The effect of normal force and roughness on friction in human finger contact

The relationship between normal force and friction force for a finger contacting a surface was examined, and also the effect of surface roughness, using a finger friction rig. To investigate the normal force–friction force relationship 12 materials were used: 3 metals, 5 plastics and 4 elastomers. Forty tests were carried out across a range of loads, for each material. The roughness tests were run on three metals (brass, steel and aluminium), with a range of surface roughness. The deformation of the finger was also monitored in the normal force tests using a high-speed video camera. The finger tested, reached a maximum area of contact at a low load: 1 N. Therefore, for forces above 1 N, the main friction mechanism was adhesion. The plateau in contact area and dominance of adhesion resulted in a linear relationship between normal force and friction force, for all loads above 1 N. The roughness tests showed that at a low surface roughness the finger dominates the interaction, so little difference is seen between materials and roughness. However, as the roughness increases the friction increases linearly, up to a point. At this point the friction does not vary as the width and height of the surface ridges are increased.