A systems based experimental approach to tactile friction.

This work focuses on the friction in contacts where the human finger pad is one of the interacting surfaces. This 'tactile friction' requires a full understanding of the contact mechanics and the behaviour of human skin. The coefficient of friction cannot be considered as a property of the skin alone, but depends on the entire tribo-system. In this work, frictional forces were measured using a commercially available load cell. Parameters such as the hydration of the skin, the normal load on the contact and the roughness of the contacting surfaces were varied, whilst keeping the other parameters constant. The tests were performed under controlled environmental conditions. The total friction force is a combination of forces related to adhesion and to deformation. A commonly made assumption is that, to describe the friction of human skin, the deformation component can be ignored and only the adhesive behaviour has to be taken into account. However, in this study it was found that the forces related to the (micro-scale) deformation of skin can have a significant contribution to the total friction force; this is valid both for dry conditions and in the presence of water, when hydration of the skin causes softening.