Difference between revisions of "Stability of Thin Films: Foams"

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A static pressure difference can be imposed between
 
A static pressure difference can be imposed between
the interior and exterior of a soap film by several means including, eg, gravity. In
+
gass-liquid-glass interfaces by several means including gravity. Therefore,
such cases the equilibrium film thickness depends on the imposed pressure difference
+
the equilibrium film thickness depends on the imposed pressure difference
as well as on the effective interface potential. When the film thickness
+
and the effective interface potential. A disjoining pressire p arises when the film thickness
does not minimize Vð‘Þ, there arises a disjoining pressure P ¼ �dV=d ‘ which
+
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
drives the system toward mechanical equilibrium. In response to a hydrostatic
+
 
pressure, the film thickness thus adjusts itself so that the disjoining pressure
 
pressure, the film thickness thus adjusts itself so that the disjoining pressure
 
balances the applied pressure and mechanical equilibrium is restored.
 
balances the applied pressure and mechanical equilibrium is restored.
The disjoining pressure versus film thickness as predicted by DLVO theory
+
 
for an aqueous film containing 1 mM of 1:1 electrolyte is shown along with
+
This paper predicts the disjoining pressure versus film thickness using DLVO theory
the effective interface potential in Figure 2. The equilibrium thickness of a
+
for an aqueous film containing 1 mM of 1:1 electrolyte. The equilibrium thickness of a
free film is where the effective interface potential is at a local minimum or,
+
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
equivalently, where the disjoining pressure vanishes with a negative slope. If
+
the same film is not free, but instead rises vertically from solution in the presence
+
of the earth’s gravitational field, its thickness will vary in response to
+
the height dependence of the hydrostatic pressure. For example, at �8 cm
+
above the solution the hydrostatic pressure in the film drops by �10 kPa and,
+
according to Figure 2, the film thickness at this height must decrease to
+
30 nm in order to be in equilibrium. Similar considerations are important for
+
 
establishing the distribution of liquid around several bubbles packed together
 
establishing the distribution of liquid around several bubbles packed together
in a foam, and hence the bubble shapes. The thin-film balance apparatus allows
+
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
 
the creation and study of a single thin film, held on a horizontal support, and at
any applied pressure. Hence, this method provides measurement of disjoining
+
any applied pressure. This method therefore provides a means to measure the disjoining
pressures vs film thickness (8,9).
+
pressures vs film thickness.
  
 
[[Image:Film Thickness vs Disjoining Pressure.jpg|thumb|Effective interface potential (left) and corresponding disjoining pressure (right)
 
[[Image:Film Thickness vs Disjoining Pressure.jpg|thumb|Effective interface potential (left) and corresponding disjoining pressure (right)
Line 52: Line 44:
 
M of 1:1 electrolyte. (Graph taken from Foams paper)]]
 
M of 1:1 electrolyte. (Graph taken from Foams paper)]]
  
Although the details of the interaction between neighboring bubble surfaces
+
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,
+
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,
it nevertheless captures the essential physics. There is a large energy barrier,
+
 
which prevents two films approaching too closely. This energy barrier may
 
which prevents two films approaching too closely. This energy barrier may
arise from electrostatic repulsion, as in the DLVO model, or it may arise from
+
arise from electrostatic repulsion or it may arise from
 
other interactions. However, its role is primarily to prevent two films from
 
other interactions. However, its role is primarily to prevent two films from
approaching sufficiently close that they fall into the deep attractive well. The
+
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
 
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
 
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
+
the disjoining pressure. Should the repulsive barrier be overcome, the films fall into the attractive minimum, whereupon they coalesce. Thus this repulsive
6 FOAMS
+
into the attractive minimum, whereupon they coalesce. Thus this repulsive
+
 
barrier provides the essential stabilization of the foam.
 
barrier provides the essential stabilization of the foam.
Based on the underlying physical chemistry of surfactants at interfaces,
 
important features of foam structure, stability, rheology, and their interrelationships
 
can be considered as ultimately originating in the molecular composition of
 
the base liquid.
 
  
 
=== Foam Structures ===
 
=== Foam Structures ===
Line 88: Line 73:
 
== Discussion ==
 
== 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 ==
 
== Reference ==
  
 
Saint-James, A., Durian, D. J., Weitz, D. A. and Updated by Staff 2012. Foams. Kirk-Othmer Encyclopedia of Chemical Technology. 1–24.
 
Saint-James, A., Durian, D. J., Weitz, D. A. and Updated by Staff 2012. Foams. Kirk-Othmer Encyclopedia of Chemical Technology. 1–24.

Latest revision as of 19:35, 12 November 2012

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.

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.

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:

  1. Firefighting
  2. Food
  3. Separations
  4. Oil Recovery
  5. Detergents
  6. Textiles
  7. 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.