foam
/ fohm /
Picture the froth on top of a beer, or the head of bubbles when you wash dishes. It is mostly air, packed into a structure of countless little bubbles held together by thin films of liquid. A foam is exactly that: a material made of gas pockets crowded together inside a liquid or a solid.
In a foam, gas bubbles are packed so tightly that they press against one another and lose their round shape, squashing into many-sided cells separated by thin walls. Those walls are the liquid, drawn into sheets and edges by the gas pushing from both sides. Because so much of the volume is just air, a foam is very light, yet the connected network of liquid walls gives it a surprising stiffness and the ability to hold a shape — at least for a while.
Foams matter in everything from cappuccino froth and bread to firefighting foam, packaging, and lightweight materials. A common misconception is that a foam always pops away quickly. Some do, because the liquid drains and the walls thin until they burst; but if the liquid is locked in place — as in a baked loaf of bread or a block of foam plastic — the bubble structure freezes into a permanent, sturdy solid.
Bread is a frozen foam: yeast fills the dough with gas bubbles, and baking sets the dough solid around them. The light, airy texture of a fresh loaf is simply the trapped bubble structure preserved forever in a now-rigid wall.
Bread's airy crumb is a liquid foam set permanently solid by baking.
Foams are usually unstable on their own — the liquid slowly drains downward and the bubbles merge into bigger ones — which is why a soap foam needs surfactant molecules to keep its thin walls from breaking too fast.