In this blog post, we’ll explore the scientific principles behind water droplets rolling off lotus leaves and how nanotechnology, which mimics this phenomenon, is being applied across various fields.
Everyone has probably experienced the inconvenience of getting completely soaked by a sudden downpour at least once. If you had been wearing clothes that didn’t get wet even in the rain, you wouldn’t have had that problem. In fact, clothes that don’t get wet do exist. These are water-repellent garments. Water repellency refers to the effect of causing water to bead up and roll off the fabric. Clothes treated in this way repel raindrops, keeping the wearer dry even in the rain. The principle behind these water-repellent fabrics was developed by mimicking the water-repellent properties of lotus leaves.
Lotus leaves do not get wet. Water droplets on a lotus leaf maintain their spherical shape, so even if the leaf is tilted slightly, they roll off. Furthermore, because of the droplets’ spherical shape, even when multiple droplets gather on the leaf, they merge and roll off together. In this process, dust particles on the leaf are washed away along with the droplets, allowing the lotus leaf to clean itself. This is known as the Lotus effect. It was first discovered by botanist Professor Wilhelm Barthlott, who observed lotus leaves and revealed that, at the nanoscale, a rough surface exhibits superhydrophobicity more effectively than a smooth surface. Superhydrophobicity refers to the property of repelling water.
This characteristic of lotus leaves has been considered symbolically significant in many cultures for thousands of years. For example, the lotus frequently appears as a symbol of purity and rebirth in Eastern philosophy and religion, and has been revered for its ability to remain clean even amidst water and mud. Similarly, various technologies invented by humans, inspired by nature, have evolved into diverse products by mimicking the lotus’s self-cleaning ability.
Water droplets are composed of tiny water molecules. A water molecule consists of one oxygen atom bonded to two hydrogen atoms. Hydrogen and oxygen are bonded by sharing electrons, but oxygen has a stronger pull on the electrons than hydrogen. Consequently, the electrons are shifted toward the oxygen atom. Since electrons carry a negative charge, the oxygen atom becomes negatively charged, while the hydrogen atom becomes positively charged. This property of a molecule having distinct poles, like a magnet, is called polarity.
Due to this polarity, water molecules possess strong attractive forces, and these strong forces destabilize the water molecules on the surface of a water droplet. The water molecules on the surface of the droplet experience strong attractive forces from other water molecules inside the droplet. Conversely, since there are no water molecules in the surrounding air, they do not experience any attractive forces from that direction. Consequently, an imbalance in intermolecular forces occurs, placing the droplet in an unstable state. Because the water molecules on the surface are unstable, they form a shape that minimizes surface area. The force acting on the surface of a water droplet to reduce its surface area is called surface tension.
For a given volume, the shape with the smallest surface area is a sphere, whereas the shape with the largest surface area is a flat plane. Therefore, the greater the surface tension, the more spherical the liquid becomes; conversely, the lower the surface tension, the flatter the liquid becomes. If the surface the water droplet is touching is a hydrophilic object that likes water, the water molecules on that surface experience both an attractive force from the surface and an attractive force toward the interior of the droplet. Because they are subjected to forces from both directions, the water molecules become stable and exhibit low surface tension, resulting in a flattened shape. Conversely, if the surface the droplet touches is a superhydrophobic material that repels water, the water molecules on the surface of the droplet are placed in an unstable state, much like water molecules on a surface in contact with air. Consequently, surface tension increases, causing the droplet to take on a round, spherical shape.
Lotus leaves are covered with countless bumps, each measuring 310 micrometers. Because of this, water droplets that fall onto a lotus leaf cannot seep into the leaf but instead float on top of these numerous bumps, much like a balloon floating on the water’s surface. This reduces the contact area between the water droplet and the lotus leaf, thereby increasing surface tension. In fact, the contact area between the water droplet and the lotus leaf is very small, accounting for only about 23% of the surface area it covers. Because of this small surface area, the water is placed in a state similar to floating in the air, and due to the high surface tension, the water droplet on the lotus leaf maintains its spherical shape. Consequently, even if the lotus leaf is tilted slightly, the water droplets roll off the leaf, and multiple droplets merge together before flowing down the leaf. At this point, dust particles on the leaf are washed away along with the droplets, allowing the lotus leaf to clean itself.
Water-repellent fabrics are designed to mimic the lotus leaf’s ability to repel water and remain dry. Another example of this principle in action is self-cleaning exterior wall paint used in the construction industry. This paint mimics the superhydrophobic surface of a lotus leaf, preventing dust and pollutants from accumulating on building exteriors. When it rains, water droplets run down the walls, naturally removing contaminants, which significantly reduces building maintenance costs.
In addition to water-repellent fabrics, there are many other products that utilize the lotus leaf effect. By leveraging the lotus leaf’s ability to self-clean, a car that doesn’t require washing has been developed. By attaching nanoparticles to the car’s surface—similar to the protrusions found on lotus leaves—water droplets roll off on their own, washing away dust in the process. Consequently, there is no need to worry about foreign substances sticking to the car’s surface, and when you do want to wash the car, simply spraying it with water without detergent makes the process eco-friendly. Furthermore, while conventional cars get dirty because rainwater leaves streaks as it runs down the surface, cars coated with nanoparticles actually become cleaner when it rains. Coating the windshield with nanoparticles improves visibility while driving in the rain and eliminates the need for windshield wipers at speeds below 100 km/h. This helps conserve energy and promotes safer driving. Furthermore, it is said that the coated surface resists dirt buildup and easily washes away pollen and dust. Such nano-coatings are already commercially available and are used not only on cars but also on smartphones, bathrooms, fences, toilets, windows, textiles, and eyeglasses.
