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.