Wikipedia:Picture of the day/November 16, 2005
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Surface tension is caused by the attraction between molecules of a liquid, due to van der Waals forces. In the bulk of the liquid, molecules are pulled in all directions, resulting in a net force of zero. At the surface, molecules are pulled inwards, but there are no liquid molecules on the outside to balance these forces, so the surface molecules are subject to an inward force of molecular attraction which is balanced by the resistance of the liquid to compression. Photo credit: W. M. Connolley |