Surface tension

Surface tension is a property of liquids that describes the energy required to create a surface. It can be measured either as force per unit length or as energy per unit area—these two measures are equivalent. When speaking in terms of energy per area, the term "surface energy" is often used, and it applies more broadly, including to solids. For liquids, we typically say "surface tension," while for solids, "surface stress" or "surface energy" are more common. One clear sign of surface tension at work is that still liquid surfaces naturally try to become as small as possible.

This happens because of the forces between molecules. Inside the liquid, a molecule is pulled equally in all directions by its neighbors, so the net force on it is zero. But molecules at the surface have fewer neighbors above them, so they are pulled inward. This inward pull creates internal pressure and forces the surface to shrink to the smallest possible area. There is also a tension running parallel to the surface that resists any force trying to stretch it, again due to the cohesive forces between molecules.

Cohesive forces act between molecules of the same type, while adhesive forces act between different types. The balance between these two determines how well a liquid wets a surface, the contact angle, and the shape of the meniscus. If cohesion is stronger (specifically, if the adhesion energy is less than half the cohesion energy), wetting is poor and the meniscus is convex at a vertical wall—like mercury in glass. If adhesion dominates (adhesion energy more than half of cohesion energy), wetting is good and the meniscus is concave, as with water in glass.

Surface tension also shapes liquid droplets. Though easily deformed, water droplets are pulled into a sphere by the imbalance of cohesive forces at their surface. In the absence of other forces, nearly all liquid drops would be spherical. This shape minimizes the necessary "wall tension" of the surface layer, as described by Laplace's law.

Another way to understand surface tension is through energy. A molecule next to a neighbor has lower energy than one alone. Interior molecules have as many neighbors as possible, so they are in a low-energy state. Surface molecules, missing some neighbors, have higher energy. To minimize its total energy, the liquid reduces the number of these high-energy surface molecules, which results in the smallest possible surface area. Consequently, a liquid surface will assume a smooth shape.

Surface tension is represented by the symbol γ (or sometimes σ or T). Its SI unit is newtons per meter, but the older cgs unit of dynes per centimeter is also common. For example, 1 dyne/cm equals 1 erg/cm², which is 0.001 N/m or 0.001 J/m².

Surface tension can be defined either by force or by energy. In force terms, γ is the force per unit length. For instance, in a rectangular frame with three fixed sides and one movable side, surface tension pulls the movable side inward. The force needed to hold it in place is proportional to the length of that side.

Because water molecules strongly attract each other through hydrogen bonds, water has a relatively high surface tension—72.8 millinewtons per meter at 20 °C—compared to most other liquids. This allows objects denser than water, like razor blades or water striders, to rest on the surface without sinking. The magnitude of surface tension depends on the forces between surface molecules, so surfactants (such as detergents) are often used to lower it, allowing a liquid to make better contact with another material. Surface tension also creates pressure inside bubbles and is responsible for many other phenomena; it is a classic, well-studied property common to all liquids.