The Superhydrophobicity Characteristic of Nelumbo nucifera (lotus)and it's applications in the Real World
Nelumbo nucifera is a genus of aquatic plants living in typically muddy habitats. It has large, showy, water lilly like flowers commonly known as lotus or sacred lotus.The name sacred lotus comes from the self-cleaning property in its leaves.This characteristic property of self cleaning is a result of extensive folding (i.e., papillose epidermal cells) and epicuticular wax crystals that jut out from the plant's surface forming a roughened microscale surface. (http://www.asknature.org/strategy/714e970954253ace485abf1cee376ad8). As water comes in contact with this roughened surface the degree of water repellency is so high that it now displays the characteristic of superhydrophobicity.Therefore,The interdependence between surface roughness, reduced particle adhesion and water
repellency is the keystone in this self-cleaning mechanism of the sacred lotus. (http://www.springerlink.com.myaccess.library.utoronto.ca/content/4m4jnt03qlvaywjl/fulltext.pdf). This phenomenon of superhydrophobicity observed in the sacred lotus is known as the "lotus effect".It can be easily demonstrated, not only in the sacred Lotus, but also in many other leaves (e.g. cabbage, reeds, indian cress, tulips) as well as in animals (e.g. wings of butterflies and dragonflies).(http://www.lotus-effekt.de/en/funktion/mikro.php).
The "lotus effect" is demonstrated by the epidermis of the lotus plant that possesses papillae with 10 to 20 µm in height and 10 to 15 µm in width on which the epicuticular wax crystalloids are imposed. Water repellency is mainly caused by this second layer of epicuticular wax crystalloids which cover the cuticular surface in a regular microrelief of about 1±5 µm in height.These superimposed waxes are hydrophobic and togethef with epidermis form a double layer..(http://www.springerlink.com.myaccess.library.utoronto.ca/content/4m4jnt03qlvaywjl/fulltext.pdf) Furthermore, The surface physics behind the Lotus-Effect can be derived from the behavior of liquids applied to solid surfaces.The wetting of a solid with water, with air as the surrounding medium, is dependent on the relation between the interfacial tensions (c) water/air (cwa), water/solid (cws) and solid/air (csa). The ratio between these tensions determines the CA h of a water droplet on a given surface and is described by Young's equation. A CA of 0° means complete wetting, and a CA of 180° corresponds to complete non-wetting.In the latter, water tends to form hemispherical droplets with a high CA.In the case of water-repellent rough surfaces, air is enclosed between the epicuticular wax crystalloids, forming a composite surface (Fig. 6). This enlarges the water/air interface while the solid/water interface is minimized (Dettre and Johnson 1964; Holloway 1970). On such a rough ``low energy'' surface, the water gains very little energy through adsorption to compensate for any enlargement of its surface. In this situation, spreading does not occur, the water forms a spherical droplet, and the CA of the droplet depends almost entirely on the surface tension of the water.Particles deposited on a waxy surface consist, in most cases, of material which is more readily wetted than hydrophobic wax components. In addition, they are in general larger than the surface microstructures and rest only on the very tips of the latter (Fig. 5). As a result, the interfacial area between both is minimized. In the case of a water droplet rolling over a particle, the surface area of the droplet exposed to air is reduced and energy through adsorption is gained. The particle is removed from the surface of the droplet only if a stronger force overcomes the adhesion between particle and water droplet (Adamson 1990). On a given surface, this is the case if the adhesion between particle and surface is greater than the adhesion between particle and water droplet. Due to the very small interfacial area between particle and rough surface, adhesion is minimized. Therefore the particle is ``captured'' by the water droplet and removed from the leaf surface (Fig. 7).(http://www.springerlink.com.myaccess.library.utoronto.ca/content/4m4jnt03qlvaywjl/fulltext.pdf)
Biomimicry is man's way of learning from nature and fabricating the beneficial characteristics of it and developing new technologies and ideas for a better way of living. If application is successfully managed, the Lotus-effect surely is one of the most impressing examples of biomimicy of the past few years. Today, the "lotus effect" has inspired many industrial sectors such as Buildings, manufacturing, automobiles, kitchen and bath, textiles, into producing efficient self cleaning products. Some products include Lotusan® paint , GreenShield™ fabric finish, Lotus clay roofing tiles, and Mincor® TX TT textile coating.(http://www.asknature.org/strategy/714e970954253ace485abf1cee376ad8).