Surface Tension

Surface Tension

Liquids show the effects of inter-molecular forces as surface tension. Any molecule in the interior of liquid is equally attracted by neighbor molecules from all sides and it does not experience any net force.

On the other hand, any molecule at the surface of a liquid is attracted by other molecules at the surface of the liquid or below it. Due to the imbalance of forces, any molecule at the surface experiences a net inward pull. As a result the surface is under tension as if the liquid were covered with a tight skin (or stretched membrane). The phenomenon is called surface tension.

Quantitatively, the surface tension is defined as the force acting on an imaginary line of unit length drawn on the surface of the liquid and acting perpendicular to it towards the liquid side. It is represented by the Greek letter gamma, γ. Its SI unit is newton per metre (N m^–1) and CGS unit is dyne per centimetre (dyne cm^–1).

Surface Energy

Surface molecules of a liquid experience a constant inward force. Therefore, they have a higher energy than the molecules in the bulk of the liquid. Due to this reason liquids tend to have minimum number of molecules at their surface. This is achieved by minimizing the surface area.

In order to increase the surface area, more molecules must come to the surface. This can happen only if some energy is supplied or work is done. The energy supplied (or work done) for increasing the surface area of a liquid by a unit amount is known as its surface energy. Its units are joule per square metre (J m^–2 or N m^–1). Dimensions of both the surface tension and surface energy are similar.

Effect of Temperature

On raising the temperature, surface tension of a liquid decreases. It completely vanishes at the critical temperature. This happens due to the following two factors:

1. On heating, the liquids expand. This increases the inter-molecular distances.
2. On heating, the average kinetic energy of molecules and hence their chaotic motion increases.

Due to both of these factors, the inter-molecular forces become weak and the surface tension decreases.

Effect of Adding Surface Active Solutes

The solutes which get more concentrated on the surface of the liquid than in the bulk are called surface active solutes or surfactants. Alcohols are examples of such substances. Their addition to a liquid lowers its surface tension. The cleaning action of soaps and detergents is based on this fact.

Effects of Surface Tension

Surface tension results in many interesting and important properties of liquids.

• Spherical Shape of liquid drops
• Wetting and Non-wetting properties
• Capillary Action
• Curved meniscus