Water Off A Duck’s Back? Meet The Driest Material Ever


water drop bbcSource:  http://www.bbc.co.uk/news/science-environment-25004942

Imagine coming inside from the torrential rains of a thunderstorm, your face and hair dripping wet, but your clothes are bone dry.  And you didn’t need to shake your umbrella off at the door: it also hasn’t a drop of rain on it either!

Those are two of the applications being dreamt-up by a team of engineers who have created “the most waterproof material ever,” a surface 40% better at repelling drops of water than that of any surfaces known to exist in nature.  And the researchers are gleaming with excitement for its possible uses, which range from simple clothing to enhancing aircraft safety.

“We believe these are the most super-hydrophobic surfaces yet,” said Professor Kripa Varanasi, who led the team which developed the material at Massachusetts Institute of Technology in Boston.  Varanasi’s lab recently made headlines with their invention of LiquiGlide, a substance which, when applied to the inside of bottles, wrings every drop of ketchup out of the container.  LiquiGlide won the team awards, but the MIT engineers weren’t content to rest on their laurels and began work on a material with potentially even greater uses in the larger world.

Amazingly, this “super-hydrophobic” surface owes its amazing abilities not to the material itself, but engineering techniques that take advantage of the structure and physics of water droplets.  During high-photography analysis of the “driest” surfaces in nature known to exist—those of the lotus leaf, nasturtiums and the wings of the Morpho butterfly—the MIT team found that macroscopic structures had differing effects depending on the layout.  In the case of lotus leaves, water drops flattened “like a pancake” and then bounced back to a symmetrical form.  When analyzing the butterfly wings, however, droplets split into asymmetric pieces.  The disparity is owed to how the leave and wing surfaces are arranged: long parallel structures in lotus leaves, and a criss-crossing pattern covering the fragile butterfly wings.

Upon closer inspection, it was found that the tiny ridges of lotus leaves, which have inspired the “lotus effect” already employed in some manufacturing, have a surface with tiny parallel grooves providing “a high contact angle” that minimizes physical contact with the droplets themselves.  The engineers—who have published their findings in the journal Nature—were able to replicate that surface on a piece of silicon that had similar ridges, relatively larger, that increased the surface area and enhanced the repelling effect.  The team experimented with silicon surfaces before applying the same technique to metals, ceramics, and fabrics.  The results were consistently the same: a 40% increase in water’s “bouncing” off the surfaces found in nature.

However, butterflies and nasturtium leaves are different: the criss-cross arrangement makes droplets divide into multiple “pieces.”  The cause of the effect is the same: an increase in surface area.  “For years industry has been copying the lotus,” noted Varanasi.  “They should start thinking about copying butterflies and nasturtiums.”

The lotus-mimicking effect is already applicable to current manufacturing, with already existing milling machinery and fabric weaving.  But Varanasi’s team is looking at even more drastic improvement: “I hope we can manage to get a 70 to 80% reduction (in contact time). There’s a lot of room left.”

With their discovery, it may be possible to soon create products far surpassing everyday clothing: ice-resistant aircraft wings and fuselages, electrical equipment and the turbines of a wind farm.  “We have just opened a window into this world where people really think: what is super-hydrophobicity?” said Varanasi.  “Can we go beyond this?  There could be other species in the natural world which are even better.”

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