coalescence

Watch two raindrops touch on a window: they instantly merge into one bigger drop, and there is no undoing it. In an emulsion, coalescence is exactly this — neighbouring droplets that have drifted together fuse into larger droplets, and step by step the whole emulsion collapses back toward two separate layers of oil and water.

Coalescence happens when the thin film of continuous phase and emulsifier between two touching droplets drains away and ruptures, letting the droplets join. Unlike creaming or flocculation, this destroys the dispersed structure: small droplets become large ones, the droplet count falls, and eventually the emulsion cracks into a clear oil layer and a clear water layer.

Crucially, coalescence is irreversible — once droplets have merged, shaking cannot restore the original fine emulsion. It therefore represents true, terminal failure of an emulsion. The main defence is a strong, coherent interfacial film from a well-chosen emulsifying agent (or finely divided solids), which keeps droplets from making the close contact needed to rupture the film; thickening the continuous phase and reducing droplet size also help by slowing the encounters that precede coalescence.