Why whisky creates gorgeous patterns at bottom of glass

A new quest by a professional photographer and scientists has provided a deeper insight into why Whisky creates gorgeous patterns at the bottom of a glass after evaporation.

Professor Howard Stone and his Complex Fluids Group in Princeton University's Department of Mechanical and Aerospace Engineering stated that it was plausible that these were deposits at contact lines or other deposits perhaps associated with flow, which was indeed what their research indicated.

The group focused on gaining a better understanding of the composition of whisky, identifying the possible suspended material, and doing controlled model experiments to understand possible shapes and forms of deposits during evaporation.

While the drying of single-component liquids that contain small volume fractions of solids is well studied and leads to the common "coffee ring effect," it's also well known that the evaporation of alcohol-water solutions leads to interesting fluid motions, such as wine "tears" in a glass of wine or other strong liquors.

To study the flow patterns and concentration in the solution, as well as the final dried deposits from suspended particles, Hyoungsoo Kim, a postdoctoral researcher within Stone's group, and colleagues used video microscopy of drying droplets of actual whisky and compared it to video microscopy of an alcohol-water solution representative of whisky (typical whiskies are 40 percent by volume ethanol and 60 percent by volume water).

They found that initially, the droplet of alcohol-water solution creates a complex mixing flow. Ethanol evaporates first, due to the lower vapor pressure compared to water and, once the ethanol vanishes, a radial pattern can be observed. Further, as the initial ethanol concentration increases, the mobility of the receding contact line is increased as well.

And, at high ethanol concentrations, the contact line recedes and draws groups of particles along with it that are then deposited in a ring-shaped pattern.

Artist Ernie Button suspected that the distinctiveness of the rings left by whisky might have something to do with the aging process and materials soaked up through the wooden casks in which it's stored, although he doesn't see any significant difference when photographing younger vs. "more aged" versions of the same type of whisky.

The work by Stone's group might have wider implications, because the ability to control the deposition of a thin film of particles has been highly desirable for many industrial applications.