Unlike scotch whisky, when American bourbon whiskeys are diluted, they form unique web-like evaporation patterns. These differences arise in part from the way the liquors are aged: scotch is aged in re-used barrels, whereas bourbons require aging in a new, charred American white oak barrel*.
During aging, the whiskey picks up water-insoluble chemicals from the barrel. When water is added to the bourbon, it helps transport those insoluble components to the surface of a droplet, where they form a monolayer of fatty acid chains (Image 2; in green). As evaporation continues and the droplet gets smaller, the molecules at the shrinking surface collapse inward, forming the rigid web structure we see left behind. The patterns that form act as a kind of fingerprint for the bourbon. Check out some of the brand-to-brand variations over at the researchers’ Whiskey Webs site. (Image and research credit: S. Williams et al.; via Physics Today)
* In case you were wondering, this is actually a legal requirement in order to be considered bourbon. Bourbons must also be made from a grain mixture that is >50% corn.