Bioinspired Self-cleaning Materials - Biotechnologies and Biomimetics for Civil Engineering - page 218

millimeters of length: this macroscopic hierarchical structure retains air, which
induces a Cassie-Baxter wetting state (Fig. 9.9 d).
Finally, some superhydrophobic rose petals show unexpected adhesion of water
drops, which remain adherent to the surface even if tilted to 180. This effect is
driven by microstructures spacing: if microsized pillars are far enough, it is pos-
sible for water to fill the air gaps, causing a Cassie-Baxter to Wenzel transition and
increasing surface adhesion (Nosonovsky and Bhushan 2012 ).
Animals are not outdone in terms of variety of interactions with water and
hierarchical microstructuring. For instance, water striders (Gao and Jiang 2004 ),
like water spiders (Yang et al. 2008 ) and several other insects, present thin
hydrophobic wax-coated hair that allow them to trap air, analogously to a Cassie-
Baxter surface, and float on water (Fig. 9.10 a, b). Mosquito's eyes have antifog-
ging properties, generated by the hexagonal nonclose-packed nanonipples arrays
that cover hundreds of microhemispheres, which in turn are arranged in hexagonal
close-packing, as shown in Fig. 9.10 c (Gao 2012 ; Xiu and Wong 2010 ). Similarly,
cicada wings consist of wax-coated hexagonally packed pillars whose height is
approximately 250 nm and spatial distribution is slightly higher than 100 nm
(Fig. 9.10 d) (Nishimoto and Bhushan 2013 ).
Butterfly wings are covered with hierarchical scales, which induce once again
the desired self-cleaning effect, with an anisotropic structure that drives water along
the radial outward direction, while high resistance to water droplets movement is
opposed in the inward direction (Bixler and Bhushan 2012 ; Wong et al. 2013 ).
9.2.5 Gecko
Geckos are renowned in the field of biomimicry for the outstanding adhesive
properties of their toes, based on dry adhesion, which gives them the ability to
cling to any surface and detach at will. The means that allows such powerful
Fig. 9.10 a SEM image of a water strider leg showing oriented microsetae (Reprinted with
permission from Gao and Jiang 2004 ); b water spiders have thin hydrophobic (wax-coated) hairs
with nanoscale roughness which trap air and enhance hydrophobicity (Reprinted with permission
from Yang et al. 2008 ); c SEM image of a mosquito eye (Reprinted with permission from Xiu and
Wong 2010 ); d SEM image of a cicada wing (Reprinted with permission from Xiu and Wong
2010 )