in which the surface area is minimal. For a given volume, a sphere has the least possible
surface area, so drops of liquid tend to assume spherical shapes. Surface tensionis a
measure of the inward forces that must be overcome to expand the surface area of a liquid.CAPILLARY ACTION
All forces holding a liquid together are called cohesive forces.The forces of attraction
between a liquid and another surface are adhesive forces.The partial positive charges
on the H atoms of water hydrogen bond strongly to the partial negative charges on the
oxygen atoms at the surface of the glass. As a result, water adheresto glass, or is said to
wetglass. As water creeps up the side of the glass tube, its favorable area of contact with
the glass increases. The surface of the water, its meniscus,has a concave shape (Figure
13-9). On the other hand, mercury does not wet glass because its cohesive forces (due to
dispersion forces) are much stronger than its attraction to glass. Thus, its meniscus is
convex. Capillary actionoccurs when one end of a capillary tube, a glass tube with a
small bore (inside diameter), is immersed in a liquid. If adhesive forces exceed cohesive
forces, the liquid creeps up the sides of the tube until a balance is reached between adhe-
sive forces and the weight of liquid. The smaller the bore, the higher the liquid climbs.
Capillary action helps plant roots take up water and dissolved nutrients from the soil and
transmit them up the stems. The roots, like glass, exhibit strong adhesive forces for water.
Osmotic pressure (Section 14-15) also plays a major role in this process.13-5
(Right) The shape of a soap bubble is due to
the inward force (surface tension) that acts
to minimize the surface area.The surface tension of water
supports this water strider. The
nonpolar surfaces of its feet also
help to repel the water.
Droplets of mercury lying on a glass
surface. The small droplets are
almost spherical, whereas the larger
droplets are flattened due to the
effects of gravity. This shows that
surface tension has more influence
on the shape of the small (lighter)
droplets.
Figure 13-9 The meniscus, as
observed in glass tubes with water
and with mercury.
Coating glass with a silicone polymer greatly reduces the adhesion of water to the
glass. The left side of each glass has been treated with Rain-X, which contains a
silicone polymer. Water on the treated side forms droplets that are easily swept away.