Super-hydrophobic coatings represent an important and interesting field of research. This report begins with a brief discussion about the causes of superhydrophobicity and characterization of superhydrophobic surfaces. Some recent synthesis techniques and several applications of these surfaces and coatings are then described followed by a discussion of the major problems that need to be solved before these applications become widespread. To implement the results of the laboratory scale studies on an industrial scale the methods of synthesis have to be modified and adapted suitably. Although some products like hydrophobic paints, non-stick cookware are being manufactured and are available commercially, a lot of scope still remains in this field. Further research is necessary to realize this potential fully.
Introduction
1.1 Background
Superhydrophobicity is a property that governs the extreme water repellency and non-wettability of a solid surface. On such surfaces water or any other liquid forms nearly spherical droplets and not continuous films. Superhydrophobic behaviour of many substances has been known since the 18th century . 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- “the 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 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 with some 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.
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 lithography, plasma treatment, chemical vapour deposition to layer-by-layer techniques and most recently nano-particle-based (usually silica nano-particles) synthesis routes. 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, superhydrophobic paints 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 (Reference: www.lotusan.de, www.activglas.com). Intensive research activity is going on round the world to make self cleaning textiles using less expensive materials (presently techniques involving silver and titanium dioxide nano-particles have been reported). 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 rain . 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 μm high and 10 to 15 μm apart. This uneven surface is further coated with wax crystals (which are substances with low surface energy) with diameters in the 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, there have been continuous efforts directed at creating such surfaces.
1.3 Characterization
Superhydrophobic surfaces can be characterized by the following parameters :
1.3.1 Contact Angle (CA): The most common manifestation of superhydrophobicity is the formation of nearly spherical liquid drops on it. This is quantified by measuring the contact angle(C A) at the solid-liquid interface. The contact angle is the angle at which the liquid-solid interface meet and it is defined as the angle made by the tangent to the point of contact of the solid and the liquid, measured from the liquid side . It is actually determined by the thermodynamic equilibrium between all the three phases i.e. the gas, solid, liquid phases that meet along the interfaces. The highest and the lowest C As exhibited by a liquid-solid-gas combination are respectively known as the advancing and the receding angles and the difference between the two is called the Contact Angle Hysterisis.
For a surface to be designated as superhydrophobic, it must exhibit C A greater than 150o and the contact angle hysterisis must also be low.
Contact angle measurements are carried out by using an instrument known as the goniometer.
1.3.2 Angle of Slide: Another feature is that the surfaces must have a low angle of slide ( less than 10o , measured from the horizontal) i.e. the water droplets should roll down the surface very easily, similar to rolling of rigid spheres.
1.3.3 Self cleaning ability: Self-cleaning behaviour is also an extremely important behaviour of superhydrophobic coatings or surfaces. On these surfaces the adhesion between the water droplets and the dust particles is much more than that between the surface and the dust. Hence most of the dust is picked up by the rolling water drops and no residue is left behind. Hydrophilic dust particles therefore have a greater chance of removal.
1.3.4 Elastic collision of liquid droplets: Water droplets which fall on a superhydrophobic surface with some velocity can bounce back, practically without any deformation after having suffered an almost elastic collision. This is also one reason why these surfaces remain dry even after coming in contact with some liquid.
Superhydrophobicity of a solid surface is also dependent on the properties if the liquid concerned, particularly its temperature. The surface tension of a liquid decreases with rise in temperature. Hence hotter liquids can wet surfaces which exhibit superhydrophobicity towards the same liquid when it is cooler.
The aim of this seminar report is to discuss the various ways in which the interesting properties of superhydrophobic surfaces can be utilized on a large scale. In the report there is a preliminary discussion of the characterization and preparation of these surfaces which is followed by a description of some of the interesting applications of these surfaces. Though a large number of researchers are involved in investigating the various properties and applications of superhydrophobic coatings yet a lot remains to be done as far as large scale manufacture and utilization of these surfaces are concerned. The present report also looks at these aspects and suggests some areas on which future research can be directed.
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