Bioinspired Self-cleaning Materials - Biotechnologies and Biomimetics for Civil Engineering - page 217

correlated self-cleaning mechanism (Barthlott and Neinhuis 1997 ). Their study
unveiled a first level of micrometric asperities, 20-40 lm apart, and a more
complex roughness at the nanoscale induced by the presence of epicuticular wax
crystals (Fig. 9.2 ). This mixture of surface chemistry and hierarchical micro-
nanostructuring, which has then been identified on several other plants and leaves,
accounts for surface superhydrophobicity (Fig. 9.9 ), producing a water contact
angle of 160 and sliding angle lower than 4, and consequently allowing water
rolling and bouncing on the surface. (Cheng et al. 2006 ; Su and Chen 2008 ; Koch
et al. 2009 ). Since the presence of micro- and nano-asperities drives surface
wettability, Quéré in 2002 depicted the behavior or water on these surfaces by
introducing the concept of ''fakir droplets'' (Quéré 2002 ).
Other plant leaves show analogous, or similar, hierarchical arrangement of
micro and nano texture. Taro, or Colocasia esculenta, surfaces also exhibit the
same hierarchical structure, the only difference being the shape of wax nano-
structures (Koch et al. 2008 ; Xiu and Wong 2010 ). Rice leaves of all rice categories
show the same binary structure of lotus leaves, with 5-8 lm papillae parallel to the
leaf edge and nanoscale pins distributed on the whole surface sublayer: this explains
the peculiar superhydrophobicity of rice leaves, which is anisotropic—with sliding
angles of 4 in the papillae direction and 12 perpendicularly. Other examples of
self-cleaning superhydrophobic surfaces coming from the Plant kingdom include
purple setcreasea petals, indian canna leaves, perfoliate knotweed, wild pansy
flowers (Fig. 9.9 c) (Guo and Liu 2007 ; Schulte et al. 2011 ). Another interesting
plant showing superhydrophobicity is the tropical aquatic fern Salvinia molesta
(Barthlott et al. 2010 ; Bhushan 2012 ), where the effect is related to multicellular
surface structures consisting of crown-like hairs of hundreds of micrometers—up to
Fig. 9.9 a Examples of superhydrophobic daisies and grass (Copyright 2013 Giorgio Re);
b An almost ballshaped water droplet on a nonwettable plant leaf (Adapted with permission from
Blossey 2003 ); c Macro photo of a water droplet on a flower of the wild pansy (Viola tricolor)
(Adapted with permission from Schulte et al. 2011 ); d Salvinia Molesta SEM micrographs of hair