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390 Biomimetics: Biologically Inspired Technologies

Figure 15.8 Surfaces generating grip on the wet substrata. (a–d) Scanning electron micrographs of the toe pads
of the tree frog Phyllomedusa trinitatis. (Courtesy of J. Barnes, University of Glasgow.) (a) Low power view of the
terminal portions of two toes, with toe pad epithelial cells just visible, (b) expanded view of a single toe pad,
(c) medium power view of toe pad epithelium with mucous pores, (d) high power view to show detailed structure
of the columnar epithelial cells separated from each other by grooves which, in life, would be filled with mucus, and
(e) hexagonal sipes of Conti Winter Contact TS780. (Courtesy of R. Mundl, Continental AG.)

and tree frogs (Hanna and Barnes, 1990) (Figure 15.8a–d). The company promises enhanced wear
performance on dry roads, less aquaplaning and better braking on wet roads, substantially improved
lateral guidance, better grip, and more traction on ice. Fine grooves and longitudinal sipes in the
individual tread blocks, provide lateral guidance. These are intersected by lateral sipes that provide
maximum traction. This optimization ensures maximum safety on wintry roads. As lateral forces
are generated in bends, the honeycomb sipes provide more gripping edges than conventional sipes,
thus considerably improving trackholding when cornering (Barnes et al., 2002) (Figure 15.8e).

15.4 ANTI-ADHESIVE AND SELF-CLEANING SURFACES

Some biological systems have developed surfaces covered with micro and nanostructures, having
antiwetting, anti-adhesive, and self-cleaning properties. The most prominent example is the so-
called lotus-effect recently described for plant surfaces and successfully applied in numerous
industrial materials, such as paints, roof tiles, spoons, and sinks.
The majority of surfaces of vascular plants are covered by a hydrophobic cuticle, which has an
external layer consisting of so-called epicuticular waxes. The layer often contains wax crystalloids,
with dimensions ranging from hundreds of nanometers to micrometers (Figure 15.9). The roughness
of the hydrophobic plant surface decreases wettability (Holloway, 1969a,b, 1994), which is reflected
in a greater contact-angle of water droplets on such surfaces, compared to smooth surfaces of the
same chemical composition. This property of structured hydrophobic plant surfaces results in their
ability to be cleaned by rolling drops of water (Barthlott and Neinhuis, 1997, 1998). Particles
contaminating plant surfaces consist, in most cases, of material that is more readily wetted than
hydrophobic wax components. Contaminants usually rest on the tips of the surface structures, so that
the real contact area between the particles and plant surface is minimized. Thus, these particles can
be easily removed by water droplets rolling over the surface. In this case, adhesion between particles
and water droplets is greater than between particles and plant surfaces due to the reduced contact
between the particles and plant surfaces. In the case of a smooth plant surface, the real contact
area between the contaminating particles and the surface is large enough to avoid particle adherence