Phys. Rev. Fluids (in the press)

The ‘ouzo effect’ refers to the stable microemulsion that forms when anise-based liqueur is mixed with water. And it can be understood as something of a love triangle: the anethole and ethanol in ouzo are miscible, and ethanol plays well with water, but the third pairing is antagonistic, resulting in that familiar milky emulsion. Oscar Enríquez and colleagues have captured the dynamics of a water droplet introduced into the bottom of a mixture of anise oil and ethanol in a video shown in the Gallery of Fluid Motion (https://doi.org/c8gb).

Water dissolves slowly in pure anethole and mixes rapidly with ethanol. But in mixtures comprising at least two parts anethole to one part ethanol, the authors witnessed pronounced oscillations on the water’s surface. The oscillations were likely due to Marangoni stresses induced by spatial variation in the mixture’s composition, which were responsible for an accumulation of oily microdroplets within water–ethanol droplets in another study conducted by Huanshu Tan and co-workers (J. Fluid Mech. 870, 217–246; 2019).