Classification, application and formation

A foam is a dispersion of a gas in a liquid or a solid medium. Some properties of foams resemble those of emulsions, which is not surprising because a foam can be thought of as an emulsion in which the dispersed phase is a gas. Foams are, to our knowledge, never thermodynamically stable, they can only be kinetically trapped. This may be one reason why a quantitative description of foams is only poorly developed. Very good introductions into the subject are Refs. [564-566], 

Foams have a large variety of applications. Solid foams are widely used as insulating materials. Due to the presence of air bubbles they have a low thermal conductivity. Polyurethane foams and Styrofoam are examples. Styrofoam is also used as a packing material. The light weight of polymer foams makes them attractive as filling materials to stabilize otherwise hollow structures. A natural solid foam is pumice stone. Metal foams are used in the automotive and aerospace industry as light and stable materials [567], Ceramic foams are developed for electronic applications as piezoelectric transducers and low dielectric constant substrates [568], 

To make a solid foam we start from a liquid foam and induce solidification. This can be achieved by a chemical polymerization (Styrofoam), by lowering the temperature (pumice stone or a souffle), or by increasing the temperature to induce a structural transition (baking of bread). Porous solids can appear as solid foams because of their low density and their high content of gas. The difference between the two is that in a porous solid we have a bicontinuous structure while the individual cavities in a foam are closed. This is an important difference because porous solids tend to adsorb liquids due to capillary effects and then completely change their properties. 

Diisocyanites have two reacting groups and they can form chains or even networks. The two most commonly used diisocyanites are toluenediisocyanites (TDI) and diphenylmethane diisocyanates (MDI) (Fig. 12.17). With each bond formed one molecule of gaseous carbon dioxide is released. This is responsible for the foaming effect. 

The basic step in the formation of a liquid foam is the generation of bubbles. We can accomplish this by forcing gas through a nozzle. We can also nucleate bubbles from supersaturated solutions, such as in beer, or from a superheated liquid (boiling). Another way to form bubbles is by mechanical agitation such as in a washing machine or in breaking waves on the sea. 

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