If you learned that scientists have blended a darkling beetle, a cactus, and a carnivorous pitcher plant, you might imagine some unholy creation that’s all spines and scuttling legs and digestive enzymes. Instead, what Kyoo-Chul Park has made looks like … nothing at all. It’s not even alive—just a non-descript material covered in microscopic bumps. But this seemingly unremarkable surface has a remarkable ability—it excels at yanking water out of the air.
As a graduate student at MIT, Park worked on large mesh panels that could be erected in foggy areas and provide surfaces for the airborne water to condense upon. These “fog-catchers” are being tested in Chilean hillsides, as ways of providing drinking water to people in these extremely arid and remote places. When Park later moved to Joanna Aizenberg’s lab at Harvard University, he became interested in darkling beetles—animals that had come up with the same fog-catching idea, millions of years ago.
The beetles live in southern Africa’s Namib Desert, which gets less than a centimeter of rain every year. There is water here, however, in the form of fog that blows in from the Atlantic Ocean on the west. The beetles harvest the fog by sitting in it and pointing their abdomens skywards, allowing water to condense on their bodies and trickle down into their mouths. In this way, they drink their fill in one of the driest places in the world.
Many scientists have shown that the beetles have microscopic tricks for catching fog. Their backs are covered in little bumps made from water-attracting substances, with flat, water-repelling valleys between them. The bumps attract moisture and allow droplets to form, while the valleys channel the collected water away. These chemical patterns certainly help, but Park wondered if the bumps themselves, absent any water-attracting substances, might be important.
When he fashioned artificial bumps that match those on the beetle’s back, he found that water droplets would quickly grow on top of them, even if they were coated with water-repelling chemicals. And the smaller and more tightly curved the bumps, the faster the droplets grew. That’s fine, but it creates a problem for a would-be water-collecting surface: The droplets quickly max out the capacity of the bumps and stop growing. Park came up with two solutions.
First, he changed the bumps from spheres into rectangular pillars, with flat tops and curved edges. Now, droplets start condensing on the edges but eventually merge in the central plateau, freeing up the edges for yet more condensation.
Second, Park drew inspiration from cacti, whose spines are also excellent water-harvesters. By adding a ramp to his microscopic pillars, which allows the growing droplet to roll off, he was able to once again free up space for more condensation.
Park also coated the ramp with a material inspired by pitcher plants. These carnivorous plants trap insects in their vase-shaped leaves whose rims are exceptionally slippery; bugs that walk over them lose their footing and fall into the pool of digestive fluids below. In earlier work, Aizenberg developed a material based on the microscopic structure of the pitcher’s traps—an ‘omniphobic’ surface that repels water, ice, blood, crude oil, and even bacteria. She called it SLIPS—Slippery Liquid-Infused Porous Surface—and Park used it to line his ramps.
Together, these features produced a material that collects more than ten times more water than other state-of-the-art surfaces. Droplets form more quickly, reach larger sizes, and roll away more rapidly; they’ll even do so against gravity. All three features—the bumps, the ramps, and the SLIPS—are important; lose any one of them, and the material collects noticeably less water.
SLIPS Technologies, a Cambridge-based company that Aizenberg co-founded, is now trying to commercialize these surfaces. Collecting water in deserts is an obvious application. “In arid environments, there’s a lot of evaporation,” explains Park. “If we can’t collect water with fast growth and transport, we’ll lose it.” He adds that the super-condenser surfaces are also useful for power plants, desalination plants, and other operations whose heat exchangers rely on efficient condensation.
Daniel Beysens, a physicist who studies condensation at ESPCI ParisTech, says that the team’s work is impressive, but that only one of the three “bio-inspired” aspects—the SLIPS—reflect their natural counterparts. In reality, he says, droplets form more effectively between a darkling beetle’s bumps than on top of them, and they rarely run down from the tip of a cactus’s spines to their base.
Still, that doesn’t matter. “This beautiful experiment however shows that artificial surfaces inspired by nature can go beyond it, leading to new properties that are paradoxically not found in nature,” Beysens adds. “The next necessary step will be to make those surfaces robust to aging, unaffected by the unavoidable industrial or atmospheric pollution and contamination that is found in real life: a new challenge.”
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