Monday, 30th October 2017

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The lotus effect

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By The ability of leaves to clean themselves has inspired Indian scientists to develop a self-cleaning paint. So a car that receives a coat of such paint will not need washing during its lifetime. T.V. Jayan reports
  • Published 29.12.08
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Peering through the powerful microscope before him was more than enough for scientist Ayyappanpillai Ajayaghosh to gauge the magnitude of the invention his team had made. A chemist at the National Institute of Interdisciplinary Science and Technology (NIIST) in Thiruvananthapuram, Ajayaghosh was gazing at a droplet of water rolling off a postage stamp-sized glass plate that had received a coat of a material prepared in his lab.

To his amazement, he saw that as the droplet rolled down, it swept away all tiny specks of dirt that it encountered on its way, leaving the glass surface clean. “We weren’t prepared for anything like that,” Ajayaghosh says recollecting that particular day about a year ago.

What Ajayaghosh and colleagues recreated in the lab was what Nature does effortlessly. Leaves of certain plants — famous among them are the lotus and colocasia (kochu in Bengali and arbi in Hindi) — have the ability to remain spotlessly clean, despite their dusty or muddy environs.

Raindrops fall on the water-repellent leaves and form water beads that look like pearls. The glittering droplets roll off, taking along with them all dust and dirt particles. The phenomenon — rightly called the Lotus Effect — was known since the 1970s but its scientific basis was understood only in 1997 when two German botanists, Wilhelm Barthlott and Christoph Neinhuis, examined leaf surfaces of the lotus and several other plants using a highly powerful scanning electron microscope. It became clear to them that the surface of the lotus leaf has “hills and valleys” which are 3 to 10 micrometres tall (or deep in the case of a valley). The entire structure further is covered with a very fine layer of wax-like coating that repels water. While the “bumpy” structure minimises the area with which the water comes into contact, the water-repellent coating prevents the water from getting into the valleys. As a result, the water cannot coat the leaf and simply rolls off.

“The self-cleaning ability of such natural surfaces has inspired scientists to mimic this property with artificial materials,” says Ajayaghosh, who recently won India’s top science award, the Shanti Swarup Bhatnagar prize.

Mimicking such self-cleaning property in man-made materials can offer exciting possibilities in day-to-day life. A car that receives a coat of such superhydrophobic (water-repelling) paint will not need a wash during its lifetime. Windows of buildings can remain clean even after years. Ships and boats which receive such a coat on their bottoms can not only move faster, but also reduce fuel consumption as the water-repellent property reduces friction drastically.

“We weren’t the first one to do this. In fact, a few commercial products that exploit the Lotus Effect are already in the market. But what sets apart our work from similar attempts made earlier maybe the simplicity of the procedure,” Ajayaghosh says.

To mimic the intricate nature of the surface of lotus leaves the NIIST researchers used carbon nanotubes —long, hollow fibres made of carbon atoms held together in a honeycomb kind of arrangement. These cylindrical carbon molecules with diameters tens of thousand times less than human hair have several unique properties that make them suitable for a wide range of fields such as electronics, nanotechnology, materials science and architecture. The researchers subsequently gave these carbon nanotubes a nanometre-thick (one billionth of a metre) coating using a paraffin kind of organic molecule. For this, they used a process called self-assembly.

This technique of manipulating interactions among two or more kinds of molecules to form an organised structure helped the scientists attach organic molecules to the exterior of the tubes. These molecules consist of a short backbone of aromatic six-member carbon rings that supports long hydrocarbon chains. The aromatic rings in these organic molecules attach themselves to the honeycomb structure of the nanotubes, giving it a paraffin-like coating. Subsequently, the they applied a dispersion of this material to glass, metal and mica surfaces. Once dry, it gave a water-repellent coating with stable self-cleaning properties.

“The new research shows that even an irregular microstructure created by nanostructured material is sufficient to exhibit the superhydrophobic properties of lotus leaves. Earlier only the regular micro and nanostructured topography of natural systems were mimicked,” says Arunn Narasimhan, a researcher with the Indian Institute of Technology (IIT), Madras.

“Using multi-walled carbon nanotubes instead of the more expensive and more difficult to prepare single-walled carbon nanotubes in preparing superhydrophobic surfaces reduced the cost of preparation dramatically,” says Narasimhan. A gram of single-walled carbon nanotubes cost US$ 500, while the multi-walled variety costs just US$10 .