Have you ever seen small insects walking on the water? Or small objects, such as needles or clips, floating on its surface? These amazing facts are due to surface tension. Without taking this property into account, the behavior of liquids cannot be understood. The surface tension in fact governs the shape of droplets and the contact of liquids with substrates.

Surface tension is due to intermolecular forces. In the bulk of liquids, each molecule is equally pulled in all directions by neighboring molecules, resulting in a net force equal to zero. At the surface, the molecules are pulled inwards by the molecules deeper inside the liquid, but not attracted with the same intensity by the molecules in the medium over the surface (air, for instance). Therefore, all the surface molecules are subject to an inward force of molecular attraction, acting until the liquid’s resistance to compression balances it. In this respect, the liquid surface behaves as a stretched elastic membrane. We find then a tendency to diminish the surface area, and then to observe the liquid squeezing itself until it has the lowest local surface area possible.

Surface tension has the dimension of force per unit length. This means that surface tension also has a dimension of energy per unit area, a dimension preferred when we are referring to this very same physics quantity, calling it “surface energy”. This is a more general term, which applies also to solids and not just liquids. If we consider the energy point of view, we consider a molecule in contact with a neighbor in a lower state of energy than if it is free. The bulk molecules all have as many neighbors as they can have. The boundary molecules instead have fewer neighbors and are therefore in a higher state of energy. To minimize its energy state, the liquid must minimize its number of boundary molecules and therefore minimize its surface.

If we consider solids, the surface energy quantifies the disruption of intermolecular bonds that occurs when a surface is created. The unit surface energy of a material would be half of its energy of cohesion: in practice. This is true only for a surface freshly prepared in vacuum. Surfaces often change their form because they are found to be highly dynamic regions, which readily rearrange or react, so that the energy is often reduced.