Capillarity

Capillary action - It describes the ability of a liquid to flow against gravity in a narrow space such as a thin tube.

Capillarity - The phenomenon of rise & fall of liquid in a Heron capillary on account of surface tension is called capillarity.

Examples of capillary action -

1. Rise of oil in lamp is due to capillarity.
2. Moisture rises in plant by capillarity action.
3. A blotting paper absorbs ink by capillary action.
4. Moisture from the foundation of the house rises in the wall due to capillary action.
5. The action of a towel in soaking up moisture from the body is due to capillary action of cotton in towel.

Shape of meniscus (top surface in capillary tube) –

 In case of water

Consider water molecule "P" on the surface near wall. It experience two forces one is force of adhesion (R_A) & the other is force of cohesion (R_C). In order to balance these two forces the resultant force (R) is in such direction which tends liquid surface in concave shape.

 In case of mercury

In case of mercury the cohesive force is larger than the adhesive force. In order to balance these two forces, the resultant force (R) is in such direction which tends liquid surface in convex shape.

Relation between Surface tension, Capillary rise & Radius of capillary -

where r = radius of capillary 

h = rise of liquid in tube

ρ = density of liquid

g = gravitational acceleration

Θ = angle of contact 

Effect of impurity on surface tension

Surface tension ∝ 1 / organic impurity

Effect of temperature on surface tension

Surface tension ∝ 1 / temperature of liquid