Step-by-step explanation:
The observations that water rolls off a duck's back but thoroughly wets a head of human hair reveal that the surfaces of the duck feathers and human hair have different physical and chemical properties.
Duck feathers have a unique structure that helps them repel water. They are coated in a special oil that makes them hydrophobic, or water-repelling. The oil forms a layer on the surface of the feathers that prevents water from penetrating into the feather structure. Additionally, the feather structure is tightly packed and has a lot of surface curvature, which also helps to prevent water from sticking to the feathers. This is why water rolls off a duck's back.
In contrast, human hair does not have a hydrophobic coating, and its surface is relatively smooth. This means that water can easily stick to the surface of human hair and thoroughly wet it.
Overall, these observations reveal that the chemical nature of the surfaces of duck feathers and human hair are different, and that these differences have a significant impact on how they interact with water.