Measuring Surface Tension in Capillaries
As explained in the previous section and referring the adjoining figure, we see a rise in the capillary where,
The force of upward tension = Weight of the liquid column in the tube,
The above expression can be formulated as:
πd.σ.cosθ = πd2.h.ρg/4
therefore, the capillary rise can be calculated as, h = 4σcosθ/ρgd, and the surface tension can be found by solving:
σ = hρgd/4cosθ
where σ = force due to surface tension per unit length, and,
θ = angle of contact between the tube wall and the liquid surface.
With water as the liquid and the tube made up of glass, θ ~ 0,
Therefore with water in a glass tube the capillary rise becomes:
h = 4σ/ρgd,
and the generated surface tension can be evaluated through the expression:
σ = hρgd/4
In case of a liquid droplet, the drop attains a pressure within the liquid enclosed which is equal to the exerted surface tension.
Equating the above forces we get, πd.σ = p.πd2/4
Finally, the formula for measuring surface tension in the above case becomes:
σ = p.d/4
where p = pressure and d = diameter of the droplet.
The above explanations help us to understand this important tendency associated with liquids and eventually relates to us regarding what surface tension is and how it can be measured.