Page 218 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 213

Figure 11.2
Unbalanced
molecular
attractions in a
surface.

surface-tension phenomena, such as support for water insects that skate across
and depress the water surface.

11.3.3 Surface Tension and Capillary Rise

In a capillary tube, surface tension forces attach to the glass and hold the water
up, but the weight of the water causes the surface to sag downward in the middle,
somewhat like a blanket that is being held at the edges. This creates a visible
meniscus. The height of capillary rise usually is measured to the bottom of the
meniscus, even though that is not strictly correct. The walls of the capillary must
be clean and free of any hydrophobic material such as oil that would prevent the
adsorption of water molecules to the glass.

If a fluid does not bond to glass, the angle in Fig. 11.1 exceeds 908 and
capillarity pushes downward so that the meniscus is reversed. A reversed meniscus
may be observed at the top of a mercury column in a thermometer.

11.3.4 Oil on Troubled Waters

Oil is hydrophobic, or water-hating, and being less dense than water spreads over
the surface in a thin film that diffracts light into rainbow-like colors. An oil film
therefore creates two surface-tension interfaces, air-to-oil and oil-to-water. The
net increase in total surface tension acts to smooth wave action and give meaning
to the expression to ‘‘spread oil on troubled waters.’’ Plutarch’s explanation was
that oil makes water more slippery so that wind slips on by, but he had no
knowledge of surface tension. However, this misconception lives on in abortive
experiments to develop an ‘‘airplane grease.’’

11.3.5 Surface Tension and Surface Energy
Surface tension is a force per unit length, F/L. Multiplying the numerator and
2
denominator by L gives (FL) divided by (F/L ), which is work or energy per unit
area. Surface tension therefore is synonymous with surface energy, the energy
required to generate a surface. Reducing surface tension therefore reduces surface
energy.

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