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gas, liquid, and a foaming agent. The foaming agent
may consist of one or more of the following: surfactants,
macromolecules, or finely dispersed solids. The foaming
agent is needed to reduce surface tension and thereby
lower the required energy to generate the increased
surface area in a foam.
2It is well agreed in the literature that pure liquids
do not foam. If single bubbles persist at the surface of
a pure liquid, the sample surely is contaminated.
Unimolecular films with a surface concentration of 10 10
mol/cm2 can already influence foaminess behavior; thus,
special surface cleaning methods need to be applied if
a clean surface is necessary for the measurement of
foamability.
3
The stability of foams is considered against two
different processes: film thinning and coalescence (film
rupturing). Coalescence reduces the total surface area
and therefore surface energy, signifying that foams are
thermodynamically unstable; the term stable is used to
mean stable in a kinetic sense. The life of foams can
vary over many orders of magnitude, from seconds to
years.
4
Kister4 summarized the forces acting on a liquid film
surrounding a bubble in a foam structure and common
mechanisms causing foam stability (Table 1).
In the following subchapters, the most important
forces and mechanisms in foams that require further
explanation will be described.
2.1. Surface Tension. Droplets of liquids and bubbles
of gas tend to adopt a spherical shape to minimize total
surface free energy. The responsible force is surface
tension. Surface tension ç° causes a pressure difference
across a curved surface, with a higher pressure on the
concave side (inside the bubble). Because the pressure
inside the bubble is uniform, the different radii at the
Plateau border and the lamella cause different pres-
sures inside the liquid (¢P 2ç°/R) and consequently
a capillary flow (Laplace flow) to the Plateau borders
leading to thinning and rupture of the lamellae and
causing foam collapse.
2 A restoring force can be pre-
sented by the Marangoni effect.
2.2.Marangoni Effect. By throwing an oil-drenched
sponge into a lake, Marangoni discovered in 1871 thata liquid with low surface tension spreads itself on a
liquid with high surface tension.
5 In a surfactant-
containing solution, the surfactants concentrate at the
liquid surface. When liquid drains from a film, the film
is thinned at this area, causing an increase in the
surface area. The additional area is supplied by liquid
from the bulk, which is leaner in surfactant concentra-
tion. Because of a lower surfactant concentration at the
surface, the surface tension rises. The Marangoni effect
provokes a surface flow from the nonthinned (low
surface tension) area to the thinned (high surface
tension) area, which works against drainage and re-
stores the film.
The mass-transfer-induced Marangoni effect does not
need surface-active components. Here the Marangoni
stresses, surface tension gradients, are caused by varia-
tion in the liquid composition or temperature at the
surface, stabilizing or destabilizing the lamellae.6 In
distillation systems, the enhanced mass transfer in thin
films leads to a concentration of the less volatile
component. If the less volatile component has a higher
surface tension (Marangoni-positive system), the thin
film is served with liquid from the Plateau borders, thus
restoring the film. Foams stabilized by the mass-
transfer-induced Marangoni effect can be stable and
often lead to severe foaming in columns.4 An example
for a positive distillation system is methanol/water.