Ouzo effect with absinthe. Image via Wikimedia Commons.
If you've ever prepared absinthe by pouring water over a sugar cube suspended above the liquor, you've probably noticed something called the "Ouzo effect." This is the milky emulsion (a fancy way of saying a mixture made of two normally unmixable things) that results from adding water to anise-flavored liqueurs, such as absinthe, sambuca or ouzo, a popular before-dinner drink in Greece.Due to the unique properties of emulsified ouzo and other liqueurs flavored by the anise plant, a team of researchers from the Fluid Physics department at the University of Twente in the Netherlands decided to study how they evaporate. The results of their inquiry can be seen in the above video and a recent paper published in The Proceedings of National Academy of Sciences.According to the team's theory, there are four main phases to the life cycle of an evaporating ouzo droplet, composed of water, ethanol and anise-oil. In the first phase, the droplet remains transparent while the ethanol evaporates. In the second phase, the anise-oil microdroplets begin to quickly mix with the rest of the droplet—this is the ouzo effect in action. In the third phase, all of the ethanol has evaporated and you can see a water drop sitting on an anise-oil ring. Finally, the water evaporates and we are left with a small anise-oil droplet.The evaporation of pure and liquids with dispersed particles has been studied extensively in the last two decades, although the research coming out of the University of Twente is unique in the field. This is because it makes use of three different liquids which have different volatilities and mutual solubilities (the temperature required to make two liquids mix). Simply put, the way the ouzo drop evaporates is far more complex than a simple mixture of two liquids. Also, we get the feeling like the researches wanted to document this because it looks cool…which it does.The Dutch team's work is an important contribution to the scientific understanding of how liquids evaporate, which can deployed in a number of fields ranging from medical diagnostics to printing LED lights.