Lubricated Systems

Abstract

In order to reduce frictional forces and wear, lubrication has been used for thousands of years. It inhibits direct contact between two bodies and, thereby, replaces dry friction with fluid friction. The presence of liquid between two bodies influences not only the tangential, but also the normal forces: two dry sheets of glass can effortlessly be taken apart from one another, while in order to take two wet sheets apart one needs a considerable force. This phenomenon can be attributed, on the one hand, to capillary forces, and on the other, to pure hydrodynamic nature: a viscous liquid requires a given time in order to flow in the small space between two sheets. During dynamic loading, this phenomenon leads to an apparent “adhesion” between two lubricated bodies, which we describe as “viscous adhesion.”