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Pore Water Pressure, Capillary Water, and Frost Action
214 Geotechnical Engineering

11.3.6 Soaping Surface Tension
The manipulation of surface tension is at the heart of the soap and detergent
industries, which use organic compounds whose purpose is to lower the surface
tension between the unclean and that which is to be cleaned. Although the
amount of energy required to make new surfaces is reduced, energy still is
required, whether by use of a stirring mechanism in a washer or pounding clothes
on a rock.

Organic molecules in detergents are somewhat hydrophobic or water-hating so
they concentrate in the water surface. There they interrupt and weaken water-to-
water molecular attractions, making it easier to generate suds or foam. Only a tiny
amount of organic chemical can produce this effect, so foam is an early indication
of organic pollutants in a lake or river.

11.3.7 Measuring Surface Tension

A simple device that illustrates measurement of surface tension is shown in
Fig. 11.3, where the pulling force is 2LT, where L is the length of the expanding
surface and T is surface tension in units of force per unit length. The force is 2
because two surfaces are involved.

Another way to measure surface tension is from the height of rise in a capillary
tube, as in Fig. 11.1, if the inside diameter of the tube is known. The height of rise
is obtained by equating the surface tension at the circumference with the weight of
the water column in the tube:
2
2 rT cos ¼ r h g ð11:3Þ
where r is the radius of a circular capillary tube in units that are consistent with
units of T,
T is the surface tension of the liquid, which can be in millinewtons per
meter,
is the contact angle between the meniscus surface and the wall of the tube,
h is the height of rise of the liquid in meters,
is the density of the liquid, in grams per cubic meter, and
2
g is the acceleration of gravity (9.81 m/s ).

Figure 11.3
Measuring surface
tension.

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