Bioinspired Self-cleaning Materials - Biotechnologies and Biomimetics for Civil Engineering - page 209

Fig. 9.2 Scanning electron micrographs of the adaxial leaf surface of rough, water-repellent leaf
surfaces. Nelumbo nucifera (a) and Colocasia esculenta (b) are characterized by papillose
epidermal cells and an additional layer of epicuticular waxes. Brassica oleracea leaves (c) are
densely covered by wax crystalloids without being papillose, and the petal surfaces of Mutisia
decurrens (d) are characterized by cuticular folds. Bars 20 lm. Adapted with permission from
Barthlott and Neinhuis ( 1997 )
structure allows water to be collected from morning mist, creating a unique water
reservoir—the key to their surviving (Parker and Lawrence 2001 ).
To meet this aim, it is useful to remind some common concepts about surface
wettability.
The parameter that identifies the wetting properties of a surface is contact angle
h (Fig. 9.3 ), defined as the angle between the tangent to the curved water surface at
the point of contact with the solid surface and the plane of the surface on which the
drop is resting, measured through the water (Marmur 2010 ).
This parameter is the result of the equilibrium between three components of
surface tension, represented by three forces acting at the liquid-solid (c sl ), solid-
gas (c sg ), and liquid-gas interfaces (c lg ). Water droplets form high contact angles
([90) on hydrophobic materials on account of the energy increase upon contact
between surface and liquid, which favors the minimization of contact area between
the two. This is explained by the extremely low surface energies of such materials
(*10-50 mN/m). On the contrary, hydrophilic materials have a strong affinity
with water, which thus seeks to maximize its contact area, forming low contact
angles on their highly energetic surfaces, ranging from 500 to 5,000 mN/m (de
Gennes et al. 2002 ).