What do self-cleaning walls, counter tops, fabrics, and water-walking microbots all have in common? The answer is an intriguing scientific property call super hydrophobia. The property allows caterpillars to stay dry and for water striders to be able to support their weight on the water's surface.
For years scientists have puzzled at how this phenomenon worked, but now thanks to advanced computer simulations, a team of researchers the University of Nebraska-Lincoln and Japan's RIKEN institute have finally unlocked how it works.
When the research began in 2005, the RIKEN team had the world's fastest supercomputer at their disposal. In order to understand the basis of the strange behavior, researchers "rained" virtual water droplets of different sizes and at different speeds on surfaces that had pillars of various heights and widths, and with different amounts of space between the pillars. The pillars resembled the hydrophobic hairs on water striders or caterpillars.
What researchers found was that at a critical pillar length, droplets would roll off or in other words, engage in super hydrophobia (also known as the Cassie state, named for A.B.D. Cassie, who discovered it in 1942). Shorter pillars would allow the droplet to reach the surface, where they would encounter more standard hydrophobic interactions (the Wenzel state, named for Robert Wenzel, who found the phenomenon in nature in 1936).
The computer simulations were critical to the discovery for several reasons. First, they allowed a great number and variety of trials in a short time. Secondly, they provided precise control over droplet size -- greater than would be possible with mechanical apparatus. Finally, the computer simulations removed such factors as dirt, temperature and air flow, which could skew results.
Xiao Cheng Zeng, Ameritas university professor of chemistry at UNL, who helped conduct the research describes, "This kind of simulation -- we call it 'computer-aided surface design' -- can really help engineers in designing a better nanostructured surface. In the Cassie state, the water droplet stays on top and it can carry dirt away. In the Wenzel state, it's sort of stuck on the surface and lacks self-cleaning functionality. When you build a nanomachine -- a nanorobot -- in the future, you will want to build it so it can self-clean."
Professor Zeng says that the results are a significant step forward in understanding this strange phenomenon. "A lot of people study this and engineers especially like the water strider because it can walk on water,” Professor Zeng remarked. “Their legs are super hydrophobic and each leg can hold about 15 times their weight. 'Hydrophobic' means water really doesn't like their legs and that's what keeps them on top. A lot of scientists and engineers want to develop surfaces that mimic this from nature."
With the new research, he believes artificial materials can be created with similar properties, allowing self-cleaning or water walking.
A paper on the research is published in the May 4-8 online edition of the journal Proceedings of the National Academy of Sciences (PNAS).
quote: they provided precise control over droplet size
quote: removed such factors as dirt, temperature and air flow