The outer layer of human finger pads is stiff and rough at a small scale, but deforms under pressure to form stable contacts with surfaces.

The researchers used high-resolution imaging to monitor contact formation between two participants’ fingers and glass or rubber surfaces. For a hard glass surface, the area of the finger in contact with the surface was initially small and increased gradually over a period of tens of seconds. The coefficient of friction between the finger and the surface increased on a similar time scale.

For a soft rubber surface, both the contact area and the coefficient of friction reached their maximum values within 2 seconds.

Contact formation with a hard surface requires hydration of the keratin in the outer skin layer by sweat, which increases skin plasticity. Therefore, the slow contact formation observed with the glass surface reflects the rate of sweat secretion from the pores, whereas contact formation with a soft surface is not limited by hydration, according to the authors.

According to the authors, differences in the rate of contact formation might help people to distinguish different types of materials by touch and may have implications for designing tactile displays.

http://www.pnas.org/content/early/2017/09/19/1706233114