mono-layer-type boundary films were observed while at higher speeds, speed-dependent elastohydrodynamic-type films were produced. The latter were formed at entrainment velocities much lower than might be predicted from the viscometric properties of the bulk surfactant solutions and were similar to films formed by some oil-in-water emulsions at high speed. They show that water is entrained into high pressure, rolling contacts in accord to the predictions of elastic isoviscous theory
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