156 The solid-liquid interface1, Contamination of the liquid usually influences the contact angle.
2, Solid surfaces differ from liquid surfaces in that they show a far
greater degree of heterogeneity, even after careful polishing; for
example, a solid surface polished to the best optical standards is
wavy and pitted compared with a quiescent liquid surface. To
obtain a solid surface free from impurities which are likely to have
a significant effect on its properties is usually very difficult. It can,
therefore, be appreciated that any measured property of a solid
surface is subject to variability as a result of unavoidable sample
differences.- In practice, contact angles are rarely single-valued quantities, but,
for a given system, a range of metastable contact angles exists.
The observed contact angle will depend mainly on (a) whether the
liquid is advancing over a dry surface or receding from a wet
surface, and (b) the extent to which the drop is vibrated. Contact
angle hysteresis is most noticeable with chemically and/or geomet-
rically heterogeneous surfaces. The difference between advancing
and receding contact angles may be as much as 50°. Consider the
spreading of a liquid on a solid surface with a mixture of high
energy (low 8) and low energy (high 0) regions. Advancing liquid
will spread relatively easily over the high energy regions of the
surface, but tend to stick and locate the triple interface at low
energy regions, thus giving a high advancing contact angle.
Receding liquid, on the other hand, will readily vacate low energy
regions of the surface and the triple interface will tend to be
located at high energy regions, thus giving a low receding contact
angle. Contact angle hysteresis on geometrically rough surfaces
can be considered, to some extent, in terms of a constant
microscopic angle leading to different macroscopic angles accord-
ing to the local inclination of the surface. A common example of
contact angle hysteresis is given by raindrops on a dirty window-
pane. Mercury rolls off glass, and other solids, very easily because
of low contact angle hysteresis. - Liquid drops on anisotropic solid surfaces will tend to elongate in
the higher surface energy direction and the contact angle will,
therefore, vary with position.
If a moderately large area of flat solid surface is available, contact
angles are usually measured directly from a projection of a sessile
drop of the liquid. Alternatively, the tilting-plate method illustrated