Understanding the friction between human fingers and contacting surfaces

Friction tests were carried out to assess the friction between a human finger and contacting surface, in different conditions. Tests examined the effect of normal force, the area of contact, the effect of triangular and rectangular cross-section ridges and the effect of moisture. The tests found that when a finger is contacting a nominally flat surface, the friction force increases with normal force, following a two part linear relationship. This is associated with a large initial deformation of the finger, followed by a smaller scale deformation, after a certain load. The introduction of water to the contact results in an initial increase, which is followed by a decrease, in friction. There are two principal mechanisms responsible for this increase; water absorption to the stratum corneum, and capillary adhesion. These mechanisms increase friction by increasing the area of contact, and therefore the amount of adhesion. When the finger is contacting a ridged surface, triangular ridges display a higher friction force than rectangular ridges. This is thought to be due to the larger penetration depth that is possible with triangular ridges. The main mechanisms of friction for the triangular ridges are adhesion and interlocking friction. The main mechanisms of friction for the larger, rectangular ridged surfaces are adhesion, ploughing friction and a reduction in friction force due to an energy return from the finger forming back to its original shape. These tests showed that for a large friction force, surfaces should have high, narrow and widely spaced ridges. This, however, is at the expense of consistent friction across the surface. The understanding gained was then applied to the area of rugby ball design. Tests showed that the existing rugby ball surface designs with the highest friction were ones with pyramid pimples. However, rounded pimpled surfaces performed more consistently across all test conditions.