Surface Tension Definition and Causes

Surface Tension Definition

Surface tension is a physical property equal to the amount of force per unit area necessary to expand the surface of a liquid. It is the tendency of a fluid surface to occupy the smallest possible surface area. Surface tension is a principal factor in capillary ​action. The addition of substances called surfactants can reduce the surface tension of a liquid. For example, adding detergent to water decreases its surface tension. While pepper sprinkled on water floats, pepper sprinkled on water with detergent will sink.
Surface tension forces are due to intermolecular forces between the liquid's molecules at the liquid's outer boundaries.

The units of surface tension are either energy per unit area or force per unit length.

Examples of Surface Tension

• Surface tension allows some insects and other small animals, which are denser than water, to walk across its surface without sinking.
• The rounded shape of water droplets on a surface is due to surface tension.
• Tears of wine form rivulets on the glass of an alcoholic beverage (not just wine) due to the interaction between the different surface tension values of ethanol and water and the faster evaporation of alcohol compared with water.
• Oil and water separate because of the tension between two dissimilar liquids. In this case, the term is "interface tension", but it is simply a type of surface tension between two liquids.

How Surface Tension Works

At the interface between a liquid and the atmosphere (usually air), the liquid molecules are more attracted to each other than they are to the air molecules. In other words, the force of cohesion is greater than the force of adhesion. Because they two forces are not in balance, the surface may be considered to be under tension, like if it was enclosed by an elastic membrane (hence the term "surface tension". The net effect of cohesion versus adhesion is that there is an inward force at the surface layer. This is because the top layer of a molecule is not surrounded by liquid on all sides.

Water has an especially high surface tension because water molecules are attracted to each other by their polarity and able to engage in hydrogen bonding.