92168.pdf

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

plane solid surface^173 "^174. This technique can be improved by using

laser-light. It has been refined for measuring contact angles formed

by liquids on the surfaces of small-diameter filaments^174.

The contact angles of solids in a finely divided form are technically

important (e.g. flotation; page 161) but are difficult to measure.

Bartell et a/.^17 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)