Surface tension is a phenomenon that gives an elastic tendency to the fluid surface. The cohesive force between the molecules is responsible for surface tension. It occurs at the interface where the liquid is contacted with a gas or on the surface between two immiscible fluids. Such surface behaves like a membrane under tension. It is measured as the surface energy per unit area or force per unit length. It is denoted by using Greek letter σ (sigma).
Units of surface tension
SI unit - N/m
CGS - dyne/cm
Explanation of surface tensionConsider the molecule A, B, C of liquid as shown in fig. Every surrounding molecule attracts the molecule A. The net force acting on the A is zero. In molecule B, which is situated near the surface, have unbalanced forces acting on them. The resultant force acting on B is the downward direction. Similarly, the molecule C situated on the surface of liquid experience a downward pull. It causes the surface act as a membrane under tension.
Temperature effect on surface tensionSurface tension varies with temperature; for that reason when the surface tension of an interface is given, the temperature must be explicitly stated. In general surface tension decrease with the increase in temperature.
Read: Effect of temperature on viscosity of liquids and gases
Applications and importance of surface tension in daily life
- The floatation of the dense object on water is due to surface tension. For example, water striders walk on the water and floating needle, etc.
- Beading of rain water on a waxy surface such as leaves. Water clusters into drops because a sphere has the smallest possible area to the volume ratio.
- Separation of oil and water is caused by the interface tension of water and oil.
- Use of soap and detergent: they help lower the surface tension of water that helps to soak the pores of clothes readily.
- Washing with hot water - Surface tension decrease with increasing temperature. The hot water has lower surface tension than cold water. The lower surface tension better for soaking clothes.
- At the critical point the surface tension = 0
- It is a surface phenomenon, and cohesion is responsible for it.
- Surface tension inversely proportional to temperature.
- Surface tension helps to acquire least surface area possible. Rain come in drops due to surface tension.