A photographer who captures the surprisingly-beautiful designs left at the bottom of whiskey glasses by the liquor looked into the physical concepts behind the phenomenon.

Phoenix-based professional photographer and artist Ernie Button reached out to professor Howard Stone and his Complex Fluids Group in Princeton University's Department of Mechanical and Aerospace Engineering to help determine what causes whiskey's "particle patterning." The findings will be presented during the American Physical Society's Division of Fluid Dynamics (DFD) Meeting.

"It's infinitely fascinating to me that a seemingly clear liquid leaves a pattern with such clarity and rhythm after the liquid is gone," Button said. "Professor Stone and his team graciously entertained my questions and were very helpful with my understanding of this phenomenon."

Button believes the patterns are deposits at contact lines or associated with liquid flow. To make their findings the researchers worked to gain better understanding the composition of whiskey itself.

"To our surprise, there were very few recent studies about the evaporation of droplets of alcohol-water solutions and corresponding flow patterns," said Hyoungsoo Kim, a postdoctoral researcher within Stone's group.

To study these flow patterns the researchers and solution concentration the team used video microscopy of drying droplets of actual whisky and looked at how they compared to an alcohol-water solution representative of whisky.

They found droplets of the alcohol-water solution created a "complex mixing flow." Ethanol evaporates quicker than water because it has a lower vapor pressure, and once it's gone a radial pattern appears. As the initial ethanol concentration increased as is the mobility of the receding contact line, drawing in groups of particles and depositing them in a ring-like pattern.

"The alcohol-water solution shows circulation flow patterns (triggered by the Marangoni Effect), which occur during drying and influences patterns formed in evaporating whisky solutions," Kim said. "Deposits in the actual whisky come from a small amount of inherent raw materials present from the preparation process."

The researchers believe the distinct rings could also have something to do with the aging process of the liquid as it is soaked in a wooden cask, even though their study did not reveal a difference in pattern in whiskeys aged for different periods of time. The findings could help researcherslearn to control the deposition of thin film particles.

"One big restriction of the 'coffee ring effect' is that particles aren't uniformly distributed," Kim said. "We've learned through recent research papers that it's possible to make more uniform deposits by beginning with mixtures, so we'll continue to explore multiphase evaporating systems by running systematic experiments with well-controlled solutions."