MIT researchers have unraveled exactly how water birds like ducks and cormorants keep dry when diving in up to 100 feet of water. The secret is a combination of water-repelling oil the birds spread on their feathers during preening and the tightly interlocking structure of the feather’s barbs and barbules.
By testing and modeling the action of water on a feather, they were able to see that the bird’s plumage doesn’t totally repel the liquid and can actually get wet when immersed. But the bird’s preening oil increases the energy needed for water to wet the feather. When the animal leaves the water, the wetting is reversed and the water is ejected off the feather.
"If a feather gets wet, there is no need for it to dry out, in the traditional sense of evaporation,” says Robert Cohen, a chemical engineering professor on the research team. “It can dry by directly ejecting the water from its structure, as the pressure is reduced as it comes back up from its dive.”
Their findings, they say, could be useful in making surfaces with similar characteristics. If engineered with characteristics seen in diving birds, a hydrophobic coating that becomes wet would still be able to slough off the water and become almost immediately dry with a simple shake.
(After being coated with a special solution, a feather from a common shelduck repels both the oil (red) and water (blue), causing them to bead up on the surface rather than spreading out.)
Their work also showed how damaging water from something like an oil spill can be for diving water birds. Oil immersion (as seen in the right-hand boxes in the gifs above) fully wets the feather.
Andrew Parker, a researcher at London’s Natural History Museum who took part in the study, said the way the feathers of diving birds are constructed work precisely to the depths the animals submerge. “The results of this project show that the diving birds are only just adapted, partly through the microstructure of their feathers, to reach their maximum diving depths, without suffering permanent effects,” he said. “It is one of the most amazing examples of evolution and adaptation, with not a trace of overengineering.”
Images and video courtesy Justin Kleingartner and Siddarth Srinivasan/MIT.