The strongly reduced wettability of leaves is an obvious and long known phenomenon. The theory of wettability was to a large extend worked out in the last century, but in detail it is still subject to intensive research today. As a rule the following can be stated: The wettability of a material with air and water as the surrounding media depends on the ratio of the interfacial tension between water/air, material/water and material/air (Figure 1).
Figure 1: The degree of wetting of a solid can be described with the aid of the contact angle: It is determined by the interfacial tension ratio of the materials, and can be calculated via Young's equation.
The ratio of the tensions determines the contact angle of a water droplet to the surface. A contact angle of 0° indicates complete wetting, the water droplet spreading out into a mononuclear film. A contact angle of 180° means complete unwettability, the droplet making contact with the surface in only one point. Materials with a high interfacial tension are more wettable than those with a low interfacial tension, such as Teflon®.
The behaviour of water on a surface is to a large extend dependent on the roughness of the surface. The wettability of a smooth, easily wettable surface is even improved through roughening. On a smooth, hydrophobic surface, roughening results in a superhydrophobic (i.e. extremely unwettable) surface. In the latter case air is enclosed between the droplet and the microstructures.
A single water droplet applied to a waxy leaf rolls off the leaf like on a hotplate (Figure 2).
Figure 2: A droplet on a superhydrophobic surface: The dropled touches the leaf only in a few points and forms into a ball. It completely rolls off at the slightes declination.
The roughness of the surface minimizes the contact area between leaf and droplet, making a seemingly frictionless rolling possible.
