Stability of Thin Films: Foams
Entry by Grant Gonzalez, 9 Nov 2012
Keywords: Thin Films, Disjoining Forces, Foams, Surfactants
Authors: Arnaud Saint-James, Douglas J. Durian, David A. Weitz
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.
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.
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.
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.
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.
Foams have potential for application in many roles:
- Oil Recovery
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.
Saint-James, A., Durian, D. J., Weitz, D. A. and Updated by Staff 2012. Foams. Kirk-Othmer Encyclopedia of Chemical Technology. 1–24.