Super Water Repellent

Super Water Repellent

Engineers have tapped the hairs on spider legs to develop the ultimate water-repellent surface.


A near-perfect water repellent. I'm Bob Hirshon and this is Science Update.

University of Florida engineers have developed a surface so water-repellent that droplets roll off it like ball bearings. They modeled it on spider legs, which are covered with tiny, water-repelling hairs. Materials scientist Wolfgang Sigmund says they tried to improve upon the hairs' naturally crinkled, irregular pattern—but it turned out Nature knew best.

The perfect structures work, of course, but they don't work as well as structures that are actually more damaged-looking.

Sigmund says the material's nearly frictionless interaction with water may have all sorts of applications. For example, it could reduce the energy needed to pump water through plumbing. It could also be used in easy-clean coatings, since water picks up dirt and debris as it rolls off the surface. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

When engineers want to make a useful material, they often turn to nature as a model. After all, millions of years of natural selection has produced some biological materials with remarkable properties. Spider silk, for example, has a tensile strength greater than high-grade steel; tooth enamel is harder than many minerals and man-made materials; gecko feet can stick firmly to surfaces and yet lift off easily, which allows the lizards to climb walls and ceilings.

In this case, the researchers looked at tiny hairs on spider legs—hairs that create an extremely hydrophobic, or water-repellent, surface. Water-repellent legs help keep spiders from drowning, and water spiders even use their legs to carry air bubbles underwater, so they can breathe while they hunt for prey. Looking under a microscope, Sigmund's team found that a spider's leg hairs tend to be bent and irregular-looking. Assuming that this was just a sign of wear and tear, his team tried to do nature one better by making tiny plastic, hair-like structures of uniform length and shape.

As it turned out, the imperfect hairs on the spider legs worked better. It's not clear why, but somehow the random, chaotic nature of the surface seems to keep water bubbles from sticking to it. High-speed photography showed that water droplets below a certain size rolled off the surface as perfect spheres. On other water-repellent surfaces, the droplets still flatten out and distort slightly, because of residual friction between the water and the material. 

Sigmund's team says the material is closer to frictionless than any other man-made material. Unlike other water-repellent materials, it's also been created by manipulating the physical structure of the material itself, not just by mixing chemicals. As you heard, one possible application is in energy savings: if water pipes could be coated with this material, it could reduce the amount of energy needed to pump water through them. Considering how extensive the world's plumbing systems are, even a small improvement in efficiency could mean big savings. 

Now try and answer these questions:

  1. How do spiders benefit from water-repellent legs?
  2. How did the researchers try to improve on nature? What did they learn?
  3. What does it mean to say the surface was created using physics, not chemistry?
  4. What are some advantages of a highly water-repellent surface? Can you think of others not mentioned in this article?

Going Further

For Educators

Project WET by the Iowa Academy of Science, is an interdisciplinary water education program designed to supplement a school's existing curriculum. The site contains a link to a page explaining basic chemical properties of water.

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