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384 Biomimetics: Biologically Inspired Technologies

layer deforms and changes the local shape of the skin surface due to a shift of the damping fluid
located under the rubber-like layer. Such a material design seems to be able to damp turbulences.
Many fish have developed another mechanism to reduce friction (by up to 60% in some species)
in the boundary-layer. They produce skin secretions, which are usually slightly soluble in water. In
areas, where microturbulence is strong, these substances can be locally dissolved. This results in the
damping of microturbulence (Nachtigall, 1977). Due to skin secretions, fish can reach an extremely
high speed in a short time.
In aquatic vertebrates, skin, specialized for increased friction, often contains patterns with
microridges and micro-outgrowths (Fahrenbach and Knutson, 1975). Friction in the boundary-
layer of the body moving in the medium at high Reynolds numbers may be decreased due to such a
sculpturing of the surface. The grooved scales of the shark skin is an example of such a system.
Their size ranges from 200 to 500 mm. The surface of each scale contains parallel grooves between
so-called riblets directed almost parallel to the longitudinal body axis. Interestingly, grooves and
riblets of neighboring scales correspond exactly to each other so that the shark surface looks like a
pattern of parallel stripes (Reif and Dinkelacker, 1982). Experiments on flow resistance have been
carried out with smooth-bodied models and with those covered with grooves of dimensions similar
to original shark skin. The flow resistance measured in the grooved model was about 5 to 10% lower
6
than the resistance in the smooth model at a Reynolds number of 1.5 10 . The geometry of the
grooves and riblets can also influence results. Small bristles, scales, and microtrichia of the wings of
flying insects (Bocharova-Messner and Dmitriev, 1984; D’Andrea and Carfi, 1988) have similar
function (Figure 15.3). The microturbulences, generated around such structures in flight, presum-
ably build a kind of a lubricating layer of air between an air stream and the insect surface. This can
possibly decrease friction during high-speed flight. A foil covered by tiny riblet-like structures,
inspired by biological surfaces, has been suggested for aeroplane surfaces (Bechert et al., 2000).
Terrestrial animals, such as snakes, must overcome problems related to friction in contact with
solid or friable media. Friction-modifying nanostructures of the scaly surface of snakes have
recently been described (Hazel et al., 1999). These include an ordered microstructure array (Figure
15.4), presumably to achieve adaptable friction characteristics. Significant reduction of adhesive
forces in the contact areas caused by the double-ridge microfibrillar geometry provides ideal
conditions for sliding in a forward direction with minimum adhesive forces. Low surface adhesion
in these local contact points may reduce local wear and skin contamination by environmental
debris. The highly asymmetric profile of the microfibrillar ending with a radius of curvature of 20 to
40 nm may induce friction anisotropy along the longitudinal body axis and functions as a kind of
stopper for backward motion, while providing low friction for forward motion. Additionally, the
system of micropores penetrating the snakeskin may serve as a delivery system for a lubrication or
anti-adhesive lipid mixture that provides boundary lubrication of the skin.

15.3 ATTACHMENT SYSTEMS

Materials and systems preventing the separation of two surfaces may be defined as adhesives. There
are a variety of natural attachment devices based on entirely mechanical principles, while others
additionally rely on the chemistry of polymers and colloids (Gorb, 2001; Scherge and Gorb, 2001;
Habenicht, 2002). There are at least three reasons for using adhesives: (1) they join dissimilar
materials; (2) they show improved stress distribution in the joint; and (3) they increase design
flexibility (Waite, 1983). These reasons are relevant both to the evolution of natural attachment
systems and to the design of man-made joining materials.
In general, adhesive-bond formation consists of two phases: contact formation and generation of
intrinsic adhesion forces across the joint (Naldrett, 1992). The action of the adhesive can be
supported by mechanical interlocking between irregularities of the surfaces in contact. Increased
surface roughness usually results in an increased strength of the adhesive joint, due to the increased