Wetting and dewetting phenenoma are all around us. The formation of rain droplets sitting on a plant leaf or hanging from a spiders web provide familiar examples for dewetting. On the other hand, the spreading of paint and adhesives on solid surfaces or the application of cosmetics onto the human skin rely on the wetting properties of these liquids. In fact, you could not read these lines without the tear films which wet your eyes and which are stabilized by the closure of your eyelids.
Now, imagine that we leave the macroscopic world and shrink the wetting structures to smaller and smaller length scales. Will we observe the same type of behavior as on the macroscopic scale? This question poses a general challenge to the science of colloids and interfaces. It is also of technological importance since our devices become smaller and smaller and their manufacture typically involves the processing of liquids.
Researchers at the Max Planck Institute of Colloids and Interfaces (Science, Vol. 282, 1 January 1999) have discovered new wetting phenomena at surfaces which are laterally structured on the micrometer scale. These surfaces contain hydrophilic and hydrophobic domains and, thus, have a position-dependent surface tension. This leads to 2-dimensional wettability patterns which act as templates for the 3-dimensional morphology of the liquid layers.
If the hydrophilic surface domains have the shape of long stripes, the liquid forms channels along those stripes. As one increases the volume of the liquid, these channels undergo a shape instability from a homogeneous channel state to another state with a single bulge. This instability is quite different from the classical Rayleigh-Plateau instability and represents a new type of wetting transition between two different morphologies of constant mean curvature. This latter transition can only occur because the contact angles of the channel do not satisfy the classical Young equation.