92168.pdf

70 Liquid-gas and liquid-liquid interfaces

Capillary rise method

This is, when properly performed, the most accurate method

available for determining surface tensions. Since the measurements

do not involve a disturbance of the surface, slow time effects can be

followed.

2r

z?zz

Figure 4,3 Capillary rise

For the rise of a liquid up a narrow capillary

2 cosO

which, for zero contact angle, reduces to

y = Vi rh&pg

(4.6)

(4.7)

where Ap is (density of liquid — density of vapour).

For accurate work a meniscus correction should be made. In a

narrow capillary the meniscus will be approximately hemispherical:

therefore,

y = Vir(h + r I 3)Apg (4.8)

For wider capillaries one must account for deviation of the meniscus

from hemispherical shape^2.

In practice, the capillary rise method is only used when the contact

angle is zero, owing to the uncertainty in measuring contact angles