Page 168 - Introduction to Colloid and Surface Chemistry

The solid-liquid interface 151

Figure 6.3 Tilting plate method for the measurement of contact angles

in Figure 6.3 can be used; the angle of the plate is adjusted so that the
liquid surface remains perfectly flat right up to the solid surface.
Another method, originally devised by Langmuir and Schaeffer, is
based on observation of the angle at which light from a point source is
reflected from the surface of a liquid drop at its contact point with a
173 174
plane solid surface " . This technique can be improved by using
laser-light. It has been refined for measuring contact angles formed
174
by liquids on the surfaces of small-diameter filaments .
The contact angles of solids in a finely divided form are technically
important (e.g. flotation; page 161) but are difficult to measure.
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Bartell et a/. developed a method based on displacement pressures
for measuring such contact angles. The finely divided solid is packed
into a tube and the resulting porous plug is considered to act as a
bundle of capillaries of some average radius r. The pressure required
to prevent the liquid in question from entering the capillaries of this
porous plug is measured, and the equation for the pressure difference
across a curved liquid surface (5.4) is applied,

p — 2y cos 01 r (6.11)

The equivalent radius of the capillaries is then found from a similar
experiment using a liquid which completely wets the solid - i.e.
p' = 2y' / r (6.12)