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Colloids Foams A **foam** is a substance that is formed by trapping many gaseous bubble in a liquid or a solid. A foam is normally an extremely complex system consisting of polydisperse gas bubbles separated by draining films. The term foam may also refer to anything that is analogous to such a phenomenon, such as quantan foam, pokyurethane foam (foam rubber),XPS foam, Polystyrena, phenolic, or many other manufactured foams. Fine foam can be considered a type of colloid.