Surface tension

What is surface tension?

Surface tension is the tendency of a liquid’s surface to contract to the smallest possible area. It can be described in two equivalent ways: as the energy needed to create a unit area of new surface, or as a force acting along the surface per unit length. Both lead to the same unit, millinewtons per meter (mN/m), which is numerically equal to the older dynes per centimeter.

The term normally refers to a liquid in contact with a gas, usually air. The same property at the boundary between two liquids that do not mix, such as oil and water, is called interfacial tension.

Why surface tension exists

A molecule inside a liquid is pulled equally in every direction by its neighbors. A molecule at the surface has neighbors only on one side, so it is pulled inward and sits in a higher-energy position. Enlarging the surface means bringing more molecules into that position, which costs energy. The liquid therefore resists any increase in surface area, which is why small droplets are nearly spherical and why some insects can stand on water.

Water has an unusually high surface tension because its molecules are strongly attracted to one another through hydrogen bonding. The International Association for the Properties of Water and Steam gives 71.97 mN/m for pure water at 25 °C. Surface tension falls as temperature rises. Most oils and organic solvents have much lower surface tensions than water.

How surfactants lower surface tension

A surfactant molecule has a water-attracting head and a water-repelling tail. In water, surfactant molecules migrate to the surface, where the tails can point out of the liquid. Replacing water molecules at the surface lowers the surface energy, so surface tension drops as surfactant concentration rises.

The drop continues until the surface is crowded. Beyond that point, the critical micelle concentration (CMC), additional surfactant forms micelles in the bulk liquid and the surface tension stays roughly constant. The value reached at the CMC is a common measure of how effective a surfactant is.

Static and dynamic surface tension

Surface tension measured on a surface that has had time to reach equilibrium is the static, or equilibrium, value. In many real processes a new surface is created in milliseconds: a spray breaking into droplets, a droplet hitting a leaf, a coating being applied. Surfactant molecules need time to diffuse to a new surface, so the surface tension at that moment, the dynamic surface tension, can be much higher than the equilibrium value. For fast processes, how quickly a surfactant reaches the surface matters as much as how low it can take the surface tension.

Why surface tension matters

• Wetting and spreading. A liquid with lower surface tension spreads more easily over a solid, so lowering surface tension is central to spray adjuvants, coatings and cleaning.
• Soil water movement. Surface tension affects how water enters and moves through soil pores, which is the basis of soil surfactants.
• Foams and droplets. Surface tension governs how easily bubbles and droplets form and how stable they are.
• Capillary effects. The rise of liquid in a narrow tube or pore depends on surface tension, as does capillary pressure in rock.

How surface tension is measured

Instruments called tensiometers measure surface tension. Common methods include the du Noüy ring and the Wilhelmy plate, which measure the force needed to pull a probe through the surface; the pendant drop, which analyzes the shape of a hanging drop; and the maximum bubble pressure method, which is used for dynamic surface tension because it can measure surfaces only milliseconds old.

Frequently asked questions