I have been on to self-healing materials for some time, usually writing about them in my german blog or for newspapers and magazines. Self-healing is what makes biology superior to technology. Organisms don’t just have astonishing properties – materials have, too – but they retain them by constant regeneration and while doing so even adapt to changing conditions. We want, we need our advanced materials to do the same.
This is especially true for superhydrophobic coatings, which cause the Lotus Effect and hence are very promising not only for self-cleaning items, but also for anti-corrosion or anti-adhesive coatings. Everyone who ever scratched barnacles from the bottom of a ship knows how useful this could be. Since a superhydrophobic coating also reduces drag on surfaces, painting commercial ships with such a formulation would be a billion-dollar business.
The problem is of course, that superhydrophobic coatings do poorly out in the real world and tend to lose their unique properties due to sun bleaching or mechanical actions. Plants get around this by actively regenerating the coating from a reservoir within, an option not yet available to artificial materials.
This is about to change. In a current paper from Angewandte, Junqi Sun et al describe the creation of a coating based on a porous polymer layer made of polyelectrolyte complexes, which is made superhydrophobic by deposition of a fluoroalkylsilane layer. The polymer undercoat consists of layers of preformed complexes of polyallylamine hydrochloride and sulfonated PEEK which alternate with polyacrylic acid to an oveall thickness of about three microns. Those compounds were thermally cross-linked to form a durable layer that, according to the scientists, forms a foli that can be removed as a whole and transferred to other surfaces. Afterwards the fluoroalkylsilanes were deposited by CVD.
The resulting surface is superhydrophobic with a contact angle of about 160 degrees, which means that water sits as round droplets on such a surface without really interacting with the material. The trick however is that the fluoroalkylsilanes don’t just sit on the surface, but permeate the whole coating, thanks to the porosity of the matrix. That means that even severe damage to the coating can be undone, as long as there is any coating left.
Thisw is not just due to the presence of the perfluorated chains in the matrix. There is actually active repair happening, because the fluoroalkylsilanes migrate to the surface after damage. The effect is rather simple: The matrix is made out of hydrophilic hydrogels that tend to incorporate water into their structure. The hydrophobic perfluorated molecules thus get expelled from the watery matrix and migrate to the surface, restoring the superhydrophobicity.
In their experiment Junqi Sun et al. destroyed the surface repeatedly by etching with oxygen plasma, which not only destroys the surface but also makes it more hydrophilic by adding oxygen-containing functional groups. Nevertheless, within four hours after plasma etching a humid environment restores the superhydrophobicity of the surface completely. This process apparently can be repeated multiple times without the coating losing its properties.
Li, Y., Li, L., & Sun, J. (2010). Bioinspired Self-Healing Superhydrophobic Coatings Angewandte Chemie International Edition DOI: 10.1002/anie.201001258