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.
As far as applications are concerned, the presence of this transition makes it impossible to construct long homogeneous channels with a contact angle which exceeds 90 degrees. However, the bulge arising from this instability may coalesce with a neighboring channel and thus lead to a microbrigde between two channels. These bridges can be used to construct fluid microchips or microreactors. In addition, after a certain pattern of liquid channels and bridges has been created, one may stabilize it by freezing, polymerizatin or sol-gel reactions.
The Science article contains a detailed comparison between theory and experiment, compare Fig. 1. This comparison shows that, on the micrometer scale, the liquid channels can be understood in terms of position-dependent surface tensions and are not affected by line tension. However, the latter tension should have an observable effect as soon as the surface domains become sufficiently small. Thus, a lot of effort is currently devoted to the study of the corresponding morphologies on the nanometer scale.
(A) Bulge state of microchannel as determined theoretically; (B,D) Projection of the shape perpendicular to the substrate and (C,E) Location of the contact line which makes an excursion into the hydrophobic surface domains. (B,C) are obtained from the theoretical shape, (D,E) from the experimental observations.
Published: 22-12-98
Contact: Peter Lenz
Max Planck Institute of Colloids and Interfaces, Teltow/Germany
Phone: 49-3328-46-588
Fax: 49-3328-46-232
Journal
Science