Environmental Engineering Reference
In-Depth Information
Fig. 9.15 Mechanisms of self-cleaning on a superhydrophilic surfaces, b hydrophilic, super-
oleophobic surfaces (SLIPS), and c superhydrophobic surfaces, and related ability to remove
hydrophilic (1) or hydrophobic (2) contaminants
A first observation concerns the role of water, which is vital in all situations to
carry away particles from the surfaces. Furthermore, the nature of contaminants
also plays a key role: while water can easily carry away any type of contaminant,
be it inorganic, organic, oil-based, or hydrophilic—owing to the possibility of
partial contaminant degradation offered by the conjugated photocatalytic activ-
ity—superhydrophobic materials only offer water a bouncing surface, therefore
hydrophobic contaminants are hardly removed. Opposite considerations apply to
hydrophilic, superoleophobic surfaces, where hydrophilic sources of soiling are
retained by the surface.
Superhydrophilicity should therefore appear to be the most promising tech-
nology to produce effective and reliable self-cleaning surfaces; nevertheless, its
drawback lies in the need for photoactivation, which makes it purposeless in the
absence of a suitable irradiation source—in most cases, UV light is required,
limiting its use in outdoor applications. Current research is dedicated to the
development of doped TiO 2 components with visible light activity, which may find
application indoor as well.
One last note must be dedicated to durability. The use of a surface modification
technique to alter the wettability properties of building materials cannot set aside
the treatment lifetime, as such materials are generally designed for service lives of
several decades. Moreover, atmospheric dust and particles may exert some erosive
effect on the material surface. In this respect, the use of coatings with enhanced,
extremely controlled micrometric and nanometric roughness could be detrimental,
unless the coating is proved to exhibit excellent wear resistance. If a superhy-
drophilic coating is employed, this issue is mitigated, as only the complete
removal of the coating—rather than its partial damage—would hinder its func-
tioning. On the other hand, such photoactive coatings may suffer from deactivation
due to irreversible adsorption of reaction products released during contaminants
degradation.
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