Protection would finally lead to heavy metal toxicity to

Protection of materials from corrosion is a major deal now a day.
Materials like big vessels, water pipes, coolant pipes in nuclear reactors,
submerged materials in sea environment face a lot of problems due to corrosion.
Antifouling coatings are used to protect materials from corrosion. Most of
these paints contain trace metals as a component. When metals in water, metal
ions leaching are unavoidable. This would finally lead to heavy metal toxicity
to aquatic animals. These anti fouling properties are majorly found in
superhydrophobic coatings.

The superhydrophobicity is a property that
governs the extreme water repellency, self cleaning and non-wettability of a
solid surface. These super-hydrophobic surfaces exhibits water contact angle
(?150°) have received considerable attention due to their fabulous properties. On
such surfaces water or any other liquid forms nearly spherical droplets and not
continuous films because of the high roughness and low energy of the fine
surface structure traps a thin layer of air which reduces the
contact between the water droplet and the solid surfaces . Examples of
such surfaces are found extensively in the natural world as well. For example,
the formation of glistening beads of water on the leaves of many plants,
most notably on the lotus leaves has been a long observed phenomenon. In fact
this property has been given a special name
as Lotus Effect. The ability of some aquatic creatures
like ducks to constantly keep their feathers clean, the survival of some
insects like the Namibian desert beetle in the arid desert regions can all
be attributed to the superhydrophobic characteristics of the surfaces concerned.
It is only recently the that these naturally occurring
surfaces have been examined in detail ( pioneering work
in this field was done by Professor Wilhelm Barthlott of the
University of Bonn, Federal Republic of Germany in 1997) and
their morphological and chemical natures have been analyzed. Earlier the
works of Cassie,
Baxter and Wenzel had provided insights into the relation of wettability by
liquids and surface roughness of solids. This has provided us with
some insight into the origin of this remarkable behaviour and the
artificial synthesis of such materials. In fact
nature itself provides us withsome of the necessary templates that are being
used to synthesize these materials. Since the wettability of solid surfaces
affects many industrial processes, hence control over this property is
extremely necessary.

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Fig-1 superhydrophobic nature of Lotus Leaf

 

 

Fig-2 Superhydrophobic coated glass

                                               

 

 

Since the first demonstration of artificial
superhydrophobicity by Onda et al. in1996, researchers across the world have
developed many interesting techniques for the creation of such surfaces. These techniques range from sol-gel process, vapor deposition,
spray coating, electrodeposition, electroless galvanic deposition, spin
coating, layer by layer method and casting techniques. Use of fluorinated polymers
(though the environmental impacts of its usage are widely debated), silicones
and silanes as materials of low surface energy is also widespread and most
artificially roughened surfaces contain a coating of these substances to make
them superhydrophobic

 

Water repellency, self
cleansing and anti-sticking behaviour of these surfaces are very attractive features that can be exploited in a variety of applications. Self cleaning glass,
superhydr- rophobic paints, automotive industry, aerospace industry, optometrics
and other architectural coatings and textiles are some
of the potential areas of application. Some of these products
have already made their way into the commercial market. Intensive research activity is going on round the world to
make self cleaning textiles using less expensive materials
(presentlytechniques involving silver and titanium dioxide nanoparticles have been reported) 6.Research
is also directed at making these surfaces durable, long lasting and
mechanically strong. As these surfaces are water resistant, hence they also
resist the growth of microorganisms on them. Thus their anti-fouling properties
are also remarkable. Thus they are being applied as protective coatings on
marine vessels, submarines and oil rigs which are constantly exposed to harsh
saline environment and also get covered by algae and other marine organisms.
When applied as coatings on building material such as marbles and sandstone they can act as protection
from environmental pollution and acid rains. There are different applications
sectors for this superhydrophobic coatings, the

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Several reviews of these
and other applications of superhydrophobic coatings are currently
available in literature.

1.2 The causes of
superhydrophobicity

A detailed examination of
the surface morphology of the lotus leaves using techniques
like Scanning Electron Microscopy (SEM) reveals that the surface is covered by
tiny bumps or stubs which are 5 to 10 ?m high and 10 to 15 ?m apart. This
uneven surface is further coated with wax crystals (which are substances with low surface energy)
with diameters in then a nanometer range. The fine surface
structure traps a thin layer of air which reduces the contact between the
water droplet and the solid surfaces. The lotus effect is therefore a
physico-chemical property arising out of the combination of surface roughness
(on the nanometer and micrometer scale) and the presence of a coating of low
surface energy material. So it is possible to create an artificial substance
that resembles the lotus leaves externally. Since 1990, therehave been
continuous efforts directed at creating such surfaces.

 

 

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