biomimicry / nano surfaces

How lotus leaf make water droplets dance on its surface

dropThe closer we look in to the nature and the microscopic details of its structures, more we understand about the inner workings of the things that nature has designed. A microscope opens the door for us to see in to this small world. It’s not unusual that, even scientists frequently get amazed by the complexity of the structures made by the nature at this scale. I know this because I have, many number of times. They look almost as if something with greater power has deliberately designed and created them. These structures appear sophisticated enough to question our theory, that they are simply made by molecules, just trying to lower their energy.

The lotus effect

 Nature often conceals its most magnificent creations in things that are very ordinary, like lotus leafs for an example. It shows an adaptation that only few types of plants display in the plant world. No matter how much or how hard it rains, it doesn’t seem to get wet. When looked closely, it appears that water just bounces off of it. Unlike other leaves, when water droplet hits the surface of the lotus leaf, it doesn’t spread and wet the leaf. Instead, the water droplets breaks off on the surface in to hundreds of smaller droplets and bounces across the surface until they fall off.

This extreme repellency towards water is often referred to as superhydrophobicity or by the plant, lotus effect. This is characterized by the contact angle a droplet of water makes with the surface. To be classified as superhydrophobic, the droplets must make a contact angle of more than 150 degrees. Which makes it extremely difficult to wet.

wetting and non wetting contact angle dependance

Superhydrophobicity is also found in other plants, such as Tropaeolum (nasturtium), Opuntia (prickly pear), Alchemilla and certain cane species. This is a natural adaptation of the plant to gain an edge in the battle of survival. Often in the jungles and water ponds where these plants thrive, sunlight is in the limited supply. To make the matters worse, frequent downpours washes down dust and dirt that usually leaves a thin deposit which disturb the light from penetrating through the leaves. This negatively affect the photosynthesis of the plants. When the water droplets roll off across these surfaces, they pick up and carry the dirt particles off, keeping the surface exceptionally clean.

How it works

The secret to this amazing effect lies in the microscopic arrangement of the lotus leaf structure. Although, lotus leaf surface looks smooth to the naked eye, it is exceptionally rough in the micro scale. The surface of a lotus leaf is covered with small mountain like structures. Each of these mountains have waxy protrusions that sticks out almost like a cone. This waxy material has very low surface energy(material with low surface energy is poorly wetted by water). When a water droplet falls on to the surface of the leaf, it is forced to porch on the tips of these mountains. This permits the leaf to maintain a little air gap between the surface of the leaf and the water droplet. Without any anchoring forces, the droplet can only roll off from the surface.

lotus effect explained at ninithi.com

This feat is too smart enough to be picked up by the Science. Many scientists all over the world have started to build on this concept to make even more spectacular structures. These often, nature inspired materials are built at very small scale such as nano or microscale. Some lotus effect inspired, superhydrophobic materials are already included in application areas such as paint, coatings, wood, metal and roof. The most commonly used chemicals for the superhydrophobic finish is flourochemicals; material with one of the lowest surface energy among others.

Superhydrophobicity has emerged as a platform technology over the years as a technology with widespread application potential in number of applications. The superhydrophobic coatings are tested from textile materials to the hulls of the ships.

Further reading

  1. Latthe, Sanjay S., Chiaki Terashima, Kazuya Nakata, and Akira Fujishima. “Superhydrophobic surfaces developed by mimicking hierarchical surface morphology of lotus leaf.” Molecules 19, no. 4 (2014): 4256-4283.

  2. Superhydrophobicity, Learn from the Lotus Leaf, By Mengnan Qu, Jinmei He and Junyan Zhang 

  3. Sun, Manhui, Chunxiong Luo, Luping Xu, Hang Ji, Qi Ouyang, Dapeng Yu, and Yong Chen. “Artificial lotus leaf by nanocasting.” Langmuir 21, no. 19 (2005): 8978-8981.

  4. Yang, Haili, Fuxin Liang, Ying Chen, Qian Wang, Xiaozhong Qu, and Zhenzhong Yang. “Lotus leaf inspired robust superhydrophobic coating from strawberry-like Janus particles.” NPG Asia Materials 7, no. 4 (2015): e176.

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