There is a lot of hidden beauty in the world. Take, as an example, the art of photographer Ernie Button: eight years ago, he noticed the peculiar residue in the bottom of an empty whisky glass.
"The idea for this project occurred while putting a used Scotch glass into the dishwasher. I noted a film on the bottom of a glass and when I inspected closer, I noted these fine, lacey lines filling the bottom," he wrote on his website. " What I found through some experimentation is that these patterns and images that you see can be created with the small amount of Single-Malt Scotch left in a glass after most of it has been consumed. The alcohol dries and leaves the sediment in various patterns."
Using coloured lights to bring out the details -- and evoke the effect of strange, alien landscapes -- he embarked on a photo series: "Vanishing Spirits -- The Dried Remains of Single Malt Scotch".
What he discovered is that different whiskies behave differently. Peaty whiskies were unpredictable, sometimes producing rings, sometimes not. Aged whiskies produce the best patterns, whether Scotch, bourbon or Irish whiskey, while young whiskies and white whiskies produce none at all. Nor does cognac, or wine.
Curious about the phenomenon, and why it occurred for some liquids and not others, he turned to the internet -- and found