Foam
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Foams are materials formed by trapping pockets of gas in a liquid or solid.
Soap foam bubbles
Cleaning sponge
Top of a foamy drink
Schematic stress-strain curve of an elastomeric foam, demonstrating the three regions which are linear elastic, cell-wall buckling, and cell-wall fracture. The area under the curve specified represents the energy per unit volume the foam can absorb.
Beer head
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Beer head is the frothy foam on top of beer and carbonated beverages which is produced by bubbles of gas, predominantly carbon dioxide, rising to the surface. The elements that produce the head are wort protein, yeast and hop residue. The carbon dioxide that forms the bubbles in the head is produced during fermentation as yeasts break down sugar-rich molecules to produce ethanol and carbon dioxide. The carbonation can occur before or after bottling the beer. If the beer continues fermenting in the bottle, then it naturally carbonates and the head is formed upon opening and/or pouring the beer. If the beer is pasteurized or filtered then the beer must be force carbonated using pressurized gas.
A glass of beer with a large head of foam
A glass of beer with the head having receded, leaving behind noticeable lacing
A German wheat beer served in the traditional glassware. The glass has a half litre fill line and is designed with extra space specifically to accommodate the head.