You may notice sometimes, when you contain water in some container, that the surface of the water is not completely straight but slightly curved. this is called surface tension. what's surface tension?
Surface tension is a property of the surface of a liquid. It is what causes the surface portion of liquid to be attracted to a different surface, such as that of a liquid.
Surface tension is caused by the attraction of molecules to like molecules, also known as cohesion. Since the molecules on the surface of the liquid are not surrounded by like molecules on all sides, they are more attracted to their neighbouring molecules on the surface. Surface tension has the measurements of force per unit length, or of energy per unit area. The two are equivalent—but when referring to energy per unit of area, people use the term surface energy and not surface tension—which is a more general term in the sense that it applies also to solids and not just liquids.
In materials science, surface tension is used for either surface stress or surface free energy.