Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 17

FIGure 2.6 Coalescence of two bubbles with different radii.

Open

Closed

(a) (b)

FIGure 2.7a,b Equilibrium state of two bubbles of different radii (see text).

basis of lung alveoli, in which fluids (containing lipid surfactants) balance out the
expanding-contracting cycle.
It has been observed that a system with varying size bubbles collapses faster than
when bubbles (or liquid drops) are of exactly the same size. Another major conse-
quence is observed in the oil recovery phenomena. Oil production takes place (in
general) by applying gas or water injection. When gas or water injection is applied
where there are small pores, the pressure needed will be higher than that in the large-
pore zone. Thus, the gas or water will bypass the small-pore zone and leave the oil
behind (at present more than 30% to 50% of the oil in place is not recovered under
normal production methods). This obviously presents a great challenge to surface
and colloid chemists in the future. Enhanced oil recovery (EOR) investigations will
be described later in this topic (Chapter 6.2).
The capillary (Laplace) pressure determines many industrial and biological sys-
tems. The lung alveoli are dependent on the radii during the inhale-exhale process,
and the change in surface tension of the fluid lining them. In fact, many lung diseases
are related to the lack of surface pressure and capillary pressure balance. Blood flow
through arteries of different diameter throughout the body is another system where
the Laplace pressure is of much interest for analytical methods (such as heart func-
tion and control).
In industry, the magnitude of surface tension can be monitored with the help
of bubble pressure. Air bubbles are pumped through a capillary into the solution,
and the pressure measured is calibrated to known surface tension solutions. Using
a suitable computer, one can then estimate surface tension values very accurately.
Commercial apparatus are also available to monitor surface tension.
The consequence of Laplace pressure is very important in many different pro-
cesses. One example is that, when a small drop comes into contact with a large drop,
the former will merge into the latter. Another aspect is that vapor pressure over a
curved liquid surface, p cur , will be larger than on a flat surface, p lat . A relation between
pressure over curved and flat liquid surfaces was derived (Kelvin equation):