Fluid Repellency
Ducks are generally recognized as having perfected a means of repelling water. A classic paper, by A.B.D. Cassie and S. Baxter (Trans. Faraday Soc. 40, 1546, 1944) explains that ducks accomplish this by a particular micro-structure of their feathers and not by some chemical coating. A duck’s feather consists of barbs on either side of a main shaft. Along the barbs there are fine fibers extending out on either side. On one side the fibers have notches while on the other side there are hooks. This arrangement allows an engagement of the fibers from neighboring barbs to bond together forming a connected structure.
Non-Wetting Flow over
a Barb Fiber Structure
The barb-fiber
structure has a large amount of open space. The diameter of the fibers is about
8μm but the distance between adjacent, parallel fibers is
about 5 diameters, center to center. Experiments show that the static contact
angle for water on the solid portion of a duck’s feather is on the order
of 100°. With this combination of a non-wetting static contact angle and
regular micro-structure, water placed on the feathers will easily roll off without
penetrating through them.