Water off a Duck's Back Duck feathers, and especially those on the back, have the remarkable property that water does not stick to them. It simply rolls off like a marble. Via: John Haslam (Flickr) Feathers are mostly made of keratin, and while keratin is a hydrophobic material, this alone does not explain the remarkable repelling abilities of a duck. Consider that mammals' hair is made of the same material, but very easily becomes soaked in water. It was once believed that the repellance comes from preening oil which coats the surface of the feathers, but this again is insufficient to model the repellance of duck feathers. Water repellance of a material can be measured by the water contact angle- the angle between the surface of a drop of water and the surface that it is on. The more hydrophobic a material is, higher this number is. Materials such as wood are extremely hydrophillic, and have an angle close to zero. A material with no attraction at all to water would have an angle of 180.  Oils typically have an angle of around 120, while duck feathers are typically near 150. Oil cannot alone be the reason for the hydrophobic nature of duck feathers. The reason that duck feathers exhibit such high repellance is in the nanostructure of the material. The surfaces of duck feathers are very uneven, which creates air pockets on the surface. SEM photograph of a duck's feather at various scales. Via Yuyang Liu  Water exhibits almost no attraction to air, which causes very low friction between the surface and the water. This allows water to slip over the surface of the feathers with ease. Another source of repellance in duck feathers comes from the cohesive nature of water. The water molecules are held together by cohesive forces between the water molecules due to hydrogen bonding. The attractive force between the keratin material of the feather and the water molecules cannot overcome that of the cohesiveness of the water droplet, because the water must conform to the surface of the feather. Because the surfaces have complicated nanostructures, the water cannot be forced into the tiny gaps in the feathers by only adhesion and gravity. The water droplet stays together as a result, and the duck's feathers stay dry.