View source for Stability of Thin Films: Foams

Entry by Grant Gonzalez, 9 Nov 2012

= Foams =

'''Keywords''': Thin Films, Disjoining Forces, Foams, Surfactants

'''Authors''': Arnaud Saint-James, Douglas J. Durian, David A. Weitz

== Summary ==

This paper examines the structures of foams, a dispersion of gas within a smaller volume of liquid. Thin films form the interfaces between the gas and liquid faces and are stabilized by surfactants. 

[[Image:Foams.jpg|thumb|Photograph illustrating the microstructure of the foam that still persists 2 h after
shaking an aqueous solution containing 5% sodium dodecylsulfate. The bubble shapes are
more polyhedral near the top, where the foam is dry, and more spherical near the bottom,
where the foam is wet. The average bubble size is about 2 mm. (Photo taken from Foams paper)]]

Bubbles are dispersed such that liquid thin films are surrounded by gas on both sides, forming a gas-liquid-gas interface. Neighboring bubble surfaces in a foam interact
through a variety of forces that depend on the composition and thickness of liquid
between them, and on the physical chemistry of their liquid–vapor interfaces. As a result, various interactions affect the stability of thin films. Foam stability therefore is primarily determined by the repulsion of bubble-bubble surfaces, allowing for a layer of liquid to be dispersed between two neighboring bubbles as short distances. Otherwise, the interaction terms will combine and form one bubble. As a result, foams can be considered (while determining stability)as vapor–liquid–vapor film structures formed between neighboring bubbles while considering the interface essentially
flat.

=== Disjoining Forces ===

A static pressure difference can be imposed between
gass-liquid-glass interfaces by several means including gravity. Therefore, 
the equilibrium film thickness depends on the imposed pressure difference
and the effective interface potential. A disjoining pressire p arises when the film thickness
does not minimize the potential energy as a function of length, there arises a disjoining pressure p. This disjoining pressure drives the system toward mechanical equilibrium. In response to a hydrostatic
pressure, the film thickness thus adjusts itself so that the disjoining pressure
balances the applied pressure and mechanical equilibrium is restored.

This paper predicts the disjoining pressure versus film thickness using DLVO theory
for an aqueous film containing 1 mM of 1:1 electrolyte. The equilibrium thickness of a
free film occurs when the effective interface potential is at a local minimum or when the disjoining pressure vanishes with a negative slope. Similar considerations are important for
establishing the distribution of liquid around several bubbles packed together
in a foam, and hence the bubble shapes. As presented in this paper, a thin-film balance apparatus allows
the creation and study of a single thin film, held on a horizontal support, and at
any applied pressure. This method therefore provides a means to measure the disjoining
pressures vs film thickness.

[[Image:Film Thickness vs Disjoining Pressure.jpg|thumb|Effective interface potential (left) and corresponding disjoining pressure (right)
vs film thickness as predicted by DLVO theory for an aqueous soap film containing 1 m
M of 1:1 electrolyte. (Graph taken from Foams paper)]]

The paper then continues to discuss that the interaction between neighboring bubble surfaces
in a thin flat film may not be accurately described by the simplest DLVO theory. However, their model nevertheless captures the forces at work. Specifically the large energy barrier,
which prevents two films approaching too closely. This energy barrier may
arise from electrostatic repulsion or it may arise from
other interactions. However, its role is primarily to prevent two films from
approaching sufficiently close that they fall into the deep attractive well. Therefore, it acts to stablize the foam. The
degree to which the two films are forced together by external forces determines
how high up the energy barrier they are forced; this is in turn parameterized by
the disjoining pressure. Should the repulsive barrier be overcome, the films fall into the attractive minimum, whereupon they coalesce. Thus this repulsive
barrier provides the essential stabilization of the foam.

=== Foam Structures ===

Foams depending on the concentration of liquid to vapor form various structures, depending on the "wetness" of the system. In very wet foams, a forth is formed as excess air bubbles rise to the surface of the liquid and pop. In wet foams, spherical bubbles are formed due to sufficiently strong repulsive interactions; as bubbles rise to the surface, they pack together. In dry foams, polyhedral bubbles result from the severe distortion of spherical bubbles resulting from the lack of liquid in the system.

=== Possible Applications ===

Foams have potential for application in many roles:

# Firefighting
# Food
# Separations
# Oil Recovery
# Detergents
# Textiles
# Cosmetics

== Discussion ==

This paper explains the formation of foams and how their stability can be examined by treating the interactions of the gas-liquid-gas interfaces as thin films. The paper is a great analysis of a real world system with possible applications. Furthermore, the paper links many topics this course discuses such as stability of thin films and surfactants.

== Reference ==

Saint-James, A., Durian, D. J., Weitz, D. A. and Updated by Staff 2012. Foams. Kirk-Othmer Encyclopedia of Chemical Technology. 1–24.