In nature, plants are exposed to different kinds of contamination. Most of them are inorganic (different kinds of dust, soot), others biotic (such as spores and conidia of fungi, honeydew, algae). They also differ in wettability, thus wether they are taken up by a water droplet, or stick to its surface. This results in different cleaning properties. In rough, unwettable leaves not only the adhesion between water droplets and surface, but also the adhesion between particles and surface is reduced. If a water droplet rolls over such a dirt particle, the particle is wetted and adheres to the surface of the droplet. Because of the reduced adhesion of the particle to the surface, it is thus taken away and removed from the leaf.
With contamination of hydrophilic dirt, like mud, it shows that they are taken up and securely bound by the water droplet. A clean path becomes visible, where the dirt is taken up by the droplet (figure 1).
Figure 1: A droplet takes up the particles loosely covering the leaf while rolling off, thus cleaning the surface.
Using a hydrophobic substance instead of a hydrophilic as contamination, the droplet/water interfacial behaviour of the dirt changes fundamentally. Against all expectations a water droplet also removes those particles, although they should rather adhere to the hydrophobic surface, than to the droplet. In this case the particles are not taken up by the droplet, but rather stick to its surface, distributing evenly.
It is amazing, that even hydrophobic substances are removed from hydrophobic surfaces by a water droplet. Looking at the microscopic conditions the mechanism becomes clearer. The particles only touch the tips of the wax-crystals. The contact area, and in consequence the adhesion, is thus extremely small. Adhesion between droplet and particle is stronger than adhesion between particle and wax layer. The particle sticks to the surface of the water (figure 2).
Figure 2: Even an oleophobic color that is used by the police to mark bank notes can be rinsed of from the surface with a little water thanks to the Lotus-effect®.
The Lotus-effect is not a coincidental phenomenon, but "wanted" by the plant. Besides the inorganic contamination, which has different negative effects on the living tissue (e.g. stronger heating under sun exposure, acid effects, blocking of stomata), organic contamination in form of spores, bacteria or algae play a much more important role. The most elegant way to cope with it is the Lotus-effect. It prevents pathogen from bounding to the surface. Spores are washed off with every rainfall, and in case it doesn't rain for a while, they lack the water needed for germination (figure 3).
Figure 3: The most important reason for the Lotus-effect® in nature is protection against pathogenic organic contamination like bacteria or spores. These are regularly removed from the leaves by rainfall.
