New Artificial Surface Attracts or Repels Water on Demand

In nature, different structures have different attitudes toward water, according to their chemical composition and geometry. Some are hydrophilic (they attract water molecules) and some hydrophobic (they repel them). But so far, no material has been known to be able to shift between the two states.

Now, scientists at the National Institute of Standards and Technology (NIST) have created an easy method of constructing test surfaces that have variable affinities for water, so that the same surface can range from superhydrophilic to superhydrophobic, and everything in between.

The researchers have come up with a new and flexible approach based on ultraviolet light and photosensitive materials, but their inspiration came from observing one of nature's champion at collecting water where it's really scarce.

It's called the Stenocara beetle, living in the Namib Desert of Africa, one of the driest places on Earth. It survives by raising its warty-looking wing covers into the breeze, these beeig highly hydrophilic and able to collect every water molecule that strikes them. Then water is pushed upwards and collected on the bumps, where it condenses into tiny drops that roll into the insect's mouth.

To mimic the beetle's ability to quickly shift its surface structure and chemistry from hydrophobic to hydrophilic, the scientists coated the experimental surface with a matrix of silica granules about 11 nanometers across. The physical disposition of the particles makes every water droplet placed on the surfaces sag into the gaps before it is stopped by the surface tension.

The second step was to coat the silica granules with a chemical compound that altered their water affinity, just like a waxy substance makes some of the beetle's microparticles hydrophobic.

But the real innovation comes from using an optical technique that is so easy to modulate that it can be performed in the surrounding air: they again coated the granules with a light-sensitive material and exposed them to UV light.

The result was a premiere, as that area became more and more hydrophilic as the exposure time and intensity increased.

There are already potential applications for the new method, like paints or adhesives testing surfaces and even collecting water in dry regions to open-air microchannel devices.