Foam Formation Thin Liquid Films

In general, ordinary foams from detergent solutions are thick initially (micrometer), and as fluid flows away due to gravity or capillary forces or surface evaporation), the film becomes thinner (few hundred A). Foams are of essential part in many processes, both in industry and biology. 

Outer monolayer of detergent molecule Some amount of water Inner monolayer of detergent molecule Air on outer side 

The thickness of water phase can vary from over 100 pm to less than 100 nm (Birdi, 2010a Scheludko, 1966). Foams are thermodynamically unstable, since there is a decrease in total free energy when they collapse. As the thickness of the film decreases to around the wavelength of light (nm), one starts to observe rainbow colors (arising from interference). The TLF at even smaller in thickness (50 A or 5 nm). 

As regards foam stability, it was recognized that the surface tension under film deformation must always change in such a way as to resist the deforming forces. Thus, tension in the film where expansion takes place will increase, while it will decrease in the part where contraction takes place. There is, therefore, a force tending to restore the original condition. The film elasticity, Ej, has been defined as 

One of the most important applications is the bubble formation and stability in champagne. The size and the number of bubbles is found to be important for the impact of taste. The stability has also much impact on the looks and taste as well. The taste of impact is related to the size and number of bubbles, as well as how long the bubbles are stable. 

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Foam formation in a boiler is primarily a surface active phenomena, whereby a discontinuous gaseous phase of steam, carbon dioxide, and other gas bubbles is dispersed in a continuous liquid phase of BW. Because the largest component of the foam is usually gas, the bubbles generally are separated only by a thin, liquid film composed of several layers of molecules that can slide over each other to provide considerable elasticity. Foaming occurs when these bubbles arrive at a steam-water interface at a rate faster than that at which they can collapse or decay into steam vapor. 

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

A foam consists of a high volume fraction of gas dispersed in a liquid where the liquid forms a continuous phase. Wet foams with a high water content, e.g. immediately after the formation, can have more or less spherical bubbles. As a consequence of a drainage process of the foam lamellae, the wet foam loses water with time. Due to the resulting high volume fraction of gas, the bubbles are no longer spherical but they are deformed into a polyhedral shape. The polyhedra are separated from each other by thin liquid films. The intersection lines of the lamella are termed plateau borders (see Figure 3.28). 

Thin liquid films bordering a gas phase on both sides, or the so-called free films, are one of the oldest objects of research in the physical chemistry of disperse systems. The reason is probably the ease of their formation, simplicity, uniformity of surfaces, etc. Thin films, including foam films, are an efficient and useful model for the study of many surface phenomena. 

Microscopic foam films are most successfully employed in the study of surface forces. Since such films are small it is possible to follow their formation at very low concentrations of the amphiphile molecules in the bulk solution. On the other hand, the small size permits studying the fluctuation phenomena in thin liquid films which play an important role in the binding energy of amphiphile molecules in the bilayer. In a bilayer film connected with the bulk phase, there appear fluctuation holes formed from vacancies (missing molecules) which depend on the difference in the chemical potential of the molecules in the film and the bulk phase. The bilayer black foam film subjected to different temperatures can be either in liquid-crystalline or gel state, each one being characterised by a respective binding energy. 

Thin liquid films in foam and emulsion systems are usually stabilised by soluble surfactants. During the formation of such films the flow-out process of liquid disturbs the surfactant equilibrium state in the bulk and film surfaces. The situation of drainage of a surfactant containing liquid film between two oil droplets is shown in Fig. 3.15. (after Ivanov Dimitrov 1988). Here j" and are the bulk fluxes in the drops and the film, respectively, j and j are the fluxes due to surface diffusion or spreading caused by the Marangoni effect, respectively. 

The coalescence of disperse systems, such as foams and emulsions, and the contact of air bubbles with solid particles, e.g. in the process of flotation, takes place in two steps. The first is characterised by a flocculation of the system, the formation of thin liquid films with an equilibrium thickness. In the second step the film becomes thin enough for the interparticular attractions to overcome the film state so the two separated interfaces form a new interface. The situation where a small bubble attaches a liquid interface is shown in Fig. 2D. 1. 

The main stages in the formation and evolution of the thin liquid film between two equal size approaching foam bubbles as shown in Figure 4 can be summarized as ... 

The formation of ordered microstructure in thin liquid films offers a new mechanism for the stabilization of foams. As a proof of microstructuring in real foams, Figure 17 shows a photograph of an aqueous foam system stabilized because of the stratification in the foam bubble lamellae. The practical importance of the film microstructuring is that the lifetimes of foams with stratifying films are much longer. 

Surfactants improve foam formation by inducing film elasticity, which is helpful when the film is stretched as a gas bubble emerges from a liquid solution to become part of the foam matrix. As the film stretches, differentia surfactant adsorption at the interface leads to surface tension gradients and healing of the film (so it thins with approximately uniform thickness). The optimum surfactant concentration for foam formation (although not foam stability) is around the CMC. 

The previous definitions aid in the understanding of the chemistry and engineering of foams and are critical when dealing with the formation and stability of foam structure. Persistent or stable foams consist of a network of thin liquid films, which exhibit complex hydrodynamics. For a drilling foam to remain persistent, or stable, several mechanisms are required to prevent the loss of liquid and gas from the foam and to prevent premature collapse of the foam, when subjected to environmental stresses. 

CAS BUBBLE FORMATION AND STABILITY (THIN LIQUID FILMS AND FOAMS)... 

Thin liquid films can be formed between two coUiding emulsion droplets or between the bubbles in foam. Formation of thin films accompanies the particle-particle and particle-wall interactions in colloids. From a mathematical viewpoint, a film is thin when its thickness is much smaller than its lateral dimension. From a physical viewpoint, a liquid film formed between two macroscopic phases is thin when the energy of interaction between the two phases across the film is not negligible. The specific forces causing the interactions in a thin liquid film are called surface forces. Repulsive surface forces stabilize thin films and dispersions, whereas attractive surface forces cause film rupture and coagulation. This section is devoted to the macroscopic (hydrostatic and thermodynamic) theory of thin films, while the molecular theory of surface forces is reviewed in Section 4.4. 

Foams are disperse systems in vhich the liquid is a dispersing phase and the gas is a dispersed component. In order to obtain a solution of the consistency of foam, a two-phase liquid-gas system with the application of an appropriate surfactant must be formed (Malysa and Lunkenheimer, 2008). Due to the energy supplied to disperse the gas phase, gas bubbles surrounded by a thin liquid film containing surfactant molecules are released. In the case of pure solutions, the process of foam formation is very difficult because gas bubbles contained in the liquid or solution cannot be stabilized without a surfactant (Prud Homme and Khan, 1996). A characteristic feature of all foams is low density. 

For the mainly oil-soluble Span 20 siufactant, however, the lifetimes are much less and films rupture prematurely, in line with predictions based on Bancroft s rule. At concentrations well above the CMC where the effective volume fraction of micelles is significant (>5 vol%), thin liquid films may drain in a stepwise fashion by stratification. This phenomenon, seen initially with foam films, was explained by the formation of periodic colloidal structures inside the film that results in layering of the micelles. At a step-transition, a layer of micelles leaves the film and the film thickness decreases by approximately the effective micellar diameter. It can also occur in emulsion films shown recently for hexadecane-aqueous sodium case-inate-hexadecane systems. The step-height seen of around 20 nm is very close to the measured diameter of the casein micelles of between 20 and 25 nm. The layering ultimately increases the lifetime of a film, but a critical film area exists below which step transitions are inhibited such thick films containing layers of micelles are even more stable. 

Correspondingly, emulsion, foam and wetting films have been studied [4—10]. The model of a microscopic thin liquid film (radius 100 pm) allows one to obtain films at very low concentrations of polymeric surfactant and to study their formation and stability as well as to establish and to distinguish the surface (interaction) forces in them. 

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