Air-liquid foams

C. The remaining sol from Identification Test A, when whipped as for egg white in a kitchen-type mixer, produces a stable air/liquid foam. 

Hierarchically porous materials have been prepared using air bubbles as additional templates combined with micellar tempiating by a bubbling process that can produce air-liquid foams [64-67]. This synthesis process may allow concisely control over the cell sizes and shapes of the bubbles and a more easily... 

Emulsions are metastable colloids made out of two immiscible fluids, one being dispersed in the other, in the presence of surface active agents. The droplet volume fiaction may vary from zero to almost one dense emulsions are sometimes called biliquid foams since their structure is very similar to the cellular structure of air-liquid foams for which the continuous phase is very minor. From dilute to highly concentrated, emulsions exhibit very different internal dynamics and mechanical properties. When diluted, droplets are agitated by Brownian motion and behave as viscous Newtonian fluids, whereas when more concentrated, namely above the random close packing volume fraction which is 64% for monodisperse droplets, the internal dynamics are severely restricted and they behave as viscoelastic solids. Simple direct emulsions are composed of oil droplets dispersed in water while inverse emulsions are composed of water droplets dispersed in an oil continuous phase. In fact, emulsions are in principle made out of two immiscible phases for which the interfacial tension is therefore non-zero, and may involve other hydrophilic-like or lipophilic-like fluids in the presence of suitable surface active species, each phase being possibly comprised of numerous components. Sometimes, simple emulsions may also contain smaller droplets of the continuous phase dispersed within each droplet of the dispersed phase. Such systems are called double emulsions or multiple emulsions. ... 

Food typically is a complicated system with diverse interfaces. Stable air-water or oil-water interfaces are essential for the production of food foams and emulsions. Interface phenomena, therefore, attract great interest in the food industry. AFM provides enough resolution to visualize the interface structures, but it cannot be directly applied on air-liquid or liquid-liquid interfaces. Fortunately, the interface structure can be captured and transferred onto a freshly cleaved mica substrate using Langmuir-Blodgett techniques for AFM scan. Images are normally captured under butanol to reduce adhesion between the probe and the sample. Then, sample distortion or damage can be avoided (Morris et al, 1999). 

A stable foam is likely to have ingredients that are in a low energy state at the air-liquid interface. Substances that fit this description include proteins, emulsifiers some fats and fat components such as diglycerides monoglycerides and fatty acids. Food law uses the term emulsifier and stabiliser to cover the situation where the ingredient is stabilising an emulsion rather than helping to form it. 

The most widespread large-scale application of gas-liquid foams is in fire fighting, where air is excluded from the combustible material by a thick blanket of foam [320]. These fire-fighting foams are supplied as liquid concentrates, which can be diluted on-site to the required strength. The foam is formed from this premixture by an aerating device. Several studies have been undertaken to investigate the suitability of foams for bioremediation applications, as follows ... 

According to Tarassuk and Frankel (1955), foaming promotes lipolysis by providing (1) greatly increased surface area, (2) selective concentration of enzyme at the air-liquid interface, (3) activation of the substrate by surface denaturation of the membrane materials around the fat globules, and (4) intimate contact of the lipases and the activated substrate. 

The film is observed by a microscope using reflected light The film holder and the objective are immersed in air in the case of foam (i.e., air/liquid/air) film and in the oil phase, in the case of an O/W/O emulsion film, respectively. The film thickness can be determined by measuring the intensity of the light reflected from the film surfaces [9]. Further details of the technique will be discussed in Chapter 2. 

There are also several examples of natural surfactants and foams in the human body. The understanding of the pulmonary surfactant system, although discovered in 1929, has only been applied clinically since about 1990 for the treatment of respiratory distress syndrome. Surfactant replacement therapy may also be used in treating other forms of lung disease, such as meconium aspiration syndrome, neonatal pneumonia and congenital diaphragmatic hernia [881]. Lung surfactant, composed of phospholipids and proteins [882,883], is necessary to maintain a low surface tension at the alveolar air-liquid interface. When there is a deficiency of surfac-... 

The role of surfactants in stabilization/destabilization of foam (air/liquid dispersions) is similar to that for emulsions. This is due to the fact that foam stability/instability is determined by the surface forces operative in liquid films between air bubbles. In many industrial applications, it is essential to stabilize foams against collapse, e.g., with many food products, foam in beer, fire-fighting foam, and polyurethane foams that are used for furniture and insulation. In other applications, it is essential to have an effective way of breaking the foam, e.g., in distillation... 

In the case where foam instability is desirable, it is essential to choose surfactants that weaken the Gibbs-Marangoni effect. A more surface-active material such as a poly(alkyl) siloxane is added to destabilize the foam. The siloxane surfactant adsorbs preferentially at the air/liquid interface, thus displacing the original surfactant that stabilizes the foam. In many cases, the siloxane surfactant is produced as an emulsion which also contains hydrophobic silica particles. This combination produces a synergetic effect for foam breaking. 

