Penguins swim in some of the world’s coldest waters, yet their feathers manage to stay ice-free. Now, researchers working with Humboldt penguin feathers reveal that their anti-frosting and anti-icing properties are created by microscopic structures that repel the coolest and tiniest of water droplets. The findings are published in Journal of Physical Chemistry C this week.
Humboldt penguins, Spheniscus humboldti, live from the coast of Peru all the way to the tip of southern Chile. The temperature in some of these places drops down to -10°C (14°F) in the wintertime, and the water they swim in is part of a cold ocean current from the Atlantic that’s always less than 0°C (32°F). Their thick layer of feathers prevents the frigid seawater from penetrating down to their skin, and because they’re such fast swimmers, we used to think that water would slide off before it could freeze. But even superhydrophobic (or extremely water repellent) surfaces can get icy under high humidity or ultra-low temperatures.
To investigate, Beihang University’s Jingming Wang and colleagues gathered up body feathers of Humboldt penguins from Beijing Pacific Underwater World, cut them up into little square pieces, and then examined them using a scanning electron microscope.
The feathers, the team found, have excellent hydrophobic and anti-adhesion characteristics – even against supercooled water microdroplets. They’re comprised of rough, hierarchical, tiny structures that trap air to prevent water from adhering and coalescing. These micro- and nanostructures include a network of barbs, elaborate wrinkles on barbules projecting off the barbs, tiny interlocking hooks called hamuli on the barbules, and also specially oriented, 100-nanometer-deep grooves on the barbs (see image below).
Inspired by the feathers’ icephobic structure, the team developed an artificial replica: a polyimide nanofiber membrane with air-infused hierarchical rough structures. Their material was less icephobic than penguin feathers in tests, but with some adjustments, it could be sprayed with supercooled water droplets at an environmental temperature of -5°C (23°F) for up to four hours without frost or ice forming. The team thinks this ice-proof material could someday have applications such as electrical insulation.
Hierarchical micro- and nanostructures. 2016 American Chemical Society