Emulsion
An emulsion is a system of two immiscible liquids in which one is dispersed through the other as microscopic droplets. They are classed as direct, oil-in-water, or inverse, water-in-oil.
Most pastry emulsions are direct: ganache, mirror glaze, crémeux, custard. Butter, by contrast, is an inverse emulsion — water droplets held inside a fat phase.
Three factors govern stability: droplet size, the presence of an emulsifier, and the viscosity of the medium. The finer the droplets, the more slowly they coalesce, which is why a properly blended ganache keeps longer and does not separate.
A broken emulsion announces itself at once — the mass turns grainy, loses shine, and fat rises to the surface. The causes are usually a temperature change that was too abrupt, or a wrong ratio between the phases.
A split ganache can usually be saved: add a little warm liquid and blend, starting from the centre — the emulsion rebuilds outward from a core.