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Surface tension is a property of a liquid’s surface that causes it to resist external forces. You can see examples of it when an object denser than water is able to float anyway and in insects that ‘flit’ across water. It is caused by cohesion(intermolecular attraction) of like molecules, and is responsible for many of the behaviors of liquids.
The cohesive forces of liquid molecules are responsible for 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 molecules on top of them and are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area. It is responsible for the shape of liquid droplets. Water droplets tend to be pulled into a spherical shape by the cohesive forces of the surface layer. In the absence of other forces(gravity) drops of liquids would be perfectly spherical.
Another way to look at is this: a molecule in contact with a neighbor is in a lower state of energy than if it were alone. The interior molecules have as many neighbors as they can possibly have, but the boundary molecules do not and have a higher energy. For the liquid to minimize its energy state, the number of higher energy boundary molecules must be minimized. The minimized quantity of boundary molecules results in a minimized surface area. As a result, a surface will assume the smoothest shape it can. Since any curvature in the surface shape results in greater area, a higher energy will also result, so the surface will push back against any curvature.
Surface tension can also be defined as the force along a line of unit length, where the force is parallel to the surface but perpendicular to the line. One way to picture this is to imagine a flat soap film bounded on one side by a taut thread. The thread will be pulled toward the interior of the film. Surface tension is measured in forces per unit length and stated in newtons per meter. In thermodynamics, it is expressed as work done per unit area. This work is stored as potential energy. Consequently surface tension can be also measured as joules per square meter.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.