Every time you wash dishes or lather your hands with soap, you’re creating one of the most mysterious household compounds: foam. Foam isn’t classified as a liquid, a gas, or a solid, but it’s all three at the same time. Different substances form different types of foam that all seem to work differently. There’s not much that’s actually known about the physics of what forms this weird cohesion of matter states or how we can predict what type of foam will be formed from what type of substance.Most foams are made up primarily of gas in between bits of liquid, but there’s no mathematical formula for discovering how a foam will behave. Some foams are stiff, like shaving foams, while others are delicate, like soap bubbles. The size of the bubbles doesn’t seem to have any impact on how the foam behaves. The reason we haven’t been able to learn much about foam is a weird one.
Foam bubbles are inherently oddly shaped. The critical point of foam, defined as the moment when all the bubbles in the foam are perfectly spherical, is impossible to achieve on Earth because of gravity. Gravity pulls foam bubbles downward, and its impact is so great that even in a layer of foam just a few centimeters thick, there’s a definite difference between the shape of the bubbles at the bottom and the top. This makes it impossible to perform experiments on foam without changing what it is.