Self-cleaning walls and water-striding robots come closer to reality

May 5th, 2009 - 11:36 am ICT by ANI  

Washington, May 5 (ANI): What if scientists create self-cleaning walls and fabrics or even micro-scale robots that can walk on water? Well, researchers at the University of Nebraska-Lincoln and Japan’s RIKEN institute claim to have moved a step closer to realising such materials.

The researchers have revealed that their work is based on a study of a property called super hydrophobia, which is behind how water beads up and rolls off flowers, caterpillars and some insects, and how insects like water striders are able to walk effortlessly on water.

“A lot of people study this and engineers especially like the water strider because it can walk on water. 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,” said Xiao Cheng Zeng, Ameritas university professor of Chemistry at UNL.

Zeng and his Japanese colleagues - Takahiro Koishi of the University of Fukui and RIKEN, Kenji Yasuoka of Keio University, and Shigenori Fujikawa and Toshikazu Ebisuzaki of RIKEN - have now come up with clues to developing the long-sought super hydrophobic materials.The researcher highlight the fact that caterpillars, water striders, and the lotus achieve super hydrophobia through a two-level structure - a hydrophobic waxy surface made super hydrophobic by the addition of microscopic hair-like structures that may be covered by even smaller hairs, greatly increasing the surface area of the organism and making it impossible for water droplets to stick.

They used the superfast supercomputer at RIKEN, the fastest in the world when the research started in 2005, to design a computer simulation to perform tens of thousands of experiments that studied how surfaces behaved under many different conditions.

The team used the supercomputer to “rain” virtual water droplets of different sizes and speeds on surfaces, which had pillars of various heights and widths and different amounts of space between the pillars.

The researchers observed that there was a critical pillar height, depending on the particular structure of the pillars and their chemical properties, beyond which water droplets cannot penetrate.

According to them, if the droplet can penetrate the pillar structure and reach the waxy surface, it is in the merely hydrophobic Wenzel state, named for Robert Wenzel, who found the phenomenon in nature in 1936.

The scientists further said if it the droplet cannot penetrate the pillars to touch the surface, the structure is in the super hydrophobic Cassie state, named for A.B.D. Cassie, who discovered it in 1942, and the droplet rolls away.

“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,” Zeng said.

Using the supercomputer enabled the researchers to conduct thousands more repetitions than would have been possible in a lab, and they didn’t have to worry about variables such as dirt, temperature and airflow. The team could even control the size of droplets down to the exact number of molecules.

A research paper describing the study has been published in the online edition of the Proceedings of the National Academy of Sciences. (ANI)

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