Gas bubbles, in foams

A foam is a colloidal dispersion in which a gas is dispersed in a continuous liquid phase. The dispersed phase is sometimes referred to as the internal (disperse) phase, and the continuous phase as the external phase. Despite the fact that the bubbles in persistent foams are polyhedral and not spherical, it is nevertheless conventional to refer to the diameters of gas bubbles in foams as if they were spherical. In practical occurrences of foams, the bubble sizes usually exceed the classical size limit given above, as may the thin liquid film thicknesses. In fact, foam bubbles usually have diameters greater than 10 pm and may be larger than 1000 pm. Foam stability is not necessarily a function of drop size, although there may be an optimum size for an individual foam type. It is common but almost always inappropriate to characterize a foam in terms of a given bubble size since there is inevitably a size distribution. This is usually represented by a histogram of sizes, or, if there are sufficient data, a distribution function. 

Geometrical shape of gas bubbles in foam depends on the ratio of gas and liquid volumes, on the degree of polydispersity and on bubble packing. The results discussed below apply also for concentrated emulsions (considering density and interfacial tension). 

As it is well known, the contacts between drops (in emulsions), solid particles (in suspensions) and gas bubbles (in foams) are accomplished by films of different thickness. These films, as already discussed, can thin, reaching very small thickness. Observed under a microscope these films reflect very little light and appear black when their thickness is below 20 nm. Therefore, they can be called nano foam films. IUPAC nomenclature (1994) distinguishes two equilibrium states of black films common black films (CBF) and Newton black films (NBF). It will be shown that there is a pronounced transition between them, i.e. CBFs can transform into NBFs (or the reverse). The latter are bilayer formations without a free aqueous core between the two layers of surfactant molecules. Thus, the contact between droplets, particles and bubbles in disperse systems can be achieved by bilayers from amphiphile molecules. 

A complicated many-layer structure of foam cells is formed when gas bubbles are sparged into solutions of surfactants. According to [429], each bubble has a two-sided envelope which is a layer of the solvent containing hydrophilic polar parts of surfactant molecules (see Figure 7.2). Nonpolar hydrophobic parts of molecules on the inner surface of the envelope are oriented toward the bubble, and on the outer surface, outward the envelope. Between two cells, each of which is a capsule with envelope, there is a lamella, that is, an interlayer of a complicated structure. In the middle of the lamella, there is a liquid layer that is a continuous phase. On each of two surfaces of this layer, there is a monolayer of the surfactant. The hydrophobic parts ( tails ) of surfactants molecules in each monolayer and the tails of the envelope form two direct plate micellae [413], which separate the envelope and the continuous liquid film at the center. Thus, gas bubbles in foam are separated at least by five distinct layers. The multilayer structure of a foam lamella is well seen in photographs (e.g., see [429], p. 54). This fact is also confirmed by the ladder-type shape of the disjoining pressure... 

Ice crystals increase the viscosity of ice cream because they are solid particles suspended in the matrix cf. equation 2.11). However, because the ice crystals can interact during flow (since their volume fraction is quite high) and because they are not spherical, the viscosity is greater than equation 2.11 predicts. Similarly, fat droplets in emulsions and gas bubbles in foams with a relatively low gas-phase volume can also be... 

This closeness of 0 to zero explains the existence of a gas-oversaturated solution area in the polymer melt, when P < Pg, but the entire volume of gas remains in the solution. The degree of oversaturation, particularly upon free foaming (not in flow) can be 2- to 3-fold. In real polymer compositions, there are always solid admixtures, which have poor wetting areas. This reduces the degree of oversaturation at the interface melt-molding tool. Moreover, bubble nuclei can result from fragmentation of gas bubbles in the polymer [16]. Another factor that promotes the formation of bubble nuclei is the presence of localized hot points in the polymer melt they act as nuc-leation centres. Hot points appear either after a chemical reaction in the melt polymer [17], or in overheated areas on the surface of metal equipment [18]. Density of nucleation can be improved via introduction of various agents that reduce tension of the polymer [19]. 

Foamed polymers. Thermosets and thermoplastics formed into low density, cellular materials containing bubbles of gas. Rigid foams have their gas bubbles in closed cells, inhibiting flexibility flexible foams have the bubbles in open cells, permitting the gas to escape as the foam is flexed. 

R. Lemlich Prediction of Changes in Bubble Size Distribution Due to Interbubble Gas Diffusion in Foam. Ind. Eng. Chem. Fund. 17, 89 (1978). 

Some of the quantitative consequences of hot spot theory were presented in Ref 19. A qualitative discussion of heat flow in a compressed gas bubble hot spot was also presented in Ref 19. The necessity of having enough, but not too much, liquid or solid spray or foam within a compressed gas bubble (in order to have sufficient heat flow from the bubble to the surrounding condensed explosive) provides another hard-to-control variable in impact testing and thus increases the variability of test results... 

In order to control this disadvantageous effect, de-foamers and de-aeraters are added19. The defoamer s task is to control the foam formation due to the tensio-active substances, while the de-aerater removes gas bubbles, in solution and in the fibre structure. In this way, the contact surface between fabric and bleaching solution is kept maximal, which can only favour the rate of the process. By adding the de-aerater, lower quantities of tensio-active substances are needed, which helps to minimise foam formation. 

The Vessize program next proceeds to calculate the required horizontal vessel lengths for gas bubble or foam separation from the oil phase. Water separation from the oil phase is also calculated in the following discussion... 

Gas bubble in oil phase. Little research here has been accomplished, and very little has been published about gas bubble or foam separation from liquid. Herein I offer a good contribution to this technology, along with a plea for more field-proven data. As in the case for liquid droplet fall in the gas phase, I propose that the same equations, Eqs. (4.5), (4.6), and (4.7), be used in the oil media. This is done in these three equations, Eq. (4.7) deriving the gas bubble terminal velocity. We must, however, input a feasible and proven gas particle size Du, pm. Having accomplished several field-proven foam separation tests, the following Dm determination equation is offered. 

Electric double layers can be present at the gas/liquid interfaces between bubbles in foams. In this case, since the interfaces on each side of the thin film are equivalent, any interfacial charge will be equally carried on each side of the film. If a foam film is stabilized by ionic surfactants then their presence at the interfaces will induce a repulsive force opposing the thinning process. The magnitude of the force will depend on the charge density and the film thickness. 

Foams Free gas bubbles in oceans, lakes and rivers G/W... 

A dispersion of gas bubbles in a liquid, in which at least one dimension falls within the colloidal size range. Thus a foam typically contains either very small bubble sizes or, more commonly, quite large gas bubbles separated by thin liquid films. The thin liquid films are called lamellae (or laminae ). Sometimes distinctions are drawn as follows. Concentrated foams, in which liquid films are thinner than the bubble sizes and the gas bubbles are polyhedral, are termed polyederschaum . Low-concentration foams, in which the liquid films have thicknesses on the same scale or larger than the bubble sizes and the bubbles are approximately spherical, are termed gas emulsions , gas dispersions , or kugelschaum . See also Evanescent Foam, Froth, Aerated Emulsion. 

Condensation method for generating a foam involves creation of gas bubbles in the solution by decreasing external pressure or by increasing temperature (up to achieving a supersaturation of the solution) or as a result of a chemical reaction. 

Such an expression for the modulus of elasticity is obtained in [1], It is shown that Eq. (8.5) holds also for gas bubbles in liquid, if pa = 2o/R. Since during drainage the gas volume in the foam does not change, the pressure in the bubble remains constant. If at foam compression all cells undergo identical geometrical changes, then Eq. (8.5) remains valid for the whole foam. 

The production of polymer foams also includes a stage of air incorporation or formation of gas bubbles in the reaction medium. The gas can be trapped by the liquid phase, containing the surfactants during foam formation or be generated in the monomer-polymer... 

Figure Cl-1 shows a few examples of stmctures. The first column shows a microphoto of shaving cream at the top, with a schematic model below. This is a foam a dispersion of gas bubbles in a liquid. The photo shows about 2 mm across, so most bubbles are roughly 0.1 mm in diameter.

Foams are closely related to emulsions. In foams, the dispersed phase is not an oil or water, but it is a gas. One can use similar techniques for making foam and for making emulsions, and some of the properties are comparable. Foams are ubiquitous as well of course, the foam on beer is well known, but bread is a foam as well, as are ice cream, whipped cream, expanded polystyrene or styrofoam (which is a solid polymeric matrix with a large volume fraction of gas bubbles in it), and polyurethane foam, used, for example, for making mattresses. 

At higher EO levels, the foam volume produced by AEGS and AESo surfactants were less adversely affected by the presence of an oil phase than were other surfactants studied (Table I. Figure 1). This behavior was likely due to the formation of an oil/water emulsion which stabilized the fluid films between gas bubbles. Although foam volumes were smaller, at 75°C in three different brines, the sensitivity of AE and AES surfactants to the presence of decane decreased with increasing surfactant ethylene oxide content. 

In the early 1960 s Saunders and Frisch proposed a colloidal-chemical mechanism of open-cell formation in oligomeric foams. Later, Rossmy et al. formulated a physical mechanism of cell opening due to the effect of water vapor see Chap. 5.3). The data presented in this section explain the formation of open cells on the basis of morphological factors taking into account the type of packing of gas bubbles in oligomeric foams. 

Foam A dispersion of gas bubbles, in a liquid or solid, in which at least one dimension falls within the colloidal size range. Thus a foam typically contains either very small bubble sizes or, more commonly, quite large gas bubbles separated by thin liquid films. 

The preparation of a polymeric foam involves first the formation of gas bubbles in a liquid system, followed by the growth and stabilization of these bubbles as the viscosity of the liquid polymer increases, resulting ultimately in the solidification of the cellular resin matrix. 

Food foams are dispersions of gas bubbles in a continuous liquid or semisolid phase. Foaming is responsible for the desirable rheological properties of many foods, e.g., the texture of bread, cakes, whipped cream, ice cream, and beer froth. Thus foam stability may be an important food quality criterion. However, foams are often a nuisance for the food processor, e.g., in the production of potato starch or sugar and in the generation of yeast. Residues of antifoaming aids in molasses may drastically reduce the yield in citric acid fermentation. 

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. 

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