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Friday, April 8, 2016

Wonder Material Mimics Desert Beetles and Cacti To Suck Water Out of Thin Air

Written by William Herkewitz

This is what they need for those moisture vaporators on Tatooine in Star Wars.

Inspired by a beetle's shell, a cactus spine, and the skin of a carnivorous plant, this slick, bumpy coating is a material straight out of Dune or Star Wars. Put this water-saving coating into a rain of steady droplets it condenses airborne water vapor into liquid 10 times faster than any other known material.
Mechanical engineer Kyoo-Chul Park led the team of researchers that developed this new coating, which is outlined in a study in the journal Nature this week. Such a material would be quite useful in any machine with a heat exchanger that relies on water collection for heat transfer. "That's anything from a thermal power plant to a water distillation plant, or a humidification system and any HVAC system used in cars, trains, or airplanes," he says. But Park is also a big fan of a galaxy far, far away, so he's not limiting his imagination to just air conditioners.

Time-lapsed images of droplets condensed on slippery surfaces. The fast growing droplets on the slippery asymmetric bumps are delivered to the bottom of the slope at a size where they can then be transported by gravity, while droplets on the adjacent flat.

"I'm a big fan of Star Wars, and so you could imagine this kind of condensation system being used simply for water collection on a moisture farm, such as the one on Tatooine where Luke Skywalker was raised," Park says. "The basic idea is already here: harvesting liquid water from air-bound vapor in an extremely arid region."

Park's material was inspired by materials found in nature, on three extremely different organisms from opposite ends of the Earth. The researchers combined innovations from the bumpy shell of an African desert beetle, the skin of a carnivorous plant, and the uniquely shaped spines of a cactus. Together, facets of these biological materials encourage the condensation of big water droplets, and then hurry those droplets down the coating.

The initial inspiration came from an extraordinary and peculiar insect indeed, the Namib desert beetle. This thirsty bug survives in its arid environment by condensing water droplets on its bumpy back in the early morning. Years ago, a study observed that this condensation effect was partly due to a waxy coating found on the beetles' knobby exoskeleton. But Park wondered: What role did the bumps play?

After studying the effects of a material with the exact same sized bumps in his laboratory, Park and his colleagues discovered that the bumps are vital for gathering dew. While the process is still not entirely understood by the researchers, Park and colleageus know that water tends to condense into larger droplets at the peak of each bump. One reason that that there's just more air around the bulges' peaks. (Imagine standing on the summit of a hill versus on a flat grassy field—there's more sky around you on the hill.) That extra air means air-bound water vapor has a higher chance of coming in contact with the surface.

Rather that simply mimicing beetle bumps, Park's team ingeniously borrowed two more innovations from nature. The first was molding each bump to form a semi-triangular ramp on one side. These ramps wedge open downward, towards where the water is collected. This is a concept borrowed from certain types of cactus spines, which leverage a triangular-wedge shape to guide growing dew droplets.

Here's how the ramp works: As a water droplet grows in size, it's forced to move farther down the ramp to fit on that wedge. Because of water's high surface tension, droplets prefer to slide down the expanding ramp to fit rather than break apart. As the droplets move down the ramp, they'll pick up more tiny water droplets in their path, growing further still. Because of these ramps, droplets roll down the material faster than they would due to the pull of gravity alone. In fact, Park and his colleagues found that this effect is so strong, droplets would actually move against gravity when sliding along a skyward facing ramp.)

The third innovation by Park and colleagues was to coat their bumpy ramps with a slick finish that's similar to that found on tropical, carnivorous pitcher plants. These plants fill with a watery digestive fluid to trap inspecting insects; their slippery coating ensures water (and food) stays in the pitcher. On Park's new material, this slick coating helps water droplets zoom down the cleverly designed surface.

3D profilometry image of slippery asymmetric bumps. Blue to red color gradient corresponds to the lowest to the highest regions of the sample.

The three biological innovations work in unison. The bumps cause water to condense into big drops, the ramps on the side of the bumps force those droplets to quickly slide off, and the slick coating speeds up the whole process.

After experimentation, Park found that his team's new coating could collect "10 times more water in one hour, compared to the next best, state-of-the-art material" he says. While there's still more work to be done, Park says that further preliminary results hint at the fact that after one hour, the coating is still comparatively better at collecting water.

Hello, moisture farming.

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