Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 23

Bubble film stability can be described as follows:

Evaporation of water
Flow of water away from the film
Stability of the bubble film

Evaporation can be reduced by containing the bubble in a closed bottle. It is also
found that, in such a closed system, the bubbles remain stable for a very long time.
The drainage of water away from the film is dependent on the viscosity of the fluid.
Therefore, such additives as glycerin (or other thickening agents [polymers]) assist
in maintaining stability.

2.6 meaSurement oF SurFace tenSIon oF lIquIdS
It is thus apparent from what has been discussed so far that the measurement of the
surface tension of liquids is an important analysis. The method to use in the mea-
surement of γ depends on the system and experimental conditions (as well as the
accuracy needed). For example, if the liquid is water (at room temperature), then the
method used will be different from that if the system is molten metal (at very high
temperature, ca. 500°C). These different systems will be explained in the methods
described in this section.

2.6.1 l I q u I d d r o p W e I g h T a n d S h a p e m e T h o d

The formation of liquid drops when flow occurs through thin tubes is a common
daily phenomenon. In some cases, such as eyedrops, the size of the drop plays a sig-
nificant role in the application and dosage of the medicine. The drop formed when
liquid flows through a circular tube is shown in Figure 2.10.
In many processes (such as oil recovery, blood flow, underground water), one
encounters liquid flow through thin (micrometer diameter), noncircular-shaped
tubes, or pores. In the literature, one finds studies that address these latter systems. In
another context of liquid drop formation, for example, in an inkjet nozzle, this tech-
nique falls under a class of scientifically challenging technology. The inkjet printer
demands such quality that this branch of drop-on-demand technology is much in
the realm of industrial research. All combustion engines are controlled by oil drop
formation and evaporation characteristics. The important role of capillary forces is
obvious in such systems.
As the liquid drop grows larger, it will, at some stage, break off the tube (due to
gravity force being larger than the surface force) and will correspond to the maxi-
mum weight of the drop that can hang. The equilibrium state where the weight of the
drop is exactly equal to the detachment surface energy is given as

m m g = 2 π R γ (2.32)

where m m is the weight of the detached drop, and R is the radius of the tubing.