Electron microscopes, using the higher resolution that energetic electrons can provide, allow scientists to observe tiny structures clearly. Under an electron microscope, the surface of Lotus leaves is found to be covered by tiny bumps of each about 5-10 μm high and 10-15 μm apart. This bumpy uneven surface is itself covered with waxy crystals, measuring around 1 nm in diameter (Fig.3). The two key features: the waxy material and the tiny, bumpy surface work hand-in-hand to create the Lotus effect.
The waxy material on the surface of lotus leaves is hydrophobic. "Hydro" means water and "phobos" means fear in Greek. A material said to be hydrophobic if it does not like water, or depending on the point of view, water does not like it. Because of this property, water molecules tend to cluster together to form a droplet on a hydrophobic surface. In this way, water molecules will stay close to each other and far away from the hydrophobic surface. This water-hating effect of lotus leaves can be regarded as a chemical effect.
When a water droplet is in contact with a surface, the shape of the droplet may change depending on the nature of the surface. The water may spread onto the surface, making it "wet". In this case, the angle between the surface and the tangent of the water's surface is small, typically less than 80o (Fig. 4a). The surface is called hydrophilic or "water-loving". The angle is called the "contact angle". A smaller angle implies that water tends to spread onto the surface. Another scenario is that water clusters together and does not spread on the surface, so the surface does not get "wet". In this case the contact angle is greater than 80o (Fig. 4b). The surface is called hydrophobic or water-hating, as discussed above.
The reason why a surface is hydrophilic or hydrophobic is quite involved. For the interested students, note that two different materials or phases are separated by an interface. Here we have three materials: water, air, and the surface. Thus we have three surfaces between (i) air and water, (ii) air and the surface, and (iii) water and the surface. Whether the surface is hydrophilic or hydrophobic is determined by the competitions among the surface tensions of these three surfaces.
Adding bumps onto a flat hydrophobic surface can turn it into a super-hydrophobic surface, characterized by a large contact angle (Fig. 4b). This is a physical effect, as the contact between water and the surface is reduced by the bumpy surface and the water droplets become more spherical in shape and easier to roll off the surface. Therefore, different degrees of hydrophobicity can be engineered by modifying the roughness of the surface. Fig. 5 summarizes the four different scenes.
With the ideas of hydrophobicity and hydrophilicity, let us return to the Lotus effect. The surface of Lotus leaves turns out to be super-hydrophobic, i.e., with a very large contact angle, as a result of the surface being covered by a waxy material and the bumpy structure of the surface. Thus, water can hardly wet or spread out on the leaves and spherical water droplets form. The water droplet lies loosely stuck on the surface and the tiniest movements of the leaf will cause the water bead to roll off the surface. Dirt particles typically lie loosely on the surface, due again to the bumpy structure of the surface. As water droplets roll off the leaf, they take the dirt particles away (Fig. 6), resulting in the self-cleaning effect of the leaf (Fig. 7).
Animation: Self cleaning effect