Bioinspired Self-cleaning Materials - Biotechnologies and Biomimetics for Civil Engineering - page 208

character, these materials are included in biomimicry owing to the mechanism of
photoactivation, whose origin traces back to plants photosynthesis. It goes without
saying that in absence of light, these surfaces do not show any self-cleaning
property, which probably makes ''easy-cleaning'' a more precise definition of their
functionality.
On the other hand, proper self-cleaning should not need any external stimulus:
this can be achieved by suitable intrinsic surface chemistry, morphology, or both. In
general, these conditions always induce the opposite behavior—that is, superhy-
drophobicity, sometimes combined with oleophobicity to give superomniphobicity:
for a surface to be self-cleaning, the requisite is to strongly repel water, reducing at
maximum the water drops contact area, and therefore the chances of contamination.
This surface behavior is typical of several living organisms, from both the Plant and
the Animal kingdoms, as further described in the next paragraphs (Qu et al. 2010 ;
Genzer et al. 2008 ). Here, what most matters is the formation of hierarchical
structures at the micro or nanoscale, which force water to interact with air entrapped
between adjacent surface features more than with the surface itself (Li and Ami-
rfazli 2008 ). In this sense, the development of nanotechnology has surely boosted
laboratory and industrial research, allowing first the observation of such morpho-
logical features—and therefore the understanding of the phenomenon—and sub-
sequently their replication. The most common example of this behavior is the lotus
effect, named after the Indian lotus (Nelumbo nucifera) leaf, on which rain drops
have such a small contact area that they result in bouncing and rolling, collecting
dust and pathogens and hence cleaning the leaf surface.
Such efficient self-cleaning is ensured by a combination of a waxy, highly
hydrophobic surface and its nanostructured patterning, and is observed on several
plant leaves, as represented in Fig. 9.2 (Barthlott and Neinhuis 1997 ; Solga et al.
2007 ). Another case is a special fish scale covered by riblets that helps reducing
friction in water, which in turn decreases the adhesion of other microorganisms:
this is the case of shark skin (Schumacher et al. 2007 ).
Latest advancements in nature-mimicking nanotechnology take inspiration
from the binary periodic structure of some insects eyes, whose hierarchical
structuring induces a superhydrophobic surface state (Yang et al. 2013 ). A wider
description of natural superhydrophobic surfaces for self-cleaning is reported in
the following paragraphs.
9.1.2 Hydrophilicity and Hydrophobicity
Understanding the interaction of surfaces with water molecules is essential to
achieve the control of the material response in most applications, especially
considering the exposure to an external environment and its humidity. Having a
suitably hydrophilic or hydrophobic character plays a crucial role in nature and
decides your fate in extreme environments, as in the case of the desert Stenocara
beetle, whose hydrophilic/superhydrophobic patterned elytra with its hierarchical