The surface tension is due to cohesion between particles at the free surface. Due to molecular attraction, liquids have properties of cohesion and adhesion. Cohesion is due to the force of attraction between molecules of same liquid. Otherwise, the intermolecular attraction holds the liquid molecules together known as cohesion. This force is very small. It enables the liquid to withstand a small tensile stress.
Adhesion is defined as the force of attraction between molecules of two different liquids or between molecules of the liquid and molecules of the solid boundary surface. This property enables a liquid to stick to over another body.
Surface tension is due to the force of cohesion between liquid particles at the free surface. Liquid molecules at the interior of the liquid are generally free to move within the liquid and they move at random because these molecules are attracted equally in all directions by the other surrounding molecules and they are in equilibrium. When they reach the free surface, there are no molecules above the surface to balance the force of the molecules below it.
Consider three molecules X, Y and Z in the liquid. Molecule X is equally attracted in all directions. Therefore, it is in equilibrium condition. It exerts the equal force in all directions. The resultant force acting on the molecule is zero. Consider a molecule Y which is placed near the free surface. The upward and downward forces acting on that molecule Y are unbalanced. Obviously, the resultant force is acting in the downward direction. The molecule Z is placed at the free surface and it has a net downward force. However, such molecules are kept at the surface by the virtue of work done on these molecules at the time of formation of their surface.
This work done on the surface against inward force is called surface energy and it is generally denoted by σ (sigma). It is evident that all molecules on the free surface are exerted by a net downward force. Thus, the thin layer of molecules is formed on the free surface and it behaves as an elastic membrane under tension.
Surface tension is defined as the tensile force required to keep unit length of the surface film in equilibrium. It may also be defined as the tensile force acting on the surface of the liquid when in contact with a surface or gas between different immiscible liquids. The surface tension is same everywhere on the surface irrespective of its curvature and acts in the plane of the surface.
Some important real life examples are:
- formation of water bubbles
- formation of rain droplets
- collection of dust particles on water surface
- a small needle can gently be placed on the liquid surface without sinking
- breakup of liquid jets
- capillary rise and capillary siphoning.
The surface tension depends directly upon the intermolecular cohesion and hence, the cohesion decreases with temperature rise, the surface tension also decreases with rise in temperature. It also depends upon the following factors.
- Nature of the liquid
- Nature of the surrounding liquid
- Kinetic energy of the liquid.