Chapter 2. Prehension

Chapter 6 - During Contact 213
the hysteresis component. The adhesion component is a molecular
surface effect whereas the hysteresis component is a bulk or
macroscopic phenomenon. Both are dependent on the viscoelastic
properies of the skin. Of these, adhesion is usually the larger and
more important in elastomers; it can be reduced by a lubricant between
the two rough surfaces in relative motion (Moore, 1972). “Indeed, if
the scale of the macro-roughness is progressively diminished to a
value less than the micro-roughness (perhaps approaching molecular
dimensions), then it is clear that the hysteresis component of friction
has been replaced at some stage by adhesional friction” (Moore, 1972,
p. 234). The average molecular jump distance typical for rubber
adhesion is about 100 Angstrom units, whereas the mean wavelength
of the macro-roughness which determines the hysteresis component is
several millimetres. With the height of papillary ridges, the volar skin
of the hand lies somewhere in between, perhaps in the region of
micro-roughness. Simulating use of a keyboard, some researchers
have concluded that tactile friction in the finger pads is predominantly
adhesive (Dinc, Ettles, Calabrese and Scarton, 1990).
Moore (1972) recommends that five distinct parameters are
necessary to represent surface texture. These include factors for
macro-roughness (characteristics of typical average asperities, like
size, spacing and shape), as well as micro-roughness at asperity peaks
and height distribution of asperities. For the visible papillary ridges of
human palmar and finger skin, not all these parameters are available.
Also, there are large individual differences in the characteristics of the
hand skin. All of these factors need to be carefully investigated.
The result of high frictional forces is wear and abrasion. Nature
is kind to replace our outer skin surfaces every 12-30 days. The
generation of frictional forces between two contacting surfaces
dissipates energy, giving rise to heating effects at the interface. The
extent and implications of temperature rise have not been well
documented for hands grasping various objects, though we consider
this in the next section. A formal analysis of skin surfaces of the hand
during prehension, from a tribological9 perspective would be a
valuable contribution to our understanding of human grasping. Both
simulations and experiments are needed in this area.
We have considered the the structure of hand skin, and
mechanical implications for grasping. Pubols (1988) suggests that
many “higher order” neural and perceptual processes may be due, at
least partially, to mechanical properties of the skin which provide a
9Tribology is the study of friction.