Geoscience Reference
In-Depth Information
(ii) Natural soils, especially those with finer texture, consist of particles that are not
inert, but active as a result of electrical charges at their surfaces. In the presence of
water such surface-active particles interact, repelling each other and attracting in the
intervening spaces ions, and thus also more water to relieve the osmotic pressure; this
osmotic pressure difference, between the water in the immediate vicinity of the particles
and that in the larger pores of the soil, in turn gives rise to hydrostatic pressure differ-
ences. This may also be accompanied by swelling during wetting, and shrinking during
subsequent drying. (iii) Water molecules also behave as dipoles, and as such they may
undergo attraction by the electrical surface charges of the soil particles. (iv) When the
soil contains clay particles (especially montmorillonite), water can be held between the
“sandwich” layers of these clayey minerals in a quasi-crystalline fashion; actually, the
water is held so strongly by this mechanism that it could be debated whether the water
should be considered “free” or “chemically bound.”
Surface tension
Among these mechanisms, those involving surface tension are probably the most impor-
tant in the context of hydrology. Surface tension or capillarity acts at the interface between
two fluids. In simple terms, one can visualize the molecules in a liquid as being subject
to attraction by their neighbors. Far from boundaries the field is symmetrical and these
molecular effects are balanced. However, near the interface with another fluid whose
molecules are less attracting, the balance is broken and the molecules are pulled more
toward their own bulk. In order to increase the interfacial area of the liquid in question,
work is necessary. The surface tension is a measure of this work and, thus, of the energy
required to maintain that interface. In the case of the interface between water and air,
in the range of 0
30
◦
C, the surface tension can be estimated approximately by
≤
T
≤
10
−
3
Jm
−
2
(or N m
−
1
), with the temperature
T
in
◦
C (degrees
celsius). The word capillarity is derived from the Latin
capilla
, or hair, because surface
tension is manifested by the phenomenon of the rise of water in hair-thin glass tubes. In
the soil profile the zone above the water table, where the water content is near saturation
even though the pressure is negative, is often referred to as the
capillary fringe
(see
Figure 8.7). Similarly, whenever the soil water pressure
p
w
is negative, it is also called
capillary pressure
.
σ
=
(75.6
−
0.14
T
)
×
Equation of Laplace
As a result of surface tension, any pressure difference across the interface of two immis-
cible fluids that are in contact with each other is accompanied by a curvature of their
interface. It can readily be shown that this phenomenon is described by
1
r
2
±
1
r
1
p
=
σ
(8.3)
in which
p
w
) is the pressure difference across the interface, and
r
1
and
r
2
are the two principal radii of curvature of the interface; the plus sign between the two
terms inside the brackets applies to
synclastic
interfaces, and the minus sign to anticlastic
surfaces. A surface is said to be synclastic when the centers of the two radii of curvature
p
(
=
p
a
−
