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In order to understand how a drop of liquid will interact with a solid surface, it is important to understand the surface of the liquid. At the air-water interface is a thin region in which the water behaves differently than the rest of the solution, a property known as surface tension. In this local region, the water acts like a solid, allowing small dense objects to sit on the surface of the water without getting wet. For instance, insects such as the water strider (family Gerridae) use this principle to walk on water while young scientists make paper clips float in their science fair projects. In order for a drop of water to make a surface wet, the surface tension barrier must be broken.
Surface tension is a complex nanoscale phenomena that is still not fully understood by scientists. However, it can be explained in simple terms based on intermolecular interactions. Water molecules tend to stick to one another by hydrogen bonding and van der Waals forces. While molecules in the center of a drop of water have many neighboring molecules to interact with, those on the surface of a drop have only about half as many to choose from. Therefore, they are more likely to stick to neighboring molecules that are also on the surface. This principle can be illustrated by a chess analogy (see figure). A knight in the center of a chessboard has 8 possible moves while one on the edge has only 2-4, hence the old adage, 'a knight on the rim is dim.'
Next page > Evaporation Cooling and Surface Tension
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Keywords: surface tension wetting engine wear medical products nanoscale topography drop liquid water gerridae science fair chess analogy knight square edge corner tribology nanotechnology