SURFACE TENSION AND CAPILLARITY

What is surface tension ?

Surface tension is defined as a tensile force acting on the surface of a liquid in contact with a gas or on the surface between two immiscible liquids such that the contact surface behaves like a membrane under tension. The magnitude of this force per unit length of the free surface will have the same value as the surface energy per unit area. It is denoted by Greek letter σ{called sigma}. In MKS units, it is expressed as kgf/m while in SI units as N/m.

To understand the phenomenon of surface tension, consider three molecules A,B,C of liquids in a mass of liquids. Now, due to intermolecular attraction force, molecule A is attracted by surrounding molecules equally in all directions. Thus the resultant force acting on the molecule A will be 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 downward direction. The molecule C situated on the free surface, will experience a resultant downward force due to it's position. All the molecules on the free surface will experience a resultant downward force. Thus the free surface of the liquid acts like a thin film under tension.

Fig. Surface tension

Capillarity- It is defined as a phenomenon of rise or fall of a liquid surface in a small tube relative to the adjacent general level of liquid when the tube is held vertically in the liquid. The rise of liquid surface is known as capillary rise while the fall of the liquid surface is known as capillary depression. It is expressed in terms of cm or mm of liquid. It's value depends upon the specific weight of the liquid, diameter of the tube and surface tension of the liquid.

Also read Bernoulli's principle

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