Everyday foam

The foam of everyday experience, though different in several ways from the foam that occurs and is used in reservoirs, is worthy of some examination. Such everyday foam is a two-phase mixture of gas and liquid, in which the liquid is the continuous fluid and the gas is held in separate cells. See also the discussions in Chapters 1—3 of this book.) To display the distinctive foamlike characteristics, the volume fraction of the discontinuous phase must be greater than about 70%. At this high gas volume fraction (the so-called quality), the bubbles of gas are closely crowded together so that they cannot move independently. They also change in shape, and the walls of the cells become approximately planer, polygonal surfaces that are called lamellae or bubble-films. 

A further point of difference between C02 foam and everyday foam is the fact that in C02 foams, the gas, which is a dense fluid, acts more like a liquid than like the gas encountered in everyday foams. This means that the discontinuous phase of the foam is also capable of carrying other materials, such as surfactants, in solution. The capability imposes a new constraint on the surfactant, as we shall see. Perhaps surprisingly, however, the dense state of the gas does not cause any difficulty in the formation of the foam. In fact, it is possible to make analogous foams at low pressure with the same surfactants in water or brine, and with a light hydrocarbon like isooctane substituted for the dense C02. Despite this fact, it is common in the foam literature to refer to the discontinuous phase as gas . Similarly, the brine—surfactant solution that forms the continuous phase is often referred to simply as water . 

Contact of surfactants with the skin and mucus membranes occurs either accidentally or as a consequence of normal use. Examples of this normal and everyday use are cleaning formulations, shampoos, foam baths, and toothpastes. Again this contact is seldom made with individual surfactants, in this case alcohol sulfates and alcohol ether sulfates, but through formulated products. It is known that surfactants present significant interactions, so that mixed systems are generally less aggressive than their individual components. However, the effect of pure surfactants merits attention, particularly sodium dodecyl sulfate, which is commonly used as a reference for many studies because of its high purity and availability. 

Some important everyday items that are made from polymers with widely different properties Include billiard balls, plastic dishes, soda bottles, barrier and decorative films, egg cartons, polymeric drinking glasses, foam seats, and automotive tires. These applications for synthetic polymers have developed over about 150 years. As shown in Table 2.1, modern polymer material science and technology can be traced back to as early as 1770 [1]. Some Important advances In the understanding of polymer production were developed before World War II. 

EVA is a copolymer of ethylene with minor amounts (ca. 10 0%) of vinyl acetate. EVA has many uses as a foam rubber in everyday goods (like shoes etc.), as cable insulation and as encapsulation material in photovoltaic cells. 

It is common observation that a liquid takes the shape of a container that surrounds or contains it. However, it is also found that, in many cases, there are other subtle properties that arise at the interface of liquids. The most common behavior is bubble and foam formation. Another phenomena is that, when a glass capillary tube is dipped in water, the fluid rises to a given height. It is observed that the narrower the tube, the higher the water rises. The role of liquids and liquid surfaces is important in many everyday natural processes (e.g., oceans, lakes, rivers, raindrops, etc.). Therefore, in these systems, one will expect the surface forces to be important, considering that the oceans cover some 75% of the surface of the earth. Accordingly, there is a need to study surface tension and its effect on surface phenomena in these different systems. This means that the structures of molecules in the bulk phase need to be considered in comparison to those at the surface. 

Foam destabilization is also a factor in the packing and orientation of mixed films, which can be determined from monolayer studies. It is worth mentioning that foam formation from monolayers of amphiphiles constitutes the most fundamental process in everyday life. The other assemblies, such as vesicles and BLM, are somewhat more complicated systems, which are also found to be in equilibrium with monolayers. 

The major applications of polyurethanes in the domestic area are in the noncastable area, for example foam mattresses, cushions, and floor finishings. However, there are several everyday examples of polyurethanes in the... 

Subsequent chapters provide many examples of foams in industry and everyday life. Solid foams, dispersions of gas in a solid, are not, in general, covered in this book. 

Plastics production and utilization in terms of various forms such as pumps, billiard balls, valves, pipes, fans, nose cones, airplane canopies, hosiery, cabinets, pot handles, heat valves and other implants make one to realize the role played by them in everyday life. Plastics are produced by casting, molding, and extrusion and calendaring solid parts, lining, coatings, foams, fibers and films are also readily produced and available. 

If a gas and a liquid are mixed together in a container, and then shaken, examination will reveal that the gas phase has become a collection of bubbles that are dispersed in the liquid A foam has been formed (Figure 1). Foams have long been of great practical interest because of their widespread occurrence in everyday life. Some important kinds of familiar foams are listed in Table I. In addition to their wide occurrence, foams have important properties that may be desirable in a formulated product, such as fire-extinguishing foam, or undesirable, such as a foam in an industrial distillation tower. 

Table I. Some Examples of Foams in Everyday Experience...

Entropy Conductivity Most of our experiences of entropy are gained in everyday life, even if unconsciously. The coffee in a thermos remains hot because it is difficult for entropy to penetrate the vacuum jacket, while coffee in a cup cools down fairly quickly because entropy leaves along with the steam rising from it. Gases and foam materials whose volumes are up to 97 % gas-filled pockets strongly hinder the flow of entropy, while metal exhibits a cmiductivity of about 1,000 times that much. We will quantify this phenomenon in the following (Fig. 20.6). 

Interfacial effects are especially important in systems where interfadal area is large. This condition is met when one phase is dispersed in another as small drops or particles. With spherical particles, for example, the arearvolume ratio of the dispersed phase is (3/R), where R is the partiele radius. Qearly as R decreases with a given volume of the dispersed matraial present, interfadal area increases. When at least one dimension of each drop or particle decreases to a value in the range of 1 /rm or less, we say that a eoUoidal dispra ion exists. Foams, aerosols, and emulsions are colloidal dispersions involving fluid interfaces that are familiar from everyday life and are important in applieations ranging from food products... 

All these examples, illustrate that emulsions and foams are encountered in everybody s everyday life. 

The most powerful tool that the troubleshooter brings to his job is not his technical education but the simple experiences of everyday life. For instance, the tendency of dirty liquids to foam is well known to all of us. 

Together with numerous advantages that synthetic polymeric materials provide to society in everyday life, there is one obvious disadvantage related to the high flammability of many synthetic polymers. Polymers are used in manufacturing not only bulk parts but also films, fibers, coatings, and foams, and these thin objects are even more combustible than molded parts. 

Foamed plastics are also called cellular polymers and expanded plastics, and have played a great role in everyday life. Sponge is an open cellular polymer that is well known, but wood is the oldest form of foam. It is a naturally occurring foam of cellulose. The first commercial foam was sponge rubber that was introduced in thel910s.f Polymeric foams possess special characteristics that render them attractive for many industrial and household applications. 

Foams are cellular materials made up of soft bubbles in which gas is dispersed in a continuous liquid or solid, when the volume fraction of the gas is larger than of the liquid or solid. Foam structures occur, or are conjectured to do so, on every length scale, from the Planck scale (10 meters) to that of the large-scale structure of the universe. On the everyday length scale of your favorite pub, foam is what we get when bubbles rise... 

Surfactant molecules can be diverse in structure, and there are many examples of surfactants in everyday life, from the soaps we use to wash with, to the lipids that make up our cell membranes, and even to the proteins that help to stabilize the foam on your beer. ... 

You will be familiar with many common, everyday polymers, such as polyethylene plastic grocery bags or the polystyrene used for hard plastics and packing foams. These molecules have a linear chain structure with... 

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