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Tessa Fiorini Cohen
The Lotus Flower pops up constantly in spa adverts. And this is unsurprising, the flower has been a symbol of purity for thousands of years. Although typically found in muddy swamps, its leaves and flowers always emerge from the water spotlessly clean. This lifecycle is captured beautifully in a Buddhist quote,
“A lotus flower is born in water, grows in water and rises out of water to stand above it unsoiled.”
But Buddhists aren’t the only ones gaining inspiration from the plant. The Lotus Flower manages to keep clean in a filthy environment, without using any energy or detergents. This has also captured chemists’ imagination, and they’re copying the plant’s properties into a variety of sustainable, self-cleaning materials.
Now known as the Lotus Effect, these self-cleaning capabilities are being translated into areas as wide-ranging as house paints, to clothing, aeroplanes and even submarines. The sustainability benefits are huge – less time, energy and chemicals are needed to keep everything clean. And the key to these developments lies in the way the Lotus Flower interacts with water.
Superhydrophobic: taking it a step further
Materials can be divided into two groups, based on their water affinity. Hydrophobic materials ‘hate’ water and repel it, whereas hydrophilic materials ‘love’ water and are attracted to it. Plants’ outer surfaces are usually covered by a smooth, hydrophobic wax that repels water. But the Lotus Flower goes one step further – it’s superhydrophobic. Its waxy layer isn’t smooth, but bumpy. The bumps are waxy crystals that are so tiny, they can only be seen under high-powered, electron microscopes. However, their size belies their powerful effect.
A leaf of the Lotus Flower, as seen with the naked eye (a), and at progressive magnifications (c) and (b). 1
When water lands on a plant, it typically remains contained to a droplet and doesn’t spread, as the water molecules are repelled by the plant’s hydrophobic surface. Thanks to the bumps and valleys on a Lotus Flower, these droplets barely touch the plant – they rest on the tips of a few tiny bumps. So, they’re extremely likely to roll off, taking any dirt with them. Likewise, the dirt also rests on these tips, which makes it much easier to wash away. The result is superhydrophobicity and a spotless, self-cleaning plant.
Water droplets on a typical plant. (©Sean Cohen) and as they roll of the leaf of a Lotus Flower (©Tanakawho).
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Nanotechnology is used to transport this Lotus Effect to the material world. There are many patented methods, but they’re all based on mimicking this nano-sized surface roughness. One early use is façade paints which cover buildings with nano-scale projections and any dirt is just washed away the moment it rains. Similar coatings against biodeterioration have also been developed, which stop microorganisms, such as fungi, from growing on and damaging buildings. An added bonus is that these coatings reduce the need for toxic biocides, which would eventually seep into the surrounding environment.
Also currently on the market are clothes that shrug off tough stains, like ketchup and red wine. If you looked at these through an electron microscope, you’d see that their surfaces are covered in tiny, superhydrophobic hairs that resemble peach fuzz. Thanks to these hairs, stains just roll off the clothes and don’t penetrate the surface.
On submarines, superhydrophobic coatings are being used to reduce the drag experienced underwater, notching up speeds and fuel efficiency. Similar uses are in underwater fabrics, such as diving suits and swimsuits. The coatings also help keep electronic technology waterproof, protecting items such as mobiles and tablets from getting fried if wet.
To see the coatings in action, take a look at this video:
The Lotus Effect is just one of the many instances of biomimicry in the science world – there’s no need to reinvent the wheel when perfect solutions can be found by dipping into nature’s treasure chest, developed over millions of years of evolution. Materials inspired by the plant can now be found all around you. Invisible to the naked eye, these workhorses remain unnoticed as you pass by.