Protection of many plants, most notably on

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 ofthese paints contain trace metals as a component.

When metals in water, metalions leaching are unavoidable. This would finally lead to heavy metal toxicityto aquatic animals. These anti fouling properties are majorly found insuperhydrophobic coatings. The superhydrophobicity is a property thatgoverns the extreme water repellency, self cleaning and non-wettability of asolid surface. These super-hydrophobic surfaces exhibits water contact angle(?150°) have received considerable attention due to their fabulous properties. Onsuch surfaces water or any other liquid forms nearly spherical droplets and notcontinuous films because of the high roughness and low energy of the finesurface structure traps a thin layer of air which reduces thecontact between the water droplet and the solid surfaces . Examples ofsuch surfaces are found extensively in the natural world as well.

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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 factthis property has been given a special nameas Lotus Effect. The ability of some aquatic creatureslike ducks to constantly keep their feathers clean, the survival of someinsects like the Namibian desert beetle in the arid desert regions can allbe attributed to the superhydrophobic characteristics of the surfaces concerned.

It is only recently the that these naturally occurringsurfaces have been examined in detail ( pioneering workin this field was done by Professor Wilhelm Barthlott of theUniversity of Bonn, Federal Republic of Germany in 1997) andtheir morphological and chemical natures have been analyzed. Earlier theworks of Cassie,Baxter and Wenzel had provided insights into the relation of wettability byliquids and surface roughness of solids. This has provided us withsome insight into the origin of this remarkable behaviour and theartificial synthesis of such materials.

In factnature itself provides us withsome of the necessary templates that are beingused to synthesize these materials. Since the wettability of solid surfacesaffects many industrial processes, hence control over this property isextremely necessary. Fig-1 superhydrophobic nature of Lotus Leaf   Fig-2 Superhydrophobic coated glass                                                  Since the first demonstration of artificialsuperhydrophobicity by Onda et al. in1996, researchers across the world havedeveloped many interesting techniques for the creation of such surfaces.

These techniques range from sol-gel process, vapor deposition,spray coating, electrodeposition, electroless galvanic deposition, spincoating, layer by layer method and casting techniques. Use of fluorinated polymers(though the environmental impacts of its usage are widely debated), siliconesand silanes as materials of low surface energy is also widespread and mostartificially roughened surfaces contain a coating of these substances to makethem superhydrophobic Water repellency, selfcleansing 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, optometricsand other architectural coatings and textiles are someof the potential areas of application. Some of these productshave already made their way into the commercial market. Intensive research activity is going on round the world tomake self cleaning textiles using less expensive materials(presentlytechniques involving silver and titanium dioxide nanoparticles have been reported) 6.Researchis also directed at making these surfaces durable, long lasting andmechanically strong. As these surfaces are water resistant, hence they alsoresist the growth of microorganisms on them.

Thus their anti-fouling propertiesare also remarkable. Thus they are being applied as protective coatings onmarine vessels, submarines and oil rigs which are constantly exposed to harshsaline 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 protectionfrom environmental pollution and acid rains. There are different applicationssectors for this superhydrophobic coatings, the                        Several reviews of theseand other applications of superhydrophobic coatings are currentlyavailable in literature.1.

2 The causes ofsuperhydrophobicityA detailed examination ofthe surface morphology of the lotus leaves using techniqueslike Scanning Electron Microscopy (SEM) reveals that the surface is covered bytiny bumps or stubs which are 5 to 10 ?m high and 10 to 15 ?m apart. Thisuneven surface is further coated with wax crystals (which are substances with low surface energy)with diameters in then a nanometer range. The fine surfacestructure traps a thin layer of air which reduces the contact between thewater droplet and the solid surfaces. The lotus effect is therefore aphysico-chemical property arising out of the combination of surface roughness(on the nanometer and micrometer scale) and the presence of a coating of lowsurface energy material. So it is possible to create an artificial substancethat resembles the lotus leaves externally. Since 1990, therehave beencontinuous efforts directed at creating such surfaces.  


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