What is Surface tension?
{{Continuum mechanics|cTopic=[[Fluid mechanics]]}} '''Surface tension''' is an effect where the surface of a [[liquid]] is strong. The surface can hold up a weight, and the surface of a water droplet holds the droplet together, in a ball shape. Some small things can float on a surface because of surface tension, even though they normally could not float. Some [[insects]] (e.g. [[Gerridae|water striders]]) can run on the surface of water because of this. This property is caused by the [[molecule]]s in the liquid being attracted to each other ([[cohesion]]), and is responsible for many of the behaviors of liquids. Surface tension has the [[dimensional analysis|dimension]] of [[Force (physics)|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]]—which is a more general term in the sense that it applies also to [[solid]]s and not just liquids. In [[materials science]], surface tension is used for either [[surface stress]] or [[surface energy|surface free energy]]. ==Causes== [[Image:WassermoleküleInTröpfchen.svg|thumb|Diagram of the forces on molecules in liquid]] [[File:Surface tension March 2009-3.jpg|thumb|left|Surface tension prevents the paper clip from submerging.]] The cohesive forces among the liquid molecules cause surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some [[internal pressure]] and forces liquid surfaces to contract to the minimal area. Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. In the absence of other forces, including [[gravity]], drops of virtually all liquids would be perfectly spherica