The Lotus Effect & Hydrophobic Coatings

The lotus effect in materials science is the observed superhydrophobic (also super hydrophobic or super-hydrophobic) and self-cleaning property found with lotus plants' leaves. In some Eastern cultures, the lotus plant is a symbol of purity. Although lotuses prefer to grow in muddy rivers and lakes, the leaves and flowers remain clean. Botanists who have studied lotus leaves have found that they have a natural cleaning mechanism. This cleaning mechanism was discovered by Wilhelm Barthlott in 1982. He also holds a patent. The microscopic structure and surface chemistry of the leaves prevent them from being wetted by liquids having a contact angle of greater than 90° to an unstructured surface of the same material. With contact angles to water of up to 170°, droplets roll off a leaf's surface like mercury, taking mud, tiny insects, and contaminants with them. This is known as superhydrophobicity, or more commonly, the lotus effect. Water droplets on taro and nasturtium leaves exhibit similar behavior. Some nanotechnologists have developed treatments, coatings, paints, roof tiles, fabrics and other surfaces that can stay dry and clean themselves in the same way as the lotus leaf. This can usually be achieved using special fluorochemical or silicone treatments on structured surfaces or with compositions containing micro-scale particulates. Super-hydrophobic coatings comprising Teflon microparticles have been used on medical diagnostic slides for over 30 years. It is possible to achieve such effects by using combinations of polyethylene glycol with glucose and sucrose (or any insoluble particulate) in conjunction with a hydrophobic substance. Water repelling glass panels have also been brought onto the market for use on the roofs of conservatories. StoCoat Lotusan is an exterior coating (paint) that mimics the microstructure of the lotus leaf surface, gaining similar water-repellent and self-cleaning properties, termed the Lotus-Effect. Water does not adhere to the surface, but rolls off the paint, picking up and washing away debris in the process. By remaining dry, the coating also resists mold, mildew, and algae. Though hydrophobic, the coating is highly permeable to water vapor. Lotus effect superhydrophobic coatings applied to microwave antennas can significantly reduce rain fade and the buildup of ice and snow.

Don't forget to check out the fish swimming around on the lotus leaf here.

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Artificial Lotus Effect: Carbon nanotubes with nanoscopic paraffin coating form superhydrophobic, self-cleaning surfaces. Never wash your car again? Never clean your windows? These may well become reality if it becomes possible to produce the right coatings—coatings that imitate the self-cleaning effect of the lotus blossom. A research team led by Ayyappanpillai Ajayaghosh at the National Institute for Interdisciplinary Science and Technology (Trivandrum, India) has made significant progress toward this goal. As they report in the journal Angewandte Chemie, these scientists have successfully produced a superhydrophobic, self-cleaning surface. Their success results from carbon nanotubes having a nanometer-thick paraffin coating with the help of a rigid aromatic molecule called para-phenylenevinylene. The lotus plant has given its name to a natural self-cleaning mechanism: The extremely water-repellent (superhydrophobic) surface of its leaves causes drops of water to form spheres, which roll off the leaf, sweeping any dirt away. The lotus leaf is equipped with 3 to 10 µm “bumps” that are in turn coated with a nanoscopic water-repellent coating. The bumpy structure minimizes the area with which the water can come into contact and the water-repellent coating keeps water from getting into the valleys between the bumps. The water cannot coat the leaf and simply rolls off. The researchers started with carbon nanotubes—long, hollow fibers made of carbon atoms in a honeycomb-like arrangement. Using a self-assembly process, they attached organic molecules to the exterior of the tubes. These molecules consist of a short backbone of aromatic six-membered carbon rings that supports several long hydrocarbon chains. The aromatic rings attach themselves firmly to the honeycomb structure of the nanotubes; the hydrocarbon chains act like a paraffin-like coating. The research team applied a dispersion of these adducts to glass, metal, and mica surfaces. Once dry, the result was a water-repellent coating with stable self-cleaning properties. Electron microscopic images show that the coating does not have a regular structure like the leaves of the lotus, but does have comparable nanoscale roughness. Water has as much trouble coating these artificial surfaces as the lotus leaf. A tilt angle of 2° is sufficient to cause water droplets to roll off. Like the lotus, any dust is removed from the surface by the water droplets.