The ouzo effect (also louche effect and spontaneous emulsification) is a milky (louche) oil-in-water microemulsion that is formed when water is added to ouzo and other anise-flavored liqueurs and spirits, such as pastis, raki, arak, sambuca, absinthe and Pernod. Because such microemulsions occur with only minimal mixing and are highly stable,[1] the ouzo effect may have commercial applications.

Observation and explanation

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.

In water-immiscible solvents, oil-in-water emulsions are not stable as the oil droplets coalesce until complete phase separation is achieved at macroscopic levels. It is well known that the 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