Nucleation in a crevice (as in frames 4 and 5) cannot be ruled out; see,
however, the discussion below under ‘‘Seeding.’’
A practical example is given by the existence of trees. A tree contains
vascular channels for transport of water from the roots to the leaves. At the
surface of the leaves water evaporates, providing the driving force for water
transport. Some trees are as high as 120 m. Taking into account that a 10 m
water column will exert a pressure of about one bar under gravity, this
means that the pressure in the channels at the top of the tree would be 1
120/10¼11 bar. Despite this negative pressure, the water column does not
‘‘break’’ more frequently than once in 10 or 100 years (breaking of a water
channel is irreparable; if it occurs frequently, the tree will die). Hence
nucleation of water vapor bubbles must be extremely difficult, at least below
about 40 8 C.
Seeding. It must be concluded that gas nucleation generally does
not occur. What can happen is that bubbles form by growth of tiny gas
pockets (mostly air) already present, i.e., by a seeding mechanism. Possible
situations are
Entrapment of air by agitation. This occurs, for instance, during
kneading of bread dough, where the entrapped cells cannot escape
because of the very high viscoelasticity of the dough. The number of
cells in a dough is about 10^14 m^3 , or one cell in about 20mm cubed.
Also when beer is splashed into a glass, air bubbles are entrapped
that then grow, because CO 2 is supersaturated and diffuses to the
existing bubbles.
Persistent remnants of air bubblesthat have almost disappeared by
Ostwald ripening (Section 13.6.1). Some bubbles contain sufficient
solid particles on their surface to form on shrinkage a closely packed
layer, as depicted in Figure 14.10a. These tiny bubbles have been
called ‘‘aphrons’’. They are quite stable, and they occur in most
natural waters, albeit in very small numbers.
Gas pockets that have been left, for instance in tiny crevices. When a
soft drink supersaturated with CO 2 is poured into a plastic beaker,
bubbles are generally formed at many sites on the wall. A crevice
contains some air, and CO 2 diffuses to the air pocket, which thereby
grows. A gas bubble then forms on the wall, and it will be dislodged
by the buoyancy force acting on it, leaving a little gas pocket in the
crevice; this is illustrated in Figure 14.10b. The process repeats itself
numerous times, leading to a train of bubbles. A prerequisite is that
the contact angle wall–water–air be large. When beer is gingerly
poured into a glass, gas bubbles hardly form, at most at one or a few