Surface tension is an effect produced at the boundary of a liquid that can cause the boundary to act like a thin, elastic film. For example, surface tension enables the surface of a pool of water to support some objects that would normally sink—including needles, razor blades, and certain insects—if they are placed carefully on the water. Surface tension can also draw liquid into a thin tube, an effect known as capillary action (see Capillary action ).
Surface tension occurs because molecules near the boundary of a liquid experience different forces than do molecules in the liquid’s interior. Deep inside the liquid, each molecule is attracted to other liquid molecules all around it. Such forces typically balance in all directions. However, molecules near the boundary also interact with molecules in the neighboring material. Such forces may not balance the attractive forces of the liquid molecules, producing surface tension. At the surface of a pool of water, for example, air molecules only weakly attract water molecules. The molecules inside the pool attract the surface water molecules more strongly, resulting in an overall inward force that causes the surface to act like a thin, elastic skin.
