Bubbles

A bubble is a pocket of gas enclosed by a thin liquid film. Most common are soap bubbles in water, which are buoyant and spherical due to surface tension. The visible colors of soap bubbles come from interference of light in the thin film.

The pressure inside a spherical bubble exceeds the outside pressure by ΔP = 2γ/r, where γ is the surface tension. The film is extremely thin; stability depends on film thickness, impurities, and surfactants. Bubbles drain and thin over time and eventually rupture; when they rupture, the film breaks at a point and the gas escapes.

In everyday life, soap bubbles form; in liquids, gas bubbles can form around dissolved air, in carbonated drinks, or during boiling. Industrially, bubble columns and froth flotation use gas-liquid interfaces; in geology and volcanology, gas bubbles form in magma; in medicine, microbubbles serve as contrast agents in ultrasound.

Bubbles form by agitation or gas diffusion at the film, may grow by coalescence or gas diffusion, and rise due to buoyancy in liquids. Their wall is fragile and susceptible to rupture by contact with surfaces, impurities, or evaporation of the film.

Soap bubbles are decorative and educational; bubble wrap is a packaging material; microbubbles enable diagnostic imaging and targeted therapy; in physics, bubble chambers and cavitation studies contribute to research.