2003 HS Photo Contest Pictures
2003 High School Photo Contest Winners
Third Place
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Natural Category
Liquids sometimes form drops, and at other times they spread over a surface and wet it. In this photo both these phenomena are shown, as water comes in contact with the surfaces of a leaf. The characteristics of water droplets on a leaf are affected by the water surface tension, adhesion to the surface of the leaf, and gravity. In a single water droplet, millions of molecules are held together by their dipolar attractions, which leads to strong surface tension. If the water droplet comes in contact with a solid for which it has little or no affinity, the liquid drop can sit on the solid's surface. An example of this is visible on the left side of the photo, which features the underside of a leaf. But with strong affinity, the liquid wets the surface and spreads out in a thin layer. Such sheeting is occurring on the topside of the leaf visible on the right. When the water reaches the tip of the leaf, the strong water surface tension takes over and droplets are formed. Although leaf surfaces can be described by over 25 different characteristics, a primary surface feature is the presence or absence of hairs. Leaves with smooth leaf surfaces, such as the top surface on the right, are readily wetted, whereas tiny dense hairs on the underside of the leaf create a rough surface, allowing the drops to sit on the leaf's surface. The relative energy of the surfaces contributes to how the leaf and water interact, with this photo showing two different results.
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