Science Looks to the Butterfly to Develop Advanced Hydrophobic Surfaces

By Wesley Fenlon

Shed water like a butterfly, sting like a bee.

The Lotus effect is one of those elements of nature science has envied, and attempted to copy, for many years. The surface of the lotus leaf is incredibly water repellant, and hydrophobic coatings are based on studies of the lotus leaf and other elements in nature that demonstrate a similar effect. And now science has discovered an even more impressive hydrophobic surface in nature: a butterfly's wings.

And not just any butterfly, those butterflies do, in general, have hydrophobic wings that can help them survive a downpour. Smithsonian Mag writes that the blue morpho butterfly of South America's rainforests has special ridged patterns on the surface of its wings. Their wings are even more adept at staying dry than the lotus leaf.

How does the lotus effect work, anyway? In both the blue morpho butterfly and the lotus leaf, a ridged surface is responsible for casting off unwanted water droplets. Ask Nature offers a deeper explanation:

"Lotus leaves, for example, exhibit extensive folding (i.e., papillose epidermal cells) and epicuticular wax crystals jutting out from the plant's surface, resulting in a roughened microscale surface. As water and air adhere less well than water and solids, roughened surfaces tend to reduce adhesive force on water droplets, as trapped air in the interstitial spaces of the roughened surface result in a reduced liquid-to-solid contact area. This allows the self-attraction of the polar molecule of water to express more fully, causing it to form spheres. Dirt particles on the leaf's surface stick to these droplets, both due to natural adhesion between water and solids and because contact with the leaf surface is reduced by over 95% from the leaf's micro-topography. The slightest angle in the surface of the leaf (e.g., caused by a passing breeze) then causes the balls of water to roll off due to gravity, taking the attached dirt particles with them and cleaning the leaf without using detergent or expending energy."

MIT engineering professor Kripa Varanasi, whose research team developed LiquiGlide--known for helping ketchup ooze freely out of a bottle--has written about the potential of making even more hydrophobic surfaces by reducing the contact time between water droplet and surface. His latest paper explains how a greater number of tiny surface ridges will cause droplets to break into smaller droplets, which bounce off a surface more quickly. That minimizes surface contact and makes for a dryer pair of Dockers. Or airplane wings that won't build up dangerous frost, or perhaps windshields that clean themselves.

Varanasi's latest study reduced the surface contact time by about 40 percent. He's shooting for a 70 - 80 percent reduction.