Boundary friction

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Boundary friction occurs when a surface is partially wet but not so lubricated that there is no direct rubbing between surfaces. If two dry, smooth surfaces slide past each other, there’s a predictable amount of friction. As speed increases, this friction tends to rise up to a limit, following the Stribeck curve. If the surfaces are completely lubricated, there’s almost no direct friction at all. In real life, most contacts are somewhere in between: not completely dry, but not fully lubricated. This boundary friction can cause effects such as changes in lubrication due to shearing forces, or small vibrations as the friction level rises and falls during motion. For example, you may feel a vibration when braking on a damp road, or a cold glass with moisture might stick and then suddenly slide across the surface.