Most of us don't give much thought to drops of liquid that end up outside our drinking glasses. But physicists care a lot about liquid droplets, and study their whole lifespans—from the first splash or drip to the moment a drop disappears. Liquids that contain three different substances, though, haven't been studied as much. Detlef Lohse, a physicist at the University of Twente in the Netherlands, and his colleagues took a deep dive into one such liquid: ouzo. Ouzo is a mixture of water, ethanol, and anise oil, making its physics especially intriguing (at least to people who are intrigued by such things). Lohse explains that ouzo isn't that unusual, though. There are other liquids made up of three components that should behave in the same way as ouzo, he says: "Both other beverages like Raki or Pastis, or [three-part] liquid mixtures used in industry, medicine, or technology." Studying any such liquid could help researchers understand how they all behave—but studying booze is more fun. Lohse and his coauthors set up cameras at multiple angles to record the life cycle of a single drop of the liqueur, which they placed onto a water-repellant stage using a fine needle. It took almost 15 minutes for a drop to evaporate, leaving behind a teensy blob of oil. As the researchers recorded, they saw four stages in the droplet's life. In the first stage, while the ethanol is evaporating from the dome-shaped droplet, it still looks clear. The ethanol prefers to leave from the droplet's edges. The lowered concentration of ethanol at the rim leads microdroplets of oil to form, first there and then throughout the whole droplet, in the second phase. These oil droplets also cause the "ouzo effect," the milky appearance taken on by ouzo when it's served in a glass with water. In the third phase, a ring of oil around the droplet grows thicker and the droplet starts to shrink, turning clear again. Finally, the water is reduced to a tiny dot in a slightly less tiny blob of oil. Then it disappears completely. Recorded and sped up, the movement of the different liquids within the ouzo looks like the swirling clouds of an alien planet. "What perhaps surprised me is the violence" of the flow, Lohse says, which is "nicely visualized by the moving droplets." It almost looks too pretty to drink. https://youtu.be/1mYHlkM-Sjg Video: Physics of Fluids group, University of Twente.
Tan H, Diddens C, Lv P, Kuerten JG, Zhang X, & Lohse D (2016). Evaporation-triggered microdroplet nucleation and the four life phases of an evaporating Ouzo drop. Proceedings of the National Academy of Sciences of the United States of America PMID: 27418601
