The African darter duck is known to dive up to 35 meters without getting wet due to the microstructures of their feathers. Now, Michael Rubner, Gareth McKinley, and Robert E. Cohen from the Massachusetts Institute of Technology, Andrew Parker at the Natural History Museum, London, and their colleagues have correlated the birds’ diving behavior with the microstructures of their wings. Their goal is to apply what they learn to create friction-reducing surfaces on water-going vessels.
By placing drops of different fluids on the birds’ wings, the researchers found the “contact angle” to be very high. But those experiments were carried out in the open air. When ducks are immersed in water, the microstructures on their feathers entrain tiny pockets of air, forming an air film called a “plastron” which prevents water from wetting the feathers. Waterways, however, often contain an additional component: oils. Oils are known to wet bird feathers (observed as a low contact angle), which is why oil spills are particularly devastating to bird populations. Thus, before this technology can be successfully applied to ocean-going vessels, it is imperative to develop a mechanism for preventing oil from wetting the surfaces. As reported by the researchers in the July issue of the Journal of the Royal Society Interface (DOI: 10.1098/rsif.2014.028), the key is replacement of the preening oils on feathers with a very low-energy fluorinated polymer composite, containing molecules known as fluorodecyl polyhedral oligomeric silsesquioxanes or F-POSS.
The coated duck wings also allowed the researchers to study the role of just the microstructures in the feathers’ wetting behavior, essentially taking the variation of the ducks’ preening oil out of the picture. This helped them understand the important contribution of larger scale defect structures that also are always present in feathers. In other words, the natural gaps in the duck feathers’ micropattern prevent the duck feathers from staying dry to as great a diving depth as theory would predict. As a result, the plastron layer collapses and the feathers get wet beyond a certain diving depth.
However, the researchers’ calculations show that feathers spontaneously dewet when the bird comes back up to the surface. The researchers speculate that there is a chance that the observed “wing spreading” behavior of birds such as cormorants helps them rapidly recover dry feathers after a deep dive. “I don’t think you’ll see ships that are able to ‘stretch out their wings,’” quipped McKinley, emphasizing that this work has augmented the understanding of the defect sensitivity of these superhydrophobic microstructures, a key to designing real surfaces that perform as desired.
