Capillarity

Capillarity is defined as the phenomenon of rise or fall of a liquid in a thin tube ( capillary tube) relative to the adjacent general level of liquid. It is a surface tension effect and depends upon the relative intermolecular attraction between different substances.

Capillarity is due to both Cohesion and adhesion. The rise of liquid surface is known as capillary rise and The Fall of liquid surface is known as capillary depression. it is expressed in cm or mm of liquid.

Factors affecting Capillarity -

(1) diameter of the tube

(2) specific weight of liquid

(3) surface tension of liquid

Expression for capillary rise or fall 

Consider a capillary tube dipped in the liquid as shown in figure -

Let     σ = surface tension of liquid

         d =  diameter of capillary tube

         w = specific weight of liquid

          θ = angle of contact

         h =  rise or fall of the liquid in capillary tube

                Weight of liquid raised or lowered in the capillary tube

= [ area of tube X rise or fall] X specific weight

= [ π/4 d²h]

Vertical component of surface tension force

= σ cos(θ) X circumference of tube

=  σ cos(θ) X πd = πd σ cos(θ)

At equilibrium, the weight of the liquid column ' h' is balanced by the vertical component of surface tension force

So,  π/4 d²hw = πd σ cos(θ)

Or    h = 4 σ cos(θ) / wd