Stability of Thin Films: Foams

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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)

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. For a foam to be relatively stable, the net interaction must be sufficiently repulsive at short distances to maintain a significant layer of liquid in between neighboring bubbles. Otherwise two bubbles could approach so closely as to expel all the liquid and fuse into one larger bubble. Repulsive interactions typically become important only for bubble separations smaller than a few hundredths of a micrometer, a length small in comparison with typical bubble sizes. Thus attention can be restricted to the vapor–liquid–vapor film structure formed between neighboring bubbles, and this structure can be considered essentially flat.

In that range of thickness (from a few tens of nm to a few hundred) the thin film are usually called � common black films.

Disjoining Forces

A static pressure difference can be imposed between the interior and exterior of a soap film by several means including, eg, gravity. In such cases the equilibrium film thickness depends on the imposed pressure difference as well as on the effective interface potential. When the film thickness does not minimize Vð‘Þ, there arises a disjoining pressure P ¼ �dV=d ‘ which 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. 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 the effective interface potential in Figure 2. The equilibrium thickness of a free film is where the effective interface potential is at a local minimum or, 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 in a foam, and hence the bubble shapes. The thin-film balance apparatus allows 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 pressures vs film thickness (8,9).

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. (Photo taken from Foams paper)

Although the details of the interaction between neighboring bubble surfaces in a thin flat film may not be accurately described by the simplest DLVO theory, it nevertheless captures the essential physics. There is a large energy barrier, 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 other interactions. However, its role is primarily to prevent two films from approaching sufficiently close that they fall into the deep attractive well. 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 6 FOAMS into the attractive minimum, whereupon they coalesce. Thus this repulsive 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

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 form

Possible Applications

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

Discussion

Reference

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