Foam drainage liquid phase

To understand drainage we have to discuss the pressure inside the liquid films. At the contact line between liquid films, a channel is formed. This is called the Plateau border. Due to the small bending radius (rP in Fig. 12.18), there is a significant Laplace pressure difference between the inside of the compartment and the liquid phase. The pressure inside the liquid is significantly smaller than in the gas phase. As a result, liquid is sucked from the planar films into the Plateau s border. This is an important effect for the drainage of foams because the Plateau borders act as channels. Hydrodynamic flow in the planar films is a slow process [574], It is for this reason that viscosity has a drastic influence on the evolution of a foam. Once the liquid has reached a Plateau border the flow becomes much more efficient. The liquid then flows downwards driven by gravitation. 

The rate of foam drainage is determined not only by the hydrodynamic characteristics of the foam (border shape and size, liquid phase viscosity, pressure gradient, mobility of the Iiquid/air interface, etc.) but also by the rate of internal foam (foam films and borders) collapse and the breakdown of the foam column. The decrease in the average foam dispersity (respectively the volume) leads the liberation of excess liquid which delays the establishment of hydrostatic equilibrium. However, liquid drainage causes an increase in the capillary and disjoining pressure, both of which accelerate further bubble coalescence and foam column breakdown. 

Upon creating a pressure drop Ap in the foam liquid phase (with a porous plate, see Section 1.4) the liquid begins to flow out from the foam. During the process of foam drainage the radius of border curvature rb decreases while the capillary pressure increases, thus leading to reduction of the pressure drop Apr... 

The rate of foam drainage at large pressure drops in the foam liquid phase is much higher than that in gravitational field. 

An important aspect of foam drainage theory represents the behaviour of foam liquid phase in the initial stage of foam formation. Moreover, this behaviour is of major industrial significance, since it determines the way in which properties of frozen low expansion ratio foams and solid polymer foams, used as thermal insulation of soil, are regulated [3],... 

Depending on the type of external impact the processes in a foam could be accelerated (for example, creating pressure drop in the foam liquid phase leads to a drainage rate increase) or new processes can be initiated (for example, rupture of films that are stable if no pressure is applied). 

It is well know that the direct comparison between the methods of estimation of foam stability (in most of the cases it is determined by the foam lifetime) is not possible. Each of the existing methods involves different parameters, for example, time for destruction of a foam column of a definite height (or part of it), rate of decrease in the specific foam surface, etc. The main reason for the impossibility to make such a comparison is that foam stability is determined at different pressures in the foam liquid phase. This means that the rate of drainage as well as the time of reaching an equilibrium state of the films in the foam is different. Another reason could be attributed to the possibility both foam formation (i.e. foam volume... 

The accumulation method involving foam drying by creation of a pressure drop in the foam liquid phase is effective for extraction of some proteins from aqueous solutions. The drainage and destruction of foams from aqueous globular protein solutions (BSA, egg albumin, HSA, lysozyme and trypsin) under pressure drop have been studied in [74,76,77]. 

The studies discussed expand the use of the method for assessment of foetal lung maturity with the aid of microscopic foam bilayers [20]. It is important to make a clear distinction between this method [20] and the foam test [5]. The disperse system foam is not a mere sum of single foam films. Up to this point in the book, it has been repeatedly shown that the different types of foam films (common thin, common black and bilayer films) play a role in the formation and stability of foams (see Chapter 7). The difference between thin and bilayer foam films [19,48] results from the transition from long- to short-range molecular interactions. The type of the foam film depends considerably also on the capillary pressure of the liquid phase of the foam. That is why the stability of a foam consisting of thin films, and a foam consisting of foam bilayers (NBF) is different and the physical parameters related to this stability are also different. Furthermore, if the structural properties (e.g. drainage, polydispersity) of the disperse system foam are accounted for it becomes clear that the foam and foam film are different physical objects and their stability is described by different physical parameters. 

The gas bubbles in food foams are separated by sheets of the continuous phase, composed of two films of proteins adsorbed on the interface between a pair of gas bubbles, with a thin layer of liquid in between. The volume of the gas bubbles may make up 99% of the total foam volume. The contents of protein in foamed products are 0.1-10% and of the order of 1 mg/m2 interface. The system is stabilized by lowering the gas-liquid interfacial tension and formation of rupture-resistant, elastic protein film surrounding the bubbles, as well as by the viscosity of the liquid phase. The foams, if not fixed by heat setting of the protein network, may be destabilized by drainage of the liquid from the intersheet space, due to gravity, pressure, or evaporation, by diffusion of the gas from the smaller to the larger bubbles, or by coalescence of the bubbles resulting from rupture of the protein films. 

Role of Liquid-Phase Viscosity. As in any fluid flow process, the liquid viscosity offers resistance to flow and has a direct bearing on the rate of drainage of the liquid from foam films. The rate of film thinning will decrease as the liquid viscosity increases, and in the extreme case of very high viscosity (for example, the solidified films of latex foam), the resistance to flow can make the foam very stable. 

As mentioned earlier, heavy oil produced by solution gas drive often displays marked foaminess in wellhead samples. This feature is not surprising because the two key factors needed for nonpolar foam stability are present in the heavy-oil system. The viscosity of the liquid phase (heavy oil) is high enough to retard drainage of liquid films by capillary... 

Liquid foams, like emulsions, have a tendency to separate into distinct gas and liquid phases in order to decrease the total interfacial area. They may exist for a few seconds (e.g. champagne bubbles) or months (e.g. ice cream, provided it is kept frozen), depending on the properties of the liquid and the surface active molecule. Creaming in foams is accompanied by drainage of the liquid from between the bubbles. Since the matrix is very viscous, creaming and drainage are very slow in ice cream. 

Practical mechanisms for extending the persistence of foams can include one or several of the following conditions (1) a high viscosity in the liquid phase, which retards hydrodynamic drainage, as well as providing a cushion effect... 

A direct comparison between different methods of evaluating foaming is rarely feasible, since each method involves different parameters, and also foam stability is determined at different pressures in the foam liquid phase. These features mean that the rate of drainage and the time to reach the equilibrium state are different. However, the foam pressure drop method enables small... 

These actions combined effects may collapse the foam within minutes after air incorporation. Foams life times have been increased from several hours to days and months by the adsorption of the short chain amphiphilic molecules [50], while only a few minutes or hours stabilization results from the use of long-chain surfactants or proteins at the air-water interface [35]. Unlike other particle-stabilized foams [2], these foams percolate throughout the whole liquid phase and exhibit no drainage over days and months [49] due to the high concentration of modified particles in the initial suspension, which allows for the stabilization of very large total air-water interfacial areas. 

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). 

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