bubble

A bubble is a pocket of gas enclosed by a liquid film or another immiscible boundary. In liquids, bubbles typically have a spherical shape due to surface tension, though their shape can be distorted by flow, gravity, or external forces. Bubbles form when a gas is introduced into a liquid, when a dissolved gas comes out of solution, or during chemical reactions that release gas. The film that encloses the gas is usually a thin layer made of liquid with surface-active molecules (surfactants), which lowers surface tension and stabilizes the film.

Key properties include buoyancy: since gas is less dense than the surrounding liquid, bubbles rise and may coalesce or burst at the surface. Stability depends on film thickness, impurities, and surfactants. Bubbles can coalesce to form larger bubbles, or be broken apart by turbulence or pressure changes. In many systems, bubbles create foams—collections of bubbles separated by thin liquid films.

Common examples and uses: soap bubbles, which are mostly water and surfactants; air or gas bubbles in water in natural waters or in industrial processes; gas bubbles in molten metals or glass during manufacturing; bubble chambers used in particle physics to detect charged particles; and in finance, the term bubble describes a rapid rise in asset prices followed by a sharp decline when inflated expectations cease to hold.

Depending on context, the term bubble may also refer to other phenomena such as cavitation, where vapor-filled cavities form in a liquid under low pressure, or to the broader concept of "economic bubble" in markets.