Surface tension is defined as the force required to maintain unit length of the film in tension (or) it is the property of the liquid surface film to exert a tension. It is denoted by Greek letter σ (sigma) . In MKS units, it is expressed as kgf/m while in SI units as N/m.
The phenomenon of surface tension is explained by considering three molecules A,B,C of a liquid in a mass liquid. The molecules A is attracted in all directions equally by the surrounding molecules of the liquid. Thus the resultant force acting on the molecules A is zero. But the molecule B , which is situated near the free surface, is acted upon by upward and downward forces which are unbalanced. Thus a net resultant force on molecule B is acting in the downward direction. The molecule C, situated on the free surface of liquid, does experience a resultant downward force. All the molecules on the free surface experience a downward force. Thus the free surface of the liquid acts like a very thin film under tension of the surface of the liquid acts as through it is a elastic membrane under tension.
Factors Affecting Surface Tension
- Surface tension is caused due to cohesion and hence with an increase in temperature, surface tension value decreases for all liquids
- The value also depends upon the fluid in contact with it. In general, air is the contact fluid.