Foams

A foam is created when a mass of gas bubbles is evenly distributed throughout a quantity of liquid, in such a way that the gas occupies almost all of the final volume, surrounded by the liquid in the walls of the bubbles. If the liquid then sets solid, a very porous mass is created, which would, in principle, make an excellent filter medium. Unfortunately, in the frozen state the bubbles are not interconnected, so no fluid flow through the foam is possible.

However, if the foam is reticulated (treated with combined thermal and chemical processing), the material in the bubble walls retracts to the nodes of the original bubble network, creating a very porous mass with almost all of the pores interconnected. The pore size is determined by the original bubble size, and quite a considerable variation in this size is possible.

Thick media cartridges can then be made by fixing a layer of plastic foam to a perforated core. A major alternative use is the formation of coarse ceramic foams, as discs, to be used as filters in the removal of contaminants from molten metal.

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