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 in various industries and everyday life.
Everyone has probably had at least one experience where their clothes got completely soaked during a sudden downpour, leaving them in a bind. In such situations, it would have been much more comfortable to be wearing clothes that don’t get wet easily even when caught in the rain. In fact, water-repellent clothing is already widely used. These are clothes that have undergone a water-repellent treatment. Water repellency refers to the property that prevents water from seeping into the fabric surface and causes it to bead up and roll off. Clothing treated in this way does not get wet easily because raindrops roll off the surface even when exposed to rain. The principle behind this water-repellent fabric was developed by mimicking the lotus leaf, a prime example of a superhydrophobic structure found in nature.
Lotus leaves do not absorb water easily. Water droplets that land on a lotus leaf remain in a round, spherical shape and roll off easily even if the leaf is tilted slightly. Furthermore, when multiple droplets meet on the leaf, they merge to form a larger droplet, and in the process, any dust or contaminants accumulated on the leaf are washed away. As a result, lotus leaves can remain clean without any external assistance. This phenomenon is known as the “Lotus Effect.” It became widely known through the systematic research of German botanist Professor Wilhelm Barthlott. Upon observing the surface of lotus leaves, he discovered that it is not merely their chemical properties but their microscopic surface structure that creates strong superhydrophobicity. Here, superhydrophobicity refers to the property of repelling water very strongly.
This characteristic of lotus leaves has long held symbolic significance across various cultures. For example, the lotus frequently appears as a symbol of purity and rebirth in Eastern philosophy and religion. Because it maintains its pristine appearance even while growing in mud, it has been regarded as a sacred plant. In this way, the unique structures and functions found in nature have provided countless sources of inspiration for humans, and the lotus’s self-cleaning ability has also served as a model for the development of various cutting-edge technologies.
A water droplet is composed of countless water molecules. A water molecule consists of one oxygen atom bonded to two hydrogen atoms. Although hydrogen and oxygen share electrons to form a bond, oxygen has a stronger affinity for electrons than hydrogen. Consequently, the electrons are shifted toward the oxygen atom, resulting in the oxygen-containing region having a relatively negative charge and the hydrogen-containing region having a relatively positive charge. This property of an uneven distribution of charge within a molecule is called polarity.
Due to this polarity, relatively strong attractive forces act between water molecules. Water molecules on the surface of a water droplet are strongly attracted by the water molecules inside the droplet but do not experience the same level of attraction from the air outside. Consequently, the water molecules on the surface are subjected to an imbalance of forces. To reduce this instability, water molecules attempt to minimize their surface area as much as possible; as a result, water droplets tend to maintain a spherical shape. The force acting on the surface of the water droplet to reduce its surface area is called surface tension.
Among objects of the same volume, the shape with the smallest surface area is a sphere. Therefore, the greater the surface tension, the more the liquid tends to maintain a spherical shape; conversely, when surface tension is relatively weak, the liquid spreads out widely. If the surface the water droplet is in contact with is a hydrophilic material that attracts water, the water molecules are pulled not only by other water molecules within the droplet but also by the surface itself. In this case, the water spreads along the surface and takes on a relatively flat shape. Conversely, on hydrophobic or superhydrophobic surfaces that repel water, the water molecules cannot bond strongly with the surface. Consequently, the water droplet maintains a spherical shape on the surface and can roll around easily.
The surface of a lotus leaf is densely covered with microscopic protrusions ranging in size from a few micrometers to tens of micrometers, which are in turn covered by nanometer-scale wax crystals. Due to this dual structure, the water droplet does not come into contact with the entire leaf surface but instead rests on top of the protrusions. Simply put, a very thin layer of air forms beneath the water droplet, causing it to appear as if it is floating on air. This significantly reduces the actual contact area, allowing the droplet to maintain an even rounder shape. Consequently, even a slight tilt of the lotus leaf causes the droplet to roll off easily, and as multiple droplets merge and move, they also remove dust and contaminants from the leaf’s surface. Thanks to this process, lotus leaves can maintain their cleanliness on their own.
A prime example of mimicking the structure of lotus leaves—which repel water and remain dry—is water-repellent fabric. Today, the lotus effect is actively utilized not only in clothing but also in the field of architecture. For instance, self-cleaning exterior wall coatings and paints were developed by mimicking the superhydrophobic structure of lotus leaves. Buildings coated with these materials do not easily accumulate dust or pollutants, and when it rains, water droplets flow down the surface, carrying away contaminants with them. As a result, the costs and maintenance burden associated with cleaning building exteriors are reduced.
In addition to water-repellent fabrics, there is a wide variety of products that apply the lotus effect. Utilizing the principle by which lotus leaves naturally keep their surfaces clean, superhydrophobic coating technology for automobiles has also been developed. When nanomaterials that mimic the microscopic protrusions on the surface of lotus leaves are applied to a car’s surface, water droplets roll off easily, carrying away some dust and contaminants with them. Consequently, contaminants adhere less to the vehicle’s surface, helping to reduce the frequency of car washes. Furthermore, since raindrops run off quickly when it rains, this technology is expected to reduce traces of dirt on the vehicle’s surface. Applying this water-repellent coating to a car’s windshield allows rainwater to be removed more quickly, helping to ensure the driver’s visibility and, depending on the situation, reducing the frequency of windshield wiper use. It also helps prevent condensation and frost buildup, and facilitates the easy washing away of contaminants such as pollen and yellow dust.
The core of this technology lies not simply in repelling water, but in artificially replicating the surface structure that nature has evolved over hundreds of millions of years. This is known as biomimicry or biomimetics. Recently, advances in nanotechnology have made it possible to mimic the structure of lotus leaves with even greater precision, and research utilizing superhydrophobic surfaces is actively underway. Currently, these nano-coating technologies are being utilized not only in automobiles but also in a wide range of fields, including smartphones, building materials, bathroom fixtures, windows, textiles, eyewear, solar panels, and medical devices, and their applications are expected to expand into even broader areas in the future. The tiny structures found on lotus leaves, created by nature, are providing crucial inspiration for the advancement of cutting-edge nanotechnology.
