LINCOLN, NEBRASKA – Self-cleaning fabrics and micro-robots that walk on water are just a little closer to reality, thanks to new research into super hydrophobia from the University of Nebraska-Lincoln (UNL) and Japan’s RIKEN institute.
Super hydrophobia is the property which allows water to bead up and roll off flowers, caterpillars and some insects, and which gives insects like water-skippers the ability to walk on water.
“Engineers especially like the water strider because it can walk on water,” said Xiao Cheng Zeng, Ameritas university professor of chemistry at UNL. “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.”
Such organisms 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.
Using the superfast supercomputer at RIKEN – the fastest in the world when the research started in 2005 – the team designed a computer simulation to study how surfaces behaved under different conditions. Zeng and his colleagues used the RIKEN computer to “rain” virtual water droplets of different sizes and at different speeds on surfaces that had pillars of various heights and widths, and with different spacing between the pillars.
They learned there is a critical pillar height, depending on the particular structure of the pillars and their chemical properties, beyond which water droplets cannot penetrate. 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). 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,” Zeng said. “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 that computer modeling was more effective than experimenting that in a laboratory. It allowed for more repetitions, and eliminated variables such as dirt, temperature and air flow. It also allowed the researchers to control the size of droplets down to the exact number of molecules.
The research will be published this week in the online edition of the Proceedings of the National Academy of Sciences.