The interplay
of the forces of cohesion and adhesion explains the phenomenon
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
the liquid and solid increases and the liquid thus wets
the solid surface.
The reverse phenomenon takes place
when the force of cohesion is greater than the force of
adhesion. These adhesion and cohesion properties result
in the phenomenon of capillarity by which a liquid either
rises or falls in a tube dipped into the liquid depending
upon whether the force of adhesion is more than that of
cohesion or not (Fig.2.4).
The angle θ as shown in Fig. 2.4, is the area wetting contact angle
made by the interface with the solid surface.
Fig 2.4 Phenomenon of Capillarity
For pure water in contact with
air in a clean glass tube, the capillary rise takes place
with θ = 0 . Mercury causes capillary depression with an angle of contact
of about 1300 in a clean glass in contact with air. Since h varies
inversely with D as found from Eq. (
), an appreciable capillary rise
or depression is observed in tubes of small diameter only.
), an appreciable capillary rise
or depression is observed in tubes of small diameter only.
