Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 9

2 Capillarity and Surface

Forces (Liquids)

2.1 IntroductIon
It is common observation that a liquid takes the shape of a container that surrounds or
contains it. However, it is also found that, in many cases, there are other subtle prop-
erties that arise at the interface of liquids. The most common behavior is bubble and
foam formation. Another phenomena is that, when a glass capillary tube is dipped in
water, the fluid rises to a given height. It is observed that the narrower the tube, the
higher the water rises. The role of liquids and liquid surfaces is important in many
everyday natural processes (e.g., oceans, lakes, rivers, raindrops, etc.). Therefore, in
these systems, one will expect the surface forces to be important, considering that
the oceans cover some 75% of the surface of the earth. Accordingly, there is a need to
study surface tension and its effect on surface phenomena in these different systems.
This means that the structures of molecules in the bulk phase need to be considered
in comparison to those at the surface.
The surface molecules are under a different force field from the molecules in
the bulk phase or the gas phase. These forces are called surface forces . A liquid
surface behaves like a stretched elastic membrane in that it tends to contract. This
action arises from the observation that, when one empties a beaker with a liquid,
the liquid breaks up into spherical drops. This phenomenon indicates that drops are
being created under some forces that must be present at the surface of the newly
formed interface. These surface forces become even more important when a liquid
is in contact with a solid (such as ground-water; oil reservoir). The flow of liquid
(e.g., water or oil) through small pores underground is mainly governed by capillary
forces . Capillary forces are found to play a very dominant role in many systems,
which will be described later. Thus, the interaction between liquid and any solid will
form curved surface that, being different from a planar fluid surface, initiates the
capillary forces.
In this chapter, the basics of surface forces will be described, and examples will
be given where the system is dependent on these forces. The principles of surface
forces are the building blocks that lead to the understanding of the subject. These
forces interact at both the liquid 1 -liquid 2 and liquid-solid interfaces.

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