The interplay of the forces of the cohesion and adhesion explains the phenomena of capillarity. When a liquid is in contact with a solid, if the forces of adhesion between the molecules of the liquid and the solid are greater than the forces of cohesion among the liquid molecules themselves, the liquid molecules crowd towards the solid surface. The area of contact between solid and liquid increases and the liquid thus wets the solid surface.
The reverse phenomenon takes place when the force of cohesion is higher than the force of adhesion. These adhesion and cohesion forces result in the phenomena of capillarity by which a liquid either rises or falls in a tube dipped into the liquid depending on whether the force of adhesion is more than that of cohesion or not.
The angle ‘θ’, as shown in below figure, is the area wetting contact angle made by the interface with the solid surface.
In the above figure, ‘h’ is the height of the capillary rise.
‘D’ is the diameter of the capillary tube.
Capillarity can also be defined as the tendency of a liquid in a capillary tube to rise or fall as a result of surface tension.
Equating the weight of the column of the liquid ‘h’ with the vertical component of the surface tension force, we have,
For pure water in contact with air in a clean glass tube, the capillary rise takes place with θ = 0°. The value of θ may be different from zero in practice where cleanliness of higher order is seldom found. Mercury causes capillary depression with an angle ‘θ’ of about 130° in a clean glass in contact with air. Since height ‘h’ varies inversely with ‘D’ as found from above equation, a significant capillary rise or depression can be observed in tubes of the small diameter only.