Understanding the variation of friction coefficients of human skin as a function of skin hydration and interfacial water films

Human skin is characterised by a complex and highly variable friction behaviour. Although the variation of friction coefficients measured for skin depends on numerous parameters related to the skin itself, the surface in contact as well as contact conditions, water or sweat – either bound in the stratum corneum and manifest as skin hydration or in form of liquid films lubricating the interface – is the most important factor. Here, we analyse the variation of previous experimental data on the basis of the adhesion friction model and show how lower and upper bounds, i.e. envelope functions, can be derived for measured skin-friction coefficients. From these envelope functions, essential tribological parameters such as the interfacial shear strength and the real contact area of skin are estimated.