Liquid films foam film

We distinguish different types of liquid films. Foam films are liquid lamellae between two gas phases. They form, for example, between two bubbles. Emulsion films are films of liquid A between two immiscible liquids B. This can, for example, be a water film between two oil drops. When a solid surface is involved, we have a liquid film on top of a solid substrate. Such films form, for example, when a particle collides with a bubble. 

Kinsella (13, 14) summarized present thinking on foam formation of protein solutions. When an aqueous suspension of protein ingredient (for example, flour, concentrate, or isolate) is agitated by whipping or aeration processes, it will encapsulate air into droplets or bubbles that are surrounded by a liquid film. The film consists of denatured protein that lowers the interfacial tension between air and water, facilitating deformation of the liquid and expansion against its surface tension. 

Stability of isolated liquid films soap films foams. Pure liquids rarely, if ever, foam, but most solutions of capillary active substances do so to a greater or less extent, and long-chain colloidal electrolytes, particularly the soaps, have an almost miraculous power of rendering liquid films stable. Dewar1 has maintained soap films unbroken for over a year (one for three years ), with due precautions against evaporation, mechanical and thermal shocks, and absorption of carbon dioxide. 

As is well known, a lot of effects of surfactants, like damping of surface waves, the rate of thinning of liquid films, foaming and stabilisation of foams and emulsions, cannot just be described by a decrease in interfacial tension or by van der Waals and electrostatic interaction forces between two interfaces. The hydrodynamic shear stress at an interface covered by a surfactant adsorption layer is a typical example for the stimulation of an important surface effect. This effect, shown schematically in Fig. 3.9., is called the Marangoni effect. 

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. 

STABILITY AND WAVE MOTION OF THIN LIQUID FILMS FOAMS AND WETTABILITY... 

As will become evident from the discussion that follows, essentially only the value of Aafh) in Equation 1.32 is of interest to us. For this reason, Aa h) can be viewed as a primary characteristic describing film properties and the interactions between surfaces. Unfortunately, for solid planar parallel surfaces, such measurements are nearly impossible the experimental integration of n(fi) between macroscopic surfaces requires that the surfaces remain flat and parallel to the precision of fractions of an angstrom in the course of measuring very small forces. While this is impossible for solid surfaces, such measurements are quite possible and indeed broadly utilized in the investigation of liquid films emulsion films, foam films, and wetting films. In all of these cases, a flat and parallel state can be maintained because of the high mobility of the interfaces. 

Measurement of Disjoining Pressure Isotherms of Air-Liquid-Air Foam Films... 

FIGURE 2.9 Measurement of disjoining pressure isotherms of air-liquid-air foam films, (a) Porous frit film holder located inside hermetically sealed cell subject to applied gas pressure, Pg. Bulk liquid pressure is Pl (= Pi + PhS hyd text for definition of terms)... 

The microinterferometric method has widely been used by many authors to investigate both symmetric thin liquid films (foam and emulsion films) and asymmetric ones (wetting films). This method has been described in several papers, for example. Refs. [2, 3, 32, 33, 37] as well as in a book [1]. Here we give only a brief outline. 

A foam can be considered as a type of emulsion in which the inner phase is a gas, and as with emulsions, it seems necessary to have some surfactant component present to give stability. The resemblance is particularly close in the case of foams consisting of nearly spherical bubbles separated by rather thick liquid films such foams have been given the name kugelschaum by Manegold [175]. 

Both high bulk and surface shear viscosity delay film thinning and stretching deformations that precede bubble bursting. The development of ordered stmctures in the surface region can also have a stabilizing effect. Liquid crystalline phases in foam films enhance stabiUty (18). In water-surfactant-fatty alcohol systems the alcohol components may serve as a foam stabilizer or a foam breaker depending on concentration (18). 

AFFF, AQUEOUS FILM-FORMING FOAM Fii e-figliting foam wliicli flows on burning liquid as a film, providing rapid knock-down. 

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. 

If, by one of the above procedures, a few or even many bubbles have been introduced into a liquid, there is still no foam. In a foam, films between the bubbles are thin otherwise, the system is a gas emulsion. How, then, can a true foam be achieved If it is assumed that, because of some kind of stirring, two bubbles move to meet each other and the liquid layer between them gets thinner and thinner and if this process continues for a sufficient time, the two bubbles will touch and, eventually, coalesce. Many such encounters would destroy the foam. It is clear, therefore, that bubbles should be free to approach each other closely, but should be unable to cross the last short fraction of the initial distance. 

It is now possible to explain the origin of a critical capillary pressure for the existence of foam in a porous medium. For strongly water-wet permeable media, the aqueous phase is everywhere contiguous via liquid films and channels (see Figure 1). Hence, the local capillary pressure exerted at the Plateau borders of the foam lamellae is approximately equal to the mean capillary pressure of the medium. Consider now a relatively dry medium for which the corresponding capillary pressure in a... 

Monolayers are thus very useful in understanding various aspects of molecular packing (such as liquid crystals, etc.). With the information from area/molecule, the packing and other interaction parameters can be estimated. These monolayer studies have been found to be important in understanding the thin-liquid film (TLF) structures (bubbles, foams). 

In the beer industry, foaming behavior is vital to the product. The beer bottle is produced under C02 gas at high pressure. As soon as a beer bottle is opened, the pressure drops and the gas (CO2) is released, which gives rise to foaming. Commonly, the foam stays inside the bottle. Foaming is caused by the presence of different amphiphilic molecules (fatty acids, lipids, and proteins). The foam is very rich as the liquid film is very thick and contains a substantial aqueous phase (such foams are... 

Diverse foam structure applications In foam rubber, foamed polymers, shaving foams, milk shakes, and whipped creams, slowly draining thin liquid films (TLF) are needed. Accordingly, the rate of drainage is the most important factor in such industrial foam applications. 

Thin-liquid-film stability. The effect of surfactants on film and foam stability. Surface elasticity. Froth flotation. The Langmuir trough and monolayer deposition. Laboratory project on the flotation of powdered silica. 

In a later investigation, Kraynik and Hansen [62] demonstrated that the shear rate and liquid film viscosity greatly affect the rheological properties of foams. They studied the effect on foam properties and structure with variation of capillary number, Ca, which is the ratio of viscous to surface tension forces in the liquid films, and is given by -... 

The proteins in the liquid film should 1) be soluble in the aqueous phase, 2) readily concentrate at the liquid-air interface, and 3) denature to form cohesive layers possessing sufficient viscosity and mechanical strength to prevent rupture and coalescence of the droplet. That is, the polypeptides of the denatured proteins in the liquid film should exhibit a balance between their ability to associate and form a film and their ability to dissociate, resulting in foam instability. 

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