First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 67

Table 2.4 Surface Tension (Contact with Air) of
Selected Environmental Fluids

Surface Tension σ

(N m − 1 at 20 ◦ C)

Fluid

Acetone 0.0236

Benzene 0.0289

Ethanol 0.0236

Glycerin 0.0631

Kerosene 0.0260

Mercury 0.519

n- Octane 0.0270

Tetrachloromethane 0.0236

Toluene 0.0285

Water 0.0728

Table 2.4). The high surface tension creates a type of skin on a free surface,
which is how an object that is denser than water (e.g., a steel needle) can “float”
on a still water surface. It is the reason that insects can sit comfortably on water
surfaces. Surface tension is somewhat dependent on the gas that is contacting the
free surface. If not indicated, it is usually safe to assume that the gas is the air in
the troposphere.
Capillarity is a particularly important fluid property of groundwater flow and
the movement of contaminants above the water table. In fact, the zone immedi-
ately above the water table is called the capillary fringe . Regardless of how densely
soil particles are arranged, void spaces (i.e., pore spaces) will exist between the
particles. By definition, the pore spaces below the water table are filled exclusively
with water. However, above the water table, the spaces are filled with a mixture of
air and water. As shown in Figure 2.8, the spaces between unconsolidated mate-
rial (e.g., gravel, sand, clay) are interconnected and behave like small conduits or
pipes in their ability to distribute water. Depending on the grain size and density
of packing, the conduits will vary in diameter, ranging from large pores (i.e.,
macropores), to medium pore sizes (i.e., mesopores), to extremely small pores
(i.e., micropores).
Fluid pressures above the water table are negative with respect to atmospheric
pressure, creating tension. Water rises for two reasons: its adhesion to a surface,
and the cohesion of water molecules to one another. Higher relative surface
tension causes a fluid to rise in a tube (or a pore) and is indirectly proportional
to the diameter of the tube. In other words, capillarity increases with decreasing
tube diameter (e.g., tea will rise higher in a thin straw in a glass of iced tea than
in a fatter straw). The rise is limited by the weight of the fluid in the tube. The
rise ( h capillary ) of the fluid in a capillary is expressed as follows (Figure 2.9 gives an