Effect of finger orientation on contact stiffness and area during sliding on a smooth surface

Earlier experimental studies showed that the apparent contact area of human fingerpad shrinks and eventually reaches a steady-state value as it slides on a smooth surface, although the root causes of this reduction have not been fully understood yet. We hypothesize that finger rotation about its axial axis and the movement direction play critical roles in the area change. To test this hypothesis, we conducted an experimental study to investigate the evolution of apparent contact area between human fingerpad and a smooth flat surface under normal loading (stationary finger) and combined loading (sliding finger) conditions for 4 different internal rotations of the index finger (away from the second finger) about its axial (longitudinal) axis and 2 different sliding directions. Our results show a reduction in the contact area for radial sliding as expected, but a surprising increase in the ulnar direction for the higher finger rotations. We argue that this asymmetric behavior in contact area evolution stems from the changes in the equivalent radius of curvature and stiffening of the finger as the rotation angle increases, which manifests itself as the asymmetric stress distribution at the leading and trailing edges of the fingerpad in our finite element simulations.