The interaction between a human finger-pad and a physical surface generates not only the tangential friction needed for gripping objects but also a wide variety of perceptual experiences. Finger-surface contact behavior is known to depend on the actual contact area, the normal force with which the finger and the surface are pressed together, and the presence of liquids (such as sweat) at the interface; however, the relative contributions of these phenomena are not fully understood, nor are their interactions.
Let's think about a situation in which you are trying to pull a plastic credit card from your wallet. When your finger is dry, you may want to press on the card more to increase frictional force and prevent slip. On the other hand, less pressing force is required if your fingerpad is moist, as this moisture creates additional friction.
To investigate this problem, we are preparing a system that can simultaneously measure all three quantites (contact area, force and moisture) over time with high resolution and frame rate. We plan to conduct experiments wherein a human subject presses his or her finger on the transparent surface at the middle of the apparatus. A light illuminates the contact from below the surface, so that a camera located there can record the contact area. A force/torque sensor mounted above the surface measures the normal and tangential forces exerted by the subject. The quantity of liquid on the fingerpad is measured by a custom transparent moisture sensor that is embedded in the transparent surface at the point of fingertip contact.
By analyzing the relative contributions of the three factors, we aim to deepen our understanding of the physical phenomena that affect human manipulation and perception through the hands.