Ouzo effect

The ouzo effect (also louche effect and spontaneous emulsification) is a milky () oil-in-water emulsion that is formed when water is added to ouzo and other anise-flavored liqueurs and spirits, such as pastis, raki, arak, sambuca and absinthe. Such microemulsions occur with only minimal mixing and are highly stable.

The ouzo effect occurs when a strongly hydrophobic essential oil such as trans-anethole is dissolved in a water-miscible solvent, such as ethanol, and the concentration of ethanol is lowered by addition of small amounts of water.

Oil-in-water emulsions are not stable. Oil droplets coalesce until complete phase separation is achieved at macroscopic levels. Addition of a small amount of surfactant or the application of high shear rates (strong stirring) can stabilize the oil droplets.

In a water-rich ouzo mixture the droplet coalescence is dramatically slowed without mechanical agitation, dispersing agents, or surfactants. It forms a stable homogeneous fluid dispersion by liquid-liquid nucleation. The size of the droplets has been measured by small-angle neutron scattering to be on the order of a micron.

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