Superhydrophobic - Blog Posts

If you dropped a water balloon on a bed of nails, you’d expect it to burst spectacularly. And you’d be right – some of the time. Under the right conditions, though, you’d see what a high-speed camera caught in the animation above: a pancake-shaped bounce with nary a leak. Physically, this is a scaled-up version of what happens to a water droplet when it hits a superhydrophobic surface. 

Water repellent superhydrophobic surfaces are covered in microscale roughness, much like a bed of tiny nails. When the balloon (or droplet) hits, it deforms into the gaps between posts. In the case of the water balloon, its rubbery exterior pulls back against that deformation. (For the droplet, the same effect is provided by surface tension.) That tension pulls the deformed parts of the balloon back up, causing the whole balloon to rebound off the nails in a pancake-like shape. For more, check out this video on the student balloon project or the original water droplet research. (Image credits: T. Hecksher et al., Y. Liu et al.; via The New York Times; submitted by Justin B.)

Coating Makes Steel Tougher, Keeps Microbes From Sticking

More and more objects are getting superhydrophobic coatings that make liquids bounce right off. Surfaces with complex nanoscopic structures that prevent wetting will soon be deployed on wind turbine blades and aircraft wings to prevent ice from sticking, and even concrete is being doped with superhydrophobic compounds to help it last decades longer.

Much still needs to be done, though, to strengthen these coatings because any damage can remove the ability to repel liquids. Such an advance is hugely important since there are potentially life-saving healthcare applications if this hurdle could be overcome with a stable, nontoxic coating for steel. Just imagine if implants, scalpels and other tools used on patients had a surface impossible for infection-causing microbes to cling to.

Now, Joanna Aizenberg and her colleagues at Harvard’s Wyss Institute for Biologically Inspired Engineering have demonstrated a possible solution. They’ve been able to coat stainless steel with nanoporous tungsten oxide, which repels all liquids. What’s more, the surface is extremely tough, maintaining superhydrophobicity even after being scratched with sharp steel objects and diamond.

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