Soil Is Never Without Water - Soil: The Skin of the Planet Earth - page 106

Adhesion, known as the attraction between unlike molecules, potentially occurs
when a water molecule comes near a molecule of a solid surface. If this adhesion
has a higher value than the cohesion between water molecules, the nearby water
molecules look like they are climbing up the vertical hydrophilic surface we
observed earlier in the bottom half of Fig. 8.4a, b . On the other hand, when the adhe-
sion between water and solid surface molecules is smaller than the cohesion between
water molecules, the water molecules near the hydrophobic solid surface are
repulsed and the water forms a convex shape previously seen in Fig 8.4c with con-
tact angle above 90°. A simple observable example is a droplet of water forming a
spherical shape in order to minimize contact with a hydrophobic leaf.
Let us now continue our earlier discussion of capillarity when water begins to
move upward into a vertical glass tube after its lower end has been submerged in
water. On our oversimplifi ed molecular scale, we observe hydrophilic curving
where each water molecule of the curved surface is attracted in addition to mole-
cules placed not only on the same level and below but also to neighboring molecules
that have “climbed” higher up onto the wall to elevations above the free water hori-
zontal plane surface. Inside the tube the shape of the water surface is “U-like,” i.e.,
concave. The resulting surface pressure of this “U-like” or “cup-like” water surface
illustrated by an arrow labeled p s in Fig. 8.6 is smaller than that of the reference
pressure p r of the horizontal plane water because there are water molecules in the
capillary tube above the horizontal water plane that compensate the attractive forces
Fig. 8.6 On the left side of the fi gure, capillary depression occurs when water is not wetting the
solid walls of the tube. The length of the arrow represents the increased surface pressure p s due to
the convex shape of the water level, and this increase pushes down the curved water level in the
tube compared to the fl at plane water level with surface pressure p r . On the right side of the fi gure
when the water molecules are wetting the solid walls of the tube, capillary rise occurs since the
concave shape causes decreased surface pressure compared to fl at plain water level. This differ-
ence of surface pressures causes the water to be sucked up into the capillary tube