This blog post explores the scientific principle behind water droplets rolling off lotus leaves and how nanotechnology, inspired by this phenomenon, is being applied across various fields.
Everyone has probably experienced the frustration of getting completely soaked by a sudden downpour. Wearing clothes that stay dry in the rain would have made that situation much easier. In fact, water-repellent clothing does exist. These are garments treated with a water-repellent finish. Water repellency refers to the effect of causing water to bead up and roll off the fabric. Clothes treated this way repel raindrops, preventing them from soaking through. The principle behind this water-repellent fabric was developed by mimicking the water-resistant surface of a lotus leaf.
Lotus leaves do not get wet. Water droplets on a lotus leaf maintain their spherical shape, causing them to roll off even with the slightest tilt of the leaf. Furthermore, due to the spherical shape of the droplets, even if multiple droplets gather on the leaf, they coalesce and flow off together. At this moment, any dust sitting on the leaf is also 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 than a smooth surface. Superhydrophobicity refers to the property of repelling water.
This characteristic of lotus leaves has been symbolically significant across many cultures for thousands of years. For example, the lotus frequently appears as a symbol of purity and rebirth in Eastern philosophy and religion, revered as sacred due to its ability to remain clean even amidst water and mud. Inspired by nature, humans have developed various technologies mimicking the lotus’s self-cleaning ability, leading to the creation of diverse products.
Water droplets are composed of small water molecules. A water molecule has a structure where two hydrogen atoms are bonded to one oxygen atom. Hydrogen and oxygen share electrons to form a bond, but oxygen has a stronger pull on electrons than hydrogen. Consequently, the electrons are pulled toward the oxygen molecule. Since electrons carry a negative charge, the oxygen atom becomes negatively charged, while the hydrogen atoms become positively charged. This property of a molecule having poles, like a magnet, is called polarity.
This polarity gives water molecules strong attractive forces, and these strong forces destabilize the water molecules on the surface of a water droplet. Water molecules on the droplet’s surface experience strong attractive forces inward from other water molecules. Conversely, they experience no attractive force outward toward the surrounding air because no water molecules exist there. This creates an imbalance in the forces between molecules, placing the droplet in an unstable state. Because the surface water molecules are unstable, the water molecules 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, while conversely, the shape with the largest surface area for the same volume is a flat plane. Therefore, the greater the surface tension, the more the liquid tends to form a spherical shape; conversely, the lower the surface tension, the more the liquid tends to form a flattened shape. If the surface the water droplet contacts is hydrophilic (water-loving), the water molecules on the surface experience both an attractive force from the surface and an attractive force towards the interior of the droplet. Because they are pulled in both directions, the water molecules become stable and exhibit low surface tension, resulting in a flattened shape. Conversely, if the surface the water droplet contacts is hydrophobic, the water molecules on that surface become unstable, much like water molecules on a surface in contact with air. Consequently, surface tension increases, causing the droplet to form a spherical shape.
Lotus leaves feature numerous bumps, each about 310 micrometers in size, covering their surface. Because of these bumps, water droplets that land on the lotus leaf cannot seep into the leaf. Instead, they float on the surface of the bumps, much like a balloon floating on water. This significantly reduces the contact area between the water droplet and the lotus leaf, further increasing surface tension. The actual contact area between the water droplet and the lotus leaf is a very small value, covering only about 23% of the surface. Due to this small surface area, the water is placed in a state similar to being suspended in air, and the high surface tension causes the water droplet on the lotus leaf to maintain its spherical shape. Consequently, even a slight tilt of the lotus leaf causes water droplets to roll off its surface. Multiple droplets on the leaf merge and flow down together. At this time, dust particles resting on the leaf are also washed away along with the droplets, enabling the lotus leaf to self-clean.
Fabrics treated with a water-repellent coating mimic this lotus leaf, which repels water and remains dry. Another application of this principle is self-cleaning exterior wall paint in the construction field. This paint mimics the lotus leaf’s superhydrophobic surface, preventing dust and pollutants from accumulating on building exteriors. When it rains, water droplets flow down the wall, naturally removing contaminants, significantly reducing building maintenance costs.
Beyond water-repellent fabrics, many products utilize the lotus leaf effect. Leveraging this self-cleaning property of lotus leaves, a car that requires no washing has been developed. By attaching nano-particles to the car’s surface, similar to the protrusions on a lotus leaf, water droplets flow off naturally, washing away dust along with them. This eliminates the worry of foreign substances adhering to the car’s surface. When washing is desired, simply spraying water on the car without detergent suffices, making it environmentally friendly. Furthermore, while conventional cars develop smudges from rainwater runoff during rainfall, making them appear dirty, cars coated with nanoparticles actually become cleaner when it rains. Coating the windshield with nanoparticles improves visibility during rainy driving and eliminates the need for wipers at speeds below 100 km/h. This helps save energy and contributes to safer driving. Additionally, the coated surface resists dirt buildup, and pollen or yellow dust particles wash off easily. This nano-coating technology is already commercially available and is used not only on cars but also on smartphones, bathrooms, fences, toilets, windows, textiles, eyeglasses, and various other products.