Friction and lubrication of human skin

The in vivo friction of human skin has been measured in the dry, wet and damp states using smooth glass and polypropylene spherically tipped probes. They were selected to be representative of hydrophilic and hydrophobic countersurfaces. The data are interpreted using the adhesion model of friction, which provides an explanation for the influence of the normal load and the surface free energies of the probe materials on the frictional characteristics of the skin. In particular, explanations based on this model are given for the tendency of wet skin to exhibit stick-slip motion and of damp skin to exhibit a peak frictional force against a glass probe.