What is the Trick to Making the Most Waterproof Stuff on Earth?

It’s all in the texture. An MIT-led team of mechanical engineers is creating a super water-resistant material inspired by the wings of butterflies

Credit: Kripa Varanasi and his colleagues

To a butterfly, even the slightest torrential downpour can feel like getting pelted by a barrage of bowling balls. And as the insects take cover, the slightest residual moisture touching their wafer-thin wings can also hinder their ability to fly. Fortunately, the little critters evolved uniquely textured wings that excel at repelling water and dirt.

Scientists have known this for some time. But it’s the remarkable wingspans belonging to the blue morpho butterfly, a tropical species native to rainforest regions of Central and South America, that's of particular interest to scientists as of late. The special ridged patterns on the surface, similar to nasturtium leaves, has been found to repel liquids at a rate that surpasses even the heralded water-shedding capabilities of the lotus leaf

"For years industry has been copying the lotus. They should start thinking about copying butterflies and nasturtiums," MIT engineering professor Kripa Varanasi tells BBC News. "We believe these are the most super-hydrophobic surfaces yet."

Varanasi is best known as the head of the research team that developed LiquiGlide, a slippery surface technology that's been shown to enable ketchup to slide easily out of the bottle. His latest findings, published in the journal Nature, demonstrate how another fabricated material featuring this added wrinkle may indeed be the most water-resistant stuff on earth.

So, how does this novel material work? As shown in the video, the silicon surface features raised ridges no more than 0.1 millimeter (1/250th of an inch) high, which cause falling droplets of water to flatten like a pancake before immediately breaking apart into smaller, scattered droplets. The smaller the droplets, the faster they bounce off a surface. The amount of time the liquid is in contact with the material is considerably less this way—about a third less, in fact, than it would be with other water-repellant materials.

Credit: Screen capture from video demonstration

So what's the great advantage of a material that can stay drier than the rest? Since smaller droplets are easily repelled, less water on a surface means less likelihood of frost buildup. Commercially, such a material would be of special interest to the aviation industry. Engineers are constantly looking for ways to prevent ice from forming on planes' wings; this frozen layer can alter airflow and put the aircraft at risk of stalling. Anti-icing systems that melt the ice are already built into airplanes to combat such issues, but a superhydrophobic coating would provide an additional safeguard against the risk. The ridged texture could also be applied to the blades of wind turbines for improved performance and fabrics, which could be used to design clothing that better shields us from the elements.

For now, Varanasi and his research team are working on structural tweaks that they hope will make the material even more water-resistant. They believe increasing the number of ridges may do the trick. "I hope we can manage to get a 70 to 80 percent reduction [in contact time]," he says in a press release. “We can reduce it further.”

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