Lotus effect: self-cleaning surface

Imagine you would want to manufacture a surface from which any kind of dirt could be washed of with pure (rain) water.

Of course, there is no problem to wash off water-soluble compounds, but greasy and oily particles rather attach to all kinds of surfaces instead of being washed off by water. Now, the first step obviously is to choose a material that has the least possible interactions with any kind of other materials (as there is no material that would repel other non-charged materials).

This material is Teflon^TM. It exhibits no H-bond interactions and the smallest van der Waals interactions possible (that is why Teflon^TM is used as surface coating in non-sticking pans). But this will not do yet. In fact, even water droplets readily adhere to Teflon^TM. It appears that this problem only can be solved if one optimizes the phenomenon that has been the major challenge to the gecko, i.e. create a surface that is so rough (on a molecular level) that the dirt particles find only few contact points thus yielding very small van-der-Waals forces. As a consequence the particles will rather attach to a water droplet (with a smooth surface) even though they might not be able to interact via Hydrogen bonds. As a positive side effect the water droplets themselves will also readily run off from such a surface.

In the case of the lotus flower the surface material is wax not Teflon^TM. Wax does exhibit slightly higher van der Waals forces than Teflon^TM but just as Teflon^TM it does not engage in H-bonding. Due to its optimal surface structure the plant does show this self-cleaning “Lotus”-effect . This effect has been studied in depth by Prof. Barthlott, Dr. Neinhuis and their group at University of Bonn, Germany and they have realized that this self-cleaning effect can be used to improve many articles of daily use (e.g. clothing, carpets, dishes and container a.s.o.). Typicallly, perfluorinated materials are used for this surface treatment because - just like Teflon^TM - they exhibit no H-bonding and only weak van-der-Waals interactions.

Wax crystals on the surface of a lotus leaf
(Barthlott und Neinhuis, Planta 1997)

Water droplet with particles on its surface
(W. Barthlott)

Links:
http://en.wikipedia.org/wiki/Hydrophobicity#Superhydrophobicity
http://en.wikipedia.org/wiki/Lotus_effect
http://www.lotus-effekt.de/

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