Bioinspired Self-cleaning Materials - Biotechnologies and Biomimetics for Civil Engineering - page 220

9.3 Production Techniques and Applications

It should now be clear how technologically important the understanding and
development of self-cleaning surfaces is. Plants developed such capability to pro-
tect themselves from hazardous microorganisms such as bacteria, and to maintain
their energy source unaltered during their lifetime: in fact, covering leaves with
mud, dust, soot, or any other contaminant would hinder photosynthesis, which
requires solar energy to reach the leaf surface and be absorbed and converted by
chlorophyll-containing proteins. On the other hand, artificial self-cleaning surfaces
applied to the built environment could bring major energy savings, starting from
indirect costs—i.e., the reduction of maintenance and cleaning interventions—and
affecting also the overall material performance, with a prolonged maintaining of its
reflectance properties and therefore of the energy gain through solar irradiation. As
a consequence, a more reflective surface—especially in the infrared spectrum—
would decrease the amount of absorbed energy, and therefore the need for air
conditioning in hot season, attenuating the urban heat island phenomenon (Lev-
inson et al. 2005 ; Sleiman et al. 2011 ). Moreover, as previously cited, several
research works are now dedicated to the implementation of self-cleaning func-
tionalities on solar cells, to avoid efficiency losses due to atmospheric soiling.
Efforts in biomimicking the abovementioned self-cleaning mechanisms are
huge, and include all mechanisms described, from SLIPS to photoinduced super-
hydrophilicity, to superhydrophobic surfaces (Genzer and Marmur 2008 ; Liu and
Jiang 2012 ). Both the production techniques that can be adopted and the materials
used as starting building blocks are numerous. For instance, a comprehensive
description summarizing the realization of superhydrophobic surfaces with lotus
effect can be found in Bhushan et al. ( 2010 ); details on the use of self-cleaning
superhydrophilic coatings are given in Drelich et al. ( 2011 ), and Nakata and Fu-
jishima ( 2012 ). In the latter case, the methods most commonly applied to generate
TiO 2 , ZnO, WO 3 or SiO 2 superhydrophilic surfaces involve the deposition of a
coating, generally by sol-gel spray-coating or dip-coating deposition, as well as by
sputtering and physical (PVD) or chemical (CVD) vapor deposition (e.g., in the
case of glass), or alternatively the massive addition of nanoparticles to the substrate
itself (e.g., in the case of paints, or cementitious materials). On the other hand,
superhydrophobic surfaces are not only based on surface chemistry, but also—and
prevalently—on a hierarchical surface structure. As a consequence, production
techniques are necessarily more elaborated, and follow two approaches: either a
surface with suitable morphology is chemically modified to achieve low surface
tension, or a hydrophobic material is used to build hierarchical structures (Guo et al.
2011 ). Examples of artificial hierarchical structures are reported in Fig. 9.12 .
These approaches belong to lithography, etching, deposition and self-assembly,
and include plasma and chemical etching, electropolymerization and chemical
polymerization, nanoimprint, electrodeposition, self-assembly, as well as sol-gel
and CVD. Coatings generally consist of low surface tension polymers—such as
Teflon, PDMS or fluorinated polymers, silicon, silica, carbon nanotubes or