Role of Occlusion in Non-Coulombic Slip of the Finger Pad

Understanding how fingers slip on surfaces is essential for elucidating the mechanisms of haptic perception. This paper describes an investigation of the relationship between occlusion and the non-Coulombic slip of the finger pad, which results in the frictional force being a power law function of the normal load, with an index \( n \); Coulombic slip corresponds to \( n = 1 \). For smooth impermeable surfaces, occlusion of moisture excreted by the sweat glands may cause up to an order of magnitude increase in the coefficient of friction with a characteristic time of ~20 s. This arises because the moisture plasticises the asperities on the finger print ridges resulting in an increase in their compliance and hence an increase in the contact area. Under such steady state sliding conditions a finger pad behaves like a Hertzian contact decorated with the valleys between the finger print ridges, which only act to reduce the true but not the nominal contact area. In the limit, at long occlusion times (~50 s), it can be shown that the power law index tends to a value in the range \( {2 \mathord{\left/ {\vphantom {2 {3 \le n \le 1}}} \right. \kern-0pt} {3 \le n \le 1}} \). In contrast, measurements against a rough surface demonstrate that the friction is not affected by occlusion and that a finger pad exhibits Coulombic slip.