Capillary rise method
The methods commonly employed for the determination of surface tension :
A capillary tube of radius r is vertically inserted into a liquid. The liquid rises to a height h and forms a concave meniscus. The surface tension (y) acting along the inner circumference of the tube exactly supports the weight of the liquid column.
By definition, surface tension is force per 1 cm acting at a tangent to the meniscus surface. If the angle between the tangent and the tube wall is Ө, the vertical component of surface tension is ycosӨ
The total surface tension along the circular contact line of meniscus is 2πr times. Therefore,
Upward force = 2πrycosӨ
where is r the radius of the capillary. For most liquids, Ө is essentially zero, and cosӨ = 1. Then the upward force reduces to 2πry.
The downward force on the liquid column is due to its weight which is mass x gravity. Thus,
Downward force = hπdgr2
where d is the density of the liquid.
But,
Upward force = Downward force
2πry= hπdgr2
y = hrdg/2 dynes/cm
In order to know the value of suface tension y , the value of h is found with the help of a travelling microscope and density (d) with a pyknometer.