Dry and lubricated sliding friction for rubber on concrete: the role of surface energies.

We study the influence of lubricant fluids (water-glycerol mixtures) on rubber sliding friction for two different rubber tread compounds on a concrete surface. We find that for the lubricated contacts the sliding friction below a critical velocity v c is similar to that of the dry contact, but for v > v c the friction drops fast with increasing sliding speed. We discuss the origin of this effect and show that it is not a "normal" mixed lubrication effect but depends on surface (or interfacial) energies.