General Aspects of Biomimetic Materials - Biotechnologies and Biomimetics for Civil Engineering - page 68

The low wetting is explained by the Cassie-Baxter principle, which predicts the
formation of air bubbles between the microstructures, avoiding the spread of the
liquid. In addition to superhydrophobicity, these hierarchical architectures
decrease the contact area between the surface and any particles settled over it.
Therefore, water drops rolling off the surface carry sediments away, providing the
leaves with self-cleaning ability.
The superhydrophobicity and self-cleaning properties conferred by microto-
pographic patterns have been explored by the construction industry in the form of
self-cleaning paints (e.g., StoCoat Lotusan 4 ) or clay roof tiles (Erlus Lotus 5 )
designed to reduce building maintenance. By mimicking the microstructure of
lotus leaves, these materials display a high level of water and dirt-repellency, a
pronounced self-cleaning effect and a high resistance to soiling and growth of
mold, mildew, and algae. As a result, water (e.g., from rain) and dirt flow off
immediately and facades and roofs remains dry and attractive. The textile industry
has also been allured by the lotus effect and as a result a number of impervi-
ous, stain-free fabrics have been launched in the market (e.g., Greenshield 6 ,
Nanosphere , 7 and Mincor 8 ) which could be used for example in sunshades,
awnings, or overhangs.
Anti-fogging. On cold days, with high humidity weather, driving can become a
very dangerous and unpleasant activity due to the condensation of water over the
windshield and mirrors of one's car. Likewise, condensation of water over tiles
and walls in bathrooms after a warm bath contributes to the development of
microbial growth. Superhydrophobic surfaces inspired by plants like the lotus
cannot be used as anti-fogging since very small drops can get trapped between the
underlying microstructures (Cheng and Rodak 2005 ). Moreover, such structures
interfere with light, reducing the visibility. Inspiration to solve these problems came
from insect eyes (e.g., mosquito, moth, flies), which are maintained clean and dry,
even in moisture rich environments. The hexagonal structures that form the mos-
quito eyes (ommatidia) are covered with a pattern of smaller (*100 nm) hexagonal
features evenly spaced (*47 nm), which effectively prevent condensation, at
normal conditions (Fig. 3.5 ; Gao et al. 2007 ). Using mosquito eyes as an inspiration
and resorting to soft lithography, researchers created a superhydrophobic com-
pound eye-like microstructure consisting of arrays of micro-hemispheres covered
with nipple-like SiO 2 nanospheres surface with a contact angle of 155 (Gao et al.
2007 ). The expectation is that further developments will lead to the development of
multifunctional coatings with antifogging and easy-cleaning properties tuned
to match the requirements of applications in the realm of civil engineering (Gao
et al. 2007 ).

4

Sto Corp., Atlanta, Georgia.

5

Erlus AG, Neufharn, Germany.

6

BigSky Technologies LLC

7

Schoeller Textile AG, Sevelen, Switzerland.