Liquid-gas and liquid-liquid interfaces 11correctly. One can check for zero contact angle by allowing the
meniscus to approach an equilibrium position in turn from above and
below. With a finite contact angle, different equilibrium positions will
be noted, owing to differences between advancing and receding
contact angles (see page 156). Zero contact angle for aqueous and
most other liquids can usually be obtained without difficulty using
well-cleaned glass capillaries.
A difficulty associated with this method is that of obtaining
capillary tubing of uniform bore (slight ellipticity is not important).
Thermometer tubing is useful in this respect. Alternatively, one can
adjust the height of the reservoir liquid so as to locate the meniscus at
a particular level in the capillary where the cross-sectional area is
accurately known.
Figure 4.4 Differential capillary rise apparatusA useful variation is to measure the difference in capillary rise for
capillaries of different bore, thus eliminating reference to the flat
surface of the reservoir liquid (Figure 4.4). Since y = V^
(4.9)
2(r,-r 2 )