Why whiskey doesn’t have a ring to it

The characteristic rings at the edges of a dried drop or splash of coffee come from how the water in the solution evaporates: Water disappears faster from the edge of a drop than the center, and as liquid flows outward to replenish the loss, it carries suspended particles along. Whiskey, in contrast, leaves behind a more uniform evaporation pattern. That’s because whiskey contains alcohol, natural surfactants such as phospholipids, and macromolecules such as lignin and polysaccharides, reports a team led by Howard A. Stone, Hyoungsoo Kim, and François Boulogne of Princeton University (Phys. Rev. Lett. 2016,  DOI: 10.1103/physrevlett.116.124501). Inspired by photographer Ernie Button’s images of dried whiskey, the researchers set out to understand the liquid’s evaporation mechanism by using fluorescent markers to track fluid motion. They found that the quick evaporation of alcohol induces chaotic flow that keeps the solution well mixed. Once the alcohol is almost gone, surfactant molecules collect at the edge of the remaining solution, forcing the fluid to flow inward instead of outward. Meanwhile, macromolecules coat the underlying surface, capturing particles to create a relatively even deposition pattern. Understanding whiskey’s drying behavior could provide new insights for preparing uniform coatings, the team says.