Geoscience Reference
In-Depth Information
Fig. 8.4
Contact angle
ʳ
between water and solid: (
a
) complete wetting; (
b
) incomplete wetting;
(
c
) non-wetting
Another manifestation of the impact of a wetting angle is apparent in the bottom
half of Fig.
8.4
when we submerge one end of the solid plate vertically into water to
observe the shape of water level formed near the plate. When water wets the plate,
its surface forms an upward oriented arc as if it were climbing up to the plate. And,
whenever the water does not wet the plate, its surface forms a downward oriented
arc as if it were stepping down to the plate. When the contact angle is zero, its arc
length is one-fourth of a cylindrical circumference having one end in contact with
the plain water level and the other end in contact with the vertical solid plate. If wet-
ting is not complete, the contact angle is somewhere between 0 and 90°. Such wet-
ting behavior is known as hydrophilicity and the solid is characterized as
hydrophilic
- derived from Greek
hydros
for water and
philia
for friendship. The
lack of wetting of the solid surface by water demonstrated by contact angles greater
than 90° is known as hydrophobicity and the solid is described as hydrophobic from
Greek
phobos
for fear. It would be a false assumption that hydrophobicity does not
exist in soils. It happens when soil particles are covered by thin fi lms of humic sub-
stances or whenever humifying residuals of organic bodies cover the soil topograph-
ical surface.
Let us now submerge one end of an empty glass capillary tube vertically into
water and observe the shape and height of the level of water sucked into it. As water
rises, its curved water surface forms the shape of a cup or a hemisphere. We describe
this type of “no fl at plain” by the term
concave
that means a surface like the inside
of a sphere. This curving of the water surface brings our attention to the physical
phenomenon of “capillarity” when water molecules attract each of their neighbor-
ing water molecules to form and pull together in an organized manner called
“cohesion.”
