Hydrophobicity and Lotus Effect
Sreelakshmi P Vijayan
The word hydrophobicity is derived from two Greek words “hydro”, meaning “water”, and “phobos”, meaning “fear”. In chemistry, the substances which exhibit the property of hydrophobicity are known as hydrophobic substances. Hydrophobic substances are usually water repellant or non-wetting in nature, but at the same time they are fat or oil soluble or else we can call them as non-polar molecules. Several plant oils (coconut oil, sunflower oil, castor oil, linseed oil etc.) animal fats, ghee, butter and wax are naturally hydrophobic.
Definition
Hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water. In Contrast, hydrophilicity is the affinity toward water. Hydrophobic molecules are generally non-polar in nature whereas hydrophilic ones are polar
Need of hydrophobic surfaces?
Aren’t you guys bored of being washed out in this heavy rain. Rain drenched clothes, bags, umbrellas, shoes are still a nightmare to all of us. What if we have had a hydrophobic clothing? Or a water repellent shoe or umbrella. Or a bag with a hydrophobic coating? We could have saved a lot of our electronic gadgets having a protective hydrophobic covering. Isn’t it? So is it possible to develop a hydrophobic coating? Has someone already did that?
How to develop an artificial hydrophobicity?
Scientists have already begun their research in this area and have developed many varieties of hydrophobic surfaces, coatings, films, paints etc. from natural and synthetic raw materials.
You might have noticed the difference how water droplets roll over a lotus leaf or colocasia leaf than that on a mango or banana leaf. Why water rolls non-sticky over a lotus leaf? It’s quite interesting isn’t it? Let’s look into the science behind it.
Lotus Effect
Lotus Effect refers to self-cleaning properties that arises due to ultra-hydrophobicity as exhibited by the leaves of the lotus flower. Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet’s adhesion to that surface. Professor Barthlott discovered this extreme water repellency and unusual self-cleaning properties of the lotus leaf (Nelumbo nucifera), and coined the phrase, “Lotus Effect,”
In summary, the structure of a lotus leaf has a clever design that discourages surface wetting both through a waxy nonpolar coating (a chemical composition of the epicuticular wax with the high content of nonacosanediols) and a roughened micro structure (caused by epidermal cells or papillae) which both make surface interactions less favorable, allowing water to bead up into spheres and easily roll off the leaf.
Lotus Effect” is a cart demo which demonstrates how nature inspires nanotechnology by sharing how nanoscale features on a surface can influence a material to behave at its macroscale.
Measurement of hydrophobicity
Hydrophobicity can be measured by various analytical techniques such as hydrophobic interaction chromatography, contact angle measurement, and rose bengal measurement etc. Of which, the most easier one is contact angle analysis (optical tensiometry). If the contact angle is less than 5o the surface is super hydrophilic, if it is less than 90o the surface is hydrophilic and for a contact angle between 90-150o, the surface is hydrophobic and more than 150 o corresponds to superhydrophobic surfaces.
Check your knowledge
Hydrophobicity is the physical property exhibited by non-polar molecules by repelling water from their surfaces. The surface of such hydrophobic molecules exhibit a contact angle higher than 100o.
Contact angle is the quantitative measurement of wettability of a solid surface by a liquid. It is the angle measured through the liquid, where a liquid-vapor interface meets a solid surface.
If the contact angle of a surface exhibits an angle higher than 150o such surface acts as a superhydrophobic one.
Vegetable or plant oils, bees wax, lotus and colocasia leafs, butter, ghee etc. are hydrophobic substances. They all are non-polar in nature and when examined microscopically they all possess a micro-nano structure to provide better water repellency.
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