The reason lotus petals don’t get wet is because the microscopic bumps on their surface allow water droplets to roll off them without touching them. Water-repellent treatments that mimic this principle have applications in clothes that don’t get wet in the rain, cars that don’t need to be washed, and a variety of nano-coated products.
We’ve all been caught in a sudden rain shower and gotten soaked to the bone. If only we had been wearing clothes that didn’t get soaked in the rain. There are actually clothes that don’t get wet. These are clothes that are water-repellent. Water repellency means that water is repelled from the fabric. So, if you get caught in the rain, they’ll repel the water and you won’t get wet. The principle of these fabrics is based on the lotus petal, which is impervious to water.
Lotus petals don’t get wet. Water droplets on a lotus petal remain in the shape of droplets, so even if the petal is tilted slightly, they will run off. Also, because of the droplet shape of the water droplets, even if several water droplets gather on the leaf, they will merge and run down the leaf, washing away the dirt that has settled on the leaf with the water droplets, and the lotus petals will clean the leaf themselves. This is known as the Lotus effect. It was first discovered by botanist Prof. Wilhelm Barthlott, who observed lotus petals and realized that at the nanoscale, rough surfaces are more hydrophobic than smooth ones. Superhydrophobicity refers to the property of not liking water.
This property of lotus petals has been symbolically important to many cultures for thousands of years. For example, the lotus flower often appears in Eastern philosophy and religion as a symbol of purity and rebirth, and was revered for its ability to stay clean in water and mud. Many nature-inspired human inventions have mimicked the lotus flower’s ability to self-clean.
Water droplets are made up of tiny water molecules. A water molecule is made up of two hydrogen atoms bonded to one oxygen atom. Hydrogen and oxygen are bonded by sharing electrons with each other, but oxygen has a stronger pull on electrons than hydrogen, so the electrons are biased toward the oxygen molecule, and since electrons are negatively polarized, the oxygen atoms have negative poles and the hydrogen atoms have positive poles. When a molecule has poles like a magnet, it is called polarized.
Because of this polarity, water molecules have a strong attraction, and the strong attraction destabilizes the water molecules on the surface of the droplet. The water molecules on the surface of the droplet are strongly attracted by the same water molecules on the inside. On the other hand, there are no water molecules in the air outside, so there is no attraction. This creates an imbalance of forces between the molecules, and they are unstable. Because the water molecules on the surface are unstable, they form a shape that minimizes their surface area. The force acting on the surface of a water droplet to reduce its surface area is called surface tension.
The shape with the smallest surface area for the same volume is a sphere, and conversely, the shape with the largest surface area for the same volume is a plane. Therefore, the larger the surface tension, the more spherical the liquid, and the smaller the surface tension, the flatter the liquid. If the surface the droplet is touching is a hydrophilic object that likes water, then the water molecules on the surface are subject to both an attractive force from the surface and an attractive force from the inside of the droplet. Because the water molecules are being pulled from both sides, they are stabilized and have a small surface tension, resulting in a flattened shape. Conversely, if the surface the droplet is in contact with is a superhydrophobic object that doesn’t like water, the water molecules on the surface are unstable, just like the water molecules on the surface in contact with air. Therefore, the surface tension is large, resulting in a spherical shape.
The lotus petal has numerous 310 micrometer bumps on its surface. Because of this, water droplets that fall on the lotus petal cannot penetrate into the leaf and instead float on top of the many humps, much like a balloon floating on the surface of the water. This reduces the contact area between the water droplets and the lotus petals and increases the surface tension. In reality, the contact area of a water droplet with a lotus petal is very small, within 23% of the surface it covers. Because of this small surface, the water is in a state similar to that of air, and because of the large surface tension, the water droplets on the lotus petals retain their droplet shape. Therefore, even if the lotus petals are tilted slightly, the water droplets will run off the petals, and several droplets on the leaf will merge together and run off the leaf. The dirt that settles on the leaf is washed away with the water droplets, and the lotus petals clean themselves.
Water-repellent fabrics mimic the lotus petal’s ability to repel water and not get wet. Another example of the application of this principle in architecture is self-cleaning exterior wall paint. By mimicking the superhydrophobic surface of lotus petals, the paint prevents dust and dirt from accumulating on the exterior walls of buildings. When it rains, water droplets run down the walls and naturally remove contaminants, which can significantly reduce the maintenance costs of a building.
In addition to water-repellent fabrics, there are many other products that utilize the lotus leaf effect. This self-cleaning effect of lotus petals has been used to develop cars that don’t need to be washed. By attaching nanoparticles to the surface of the car, like the protrusions on the surface of the lotus petals, water droplets flow off spontaneously and wash away the dirt. Therefore, you don’t have to worry about getting foreign substances on the surface of the car, and if you want to wash the car, you can simply spray the car with water without detergent. In addition, when it rains, conventional cars get dirty because of the stains left behind by the rainwater, but with the nanoparticle coating, the car is actually cleaner when it rains. Coating a car’s windshield with nanoparticles improves visibility when driving in the rain and eliminates the need to use wipers at speeds below 100 kilometers per hour. This helps save energy and makes driving safer. It is also said that the coating is resistant to sticking and washes away pollen and dust. These nano-coatings are already commercialized and are used not only in cars, but also in smartphones, toilets, fences, toilets, windows, textiles, glasses, and many other products.