The volume and shape of Plateau borders depend on the expansion ratio of the foam. In a spherical monodisperse foam with close packing of bubbles all air/liquid interfaces are spherical and the liquid volume which belongs to one cell can be derived from the difference between the volumes of the corresponding polyhedron (for example, a dodecahedron) and the inscribed in it sphere, having in mind the co-ordination number of the foam cell. 

Interactions between tannins and proteins have been extensively studied (Hager-man 1989 Haslam and Lilley 1988 Haslam et al. 1992), owing to their role in haze formation, astringency perception, and nutritional and anti-nutritional effects resulting from inhibition of various enzymes and reduction of dietary protein digestion. Other effects include reduced adsorption of /3-casein at the air-liquid interface in the presence of epigallocatechin gallate with potential consequences on foam properties (Sausse et al. 2003). 

Provided that G > Gp (for liquid foams x of solutions > x of air) we obtain Eq. (39) from Eq. (41) by substitution of k instead of G. In contrast to Wagner s formula, Odelevsky s formula holds for all concentrations of the disperse phase (gas) and for all types of gas-filled systems gaseous emulsions (d < 0.74), spherical (0.74 < d< 0.9) and polyhedral ( > 0.9) foams. It requires isotropy of the matrix structures and equal diameters of the disperse phase inclusions. Therefore, the dependence of the ratio of the foam to the solution electroconductivity on the degree of foaming in the general form is given by equation... 

A greenish blue liquid foamed and fumed over the cent and over the table. The air In the neighborhood of the performance became dark red. A great colored cloud arose. . . picking (the penny) up. .. I learned another fact—nitric acid not only... 

Aqueous foams are formed from a three-dimensional network of surfactant films in air. Foams can be used as formulations for the delivery of enemas and topical products. Foams which develop in production of liquids or in ampoules are troublesome hence there is an interest in breaking foams and preventing foam formation. The breaking and prevention of liquid foams is less well understood than the stabilisation of foams, it is recognised, however, that small quantities of specific agents can reduce foam stability markedly. There are two types of such agent ... 

Examples of industrial relevance for the first two combinations are the adsorption of pollutants from waste air or water onto activated carbon. Combinations three and four can be observed at the orientation of tensid molecules on water/air interfaces (foam formation, foam stabilization) or at the interface of two immiscible liquids, (e.g. oil and water, emulsion formation). This book deals mainly with the case of liquid molecules adsorbed onto solid surfaces. For this case the following definitions are made ... 

Foam is produced when air or some other gas in introduced beneath the surface of a liquid that expands to enclose the gas with a film of liquid. Foam has a more or less stable honeycomb structure of gas cells whose walls consist of thin liquid films with approximately plane parallel sides. These two-sided films are called the lamellae of the foam. Where three or more gas bubbles meet, the lamellae are curved, concave to the gas cells, forming what is called the Plateau border or Gibbs triangles (Figure 7-1). 

Tbe basis Tor (be separation by bubbles and foam (adsorptive bubble separalion) is the difference in the surface activities of the various materials present in the solution or the suspension of interest. The material mey be cellular or colloidal substances, crystals, minerals, ionic or molecular compounds, precipitates, proteins, or bactemi, but in any case it must be surface active at the air-liquid interface (Fig. 17.1-1) These surface-active meterials tend to attach preferentially to (he air-liquid interfaces of the bubbles or fonme, As the bobbies or fonms rise throngh the columa or pool oftiquid, the attached material is removed. When this combination reachas the surface, the meierial can be removed in the relatively smell volume or collapsed foam or surface "scum. ... 

Deutsch11 found that aqueous solutions of indicator dyes acquire a different color when vigorously shaken with an immiscible liquid such as benzene. Thus the color of Bromothymol Blue at pH 7.4 changes from blue to yellow when treated in this manner. The change in color takes place at the interface of the emulsified droplets, and the original color is restored when the liquids separate. In another study, it was observed that a colorless solution of rhodamine in benzene turns a deep red when vigorously shaken to form an emulsion with water. When the phases later separate, the color almost completely disappears. The phenomenon is not limited to liquidj liquid interfaces, but also occurs at an air liquid interface a brownish-yellow solution of thymolsulfonaphthalein (pH 2.8) when shaken with air develops a reddish-violet foam. 

Liquid foams are a fine distribution of a gas (normally air) in a liquid. Thin films of liquid (the lamellae) separate the gas bubbles from one another and the gas-liquid interfacial area is quite high. Pure liquids do not foam surface-active materials must be present in order to obtain stable foam bubbles. 

In order to produce safely and go over the fire zone successfully, mixed the FR-1 polymer liquid foam inhibitor and water according to the proportion of 1 2. Use special foam device to create the inhibitor foam. And then put out fire by means of pour this material into the hidden danger areas of spontaneous combustion through compressed air pipe or inert gas injection pipe. PU s foam equipment consists of special pump, solvent tank, foam maker and air line and so on. 

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