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Foams are two-phase material systems where a gas is disbursed in a second, non-gaseous material, specifically, in which gas cells are enclosed by a distinct liquid or solid material.[1]: 6 [2]: 4 [3] The foam "may contain more or less liquid [or solid] according to circumstances",[1]: 6 although in the case of gas-liquid foams, the gas occupies most of the volume.[2]: 4 The word derives from the medieval German and otherwise obsolete veim, in reference to the "frothy head forming in the glass once the beer has been freshly poured" (cf. ausgefeimt).[1]: 1
Theories regarding foam formation, structure, and properties—in physics and physical chemistry—differ somewhat between liquid and solid foams in that the former are dynamic (e.g., in their being "continuously deformed"), as a result of gas diffusing between cells, liquid draining from the foam into a bulk liquid, etc.[1]: 1–2 Theories regarding liquid foams have as direct analogs theories regarding emulsions,[1]: 3 two-phase material systems in which one liquid is enclosed by another.[4]
In most foams, the volume of gas is large, with thin films of liquid or solid separating the regions of gas.[citation needed] A bath sponge and the head on a glass of beer are examples of foams; soap foams are also known as suds.[not verified in body]
Solid foams can be closed-cell or open-cell.[citation needed] In closed-cell foam, the gas forms discrete pockets, each completely surrounded by the solid material. In open-cell foam, gas pockets connect to each other. A bath sponge is an example of an open-cell foam:[not verified in body] water easily flows through the entire structure, displacing the air. A sleeping mat is an example of a product composed of closed-cell foam.[not verified in body]
Foams are examples of dispersed media.[not verified in body] In general, gas is present, so it divides into gas bubbles of different sizes (i.e., the material is polydisperse[not verified in body])—separated by liquid regions that may form films, thinner and thinner when the liquid phase drains out of the system films.[5][page needed] When the principal scale is small, i.e., for a very fine foam, this dispersed medium can be considered a type of colloid.[not verified in body]
Foam can also refer to something that is analogous to foam, such as quantum foam.[not verified in body]
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: CS1 maint: multiple names: authors list (link) Note, this source focuses only on liquid foams.
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: CS1 maint: multiple names: authors list (link) Note, this source also focuses on liquid foams.
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