In this blog post, we’ll examine the principles behind yogurt cup lids and water-repellent car coatings, and explore how superhydrophobic materials inspired by the nanostructure of lotus leaves are being developed and utilized.
I often enjoy eating yogurt in a cup. The real pleasure of eating yogurt this way was licking the yogurt stuck to the lid with my tongue. It may seem a bit silly to include such a highly subjective observation in an article meant to convey scientific knowledge, but the yogurt on the lid always tasted better to me. In fact, the yogurt inside the container was so delicious that I wouldn’t even bother scraping it out, yet I’d lick the yogurt clinging to the lid clean right down to the last bit. However, some products later began using lids that didn’t let the yogurt stick to them. Although it felt a bit empty when I opened the lid of those yogurts, it certainly reduced the inconvenience of yogurt getting on my hands or other places. This technology goes beyond simply creating lids that don’t get sticky with yogurt; it’s known as an application of the structure of a lotus leaf’s surface and the principle of repelling water.
You might think of it as just a lid that keeps yogurt from getting on your hands, but this lid is actually related to technology that controls the surface properties of materials to prevent liquids from adhering. Another example that utilizes a similar principle is a car on a rainy day. On rainy days, drivers turn on their windshield wipers to see clearly ahead, constantly wiping away the rainwater hitting the windshield as they drive. Applying a water-repellent coating to car windows and other surfaces prevents water from spreading widely across the glass, causing raindrops to form and roll off instead. Such water-repellent coatings are already commercially available for car windows and other applications, helping to ensure clear visibility by allowing rainwater to run off quickly. However, since these coatings cannot fully replace windshield wipers in heavy rain, at low driving speeds, or when the glass is dirty or the coating is worn, wipers remain an essential component even today.
A yogurt lid that resists yogurt residue and a water-repellent coating that prevents rainwater from spreading on windows—the common thread between these two examples is that “liquids do not adhere easily.” Some of these technologies are related to new materials and surface technologies inspired by the surface structure of lotus leaves. When developing new materials, materials engineers often study the microstructures and nanostructures of surfaces by drawing inspiration from the structures and functions of various natural objects, and the structure of the lotus leaf is a prime example of this. The phenomenon in which water droplets on a lotus leaf roll off easily, carrying away surface dust in the process, became widely known through research conducted by Wilhelm Bartholot and Christian Neuhuis in 1997 and has since been referred to as the “lotus effect.”
Upon close observation, water droplets on a lotus leaf are nearly spherical in shape, and even as they roll off, they do not easily adhere to the leaf’s surface but instead remove contaminants that were adhering to it. This property of the lotus leaf is called “superhydrophobicity.” Properties that attract water are called “hydrophilic,” while those that repel water are called “hydrophobic.” Because lotus leaves strongly repel water, they are considered a prime example of a “superhydrophobic” surface. On a superhydrophobic surface, water droplets do not spread out widely but form into near-spherical shapes; due to their small contact area and low adhesion to the surface, they roll off easily. This creates a self-cleaning effect, where water droplets carry away dust and contaminants from the surface as they roll off. In our daily lives, objects with superhydrophobic surfaces that do not get wet would be convenient in many situations; therefore, inspired by the structure and properties of lotus leaves, researchers began studying their surface structure to develop materials that remain dry or resist the adhesion of contaminants.
To the naked eye, a lotus leaf appears to have an ordinary, smooth surface, not much different from other leaves. However, when observed under a microscope, one can see that the surface is covered with countless microscopic protrusions. These microstructures, resembling peaks, appear at regular intervals, and even smaller, nanoscale structures are formed on the surface of these peaks. In other words, the surface of a lotus leaf features a hierarchical arrangement of micrometer-sized protrusions and nanometer-sized structures. Combined with a surface layer of wax—a substance with low surface energy—this arrangement prevents water from spreading easily. The combined action of these micro- and nanostructures and the surface’s chemical properties is a key factor in producing the lotus leaf’s superhydrophobicity and self-cleaning effect.
In fact, the tiny structures on the surface of a lotus leaf prevent water droplets from adhering to the entire surface, causing them to make contact primarily with the tops of the minute protrusions. Consequently, water droplets do not spread widely across the surface but instead form beads that appear to float, allowing them to roll off easily even with a slight tilt or movement. As the water droplets roll off, dust and contaminants on the surface adhere to them and are carried away, keeping the lotus leaf surface clean. This phenomenon is known as the “lotus effect,” and today, active research is underway to create various artificial superhydrophobic surfaces modeled after it.
To mimic this structure, researchers have been studying methods to process the surfaces of solids, such as silicon, to create structures with micro- and nano-scale protrusions. By forming microstructures and nanostructures of uniform size and spacing on a surface and imparting surface properties that reduce interaction with water, it is possible to create superhydrophobic surfaces similar to those of lotus leaves. Through such research, non-stick products—such as lids for pourable yogurt that prevent the yogurt from sticking—and technologies like water-repellent coatings for windows that allow rainwater to run off easily have been developed. In addition, the potential applications of superhydrophobic surface technology are being explored in various fields, including functional clothing that prevents liquids like cola from soaking in or staining when spilled, as well as surface coatings and paints that repel dust and pollutants.
Currently, it is difficult to view research on nanostructures and superhydrophobic surfaces as still in its early stages, given that it has been underway for nearly 20 years. It was not until 1997 that the superhydrophobicity and self-cleaning effects of lotus leaves became a full-fledged subject of materials science research, and since then, several decades have passed, during which a wealth of research on various artificial superhydrophobic surfaces has been accumulated. Furthermore, these technologies are not confined to laboratory research; their practical applications are being explored in various fields—including self-cleaning, anti-fouling, water repellency, corrosion prevention, and friction reduction—and some are already being utilized in commercial products.
However, the performance of superhydrophobic surfaces can degrade due to contamination or wear, and challenges remain, such as fabricating uniform structures over large areas and ensuring long-term durability. Therefore, while it is difficult to assume that all superhydrophobic materials can immediately replace existing ones, research aimed at implementing new functions by mimicking nature’s micro- and nanostructures is likely to continue in the future. While research on various new materials is underway, studies that mimic the surface structures of living organisms—such as lotus leaves—are particularly significant because they apply naturally occurring structures and functions to materials engineering. If this research continues to advance, it could lead to the development of new materials with diverse functions—beyond just surfaces that repel water and contaminants—which would help make people’s lives more convenient.
