Soil Is Never Without Water - Soil: The Skin of the Planet Earth - page 111

Fig. 8.9 Principle of hysteresis: ( a ) waterdrop moves along the solid surface; ( b ) originally dry
capillary tube is inserted into water; ( c ) capillary tube was fi rst fully immersed in water and then
partially pulled out. The length of the arrow below the curved water level represents the surface
pressure, which is different just due to the change of contact angle ʳ and thus the changed
curvature

level (Fig. 8.9b ). The wetting angle in the fi rst capillary is greater than that in the
second capillary because it is more diffi cult for water to simultaneously wet and
move along an initially dry surface. We note that the curvature of the meniscus of
the fi rst capillary is fl atter than that in the second capillary. Within the second capil-
lary that was initially submerged in water, water molecules had more time to par-
tially displace foreign molecules and more completely wet its cylindrical wall. The
difference in curvatures of the two menisci illustrates that the surface tension is
smaller in the second capillary than that pushed only into the water. The surface
pressures of the menisci p s represented by the lengths of arrows account for the
height of capillary rise in the fi rst capillary being smaller than that in the second
capillary. To be more comprehensive, we have to keep in mind that the size of a
capillary tube is not the single decisive factor on the retention of water in soil and
that the extent of hydrophilicity (or hydrophobicity) has to be considered.

8.4 From Capillary Tubes to Real Soil Pores Partly Filled

by Water

It is obvious that real soil pores have shapes distinctly different from bundles of
parallel capillary tubes. Nevertheless, we discussed parallel tube models just for
their advantage in simplicity and clear physical demonstration. We have mentioned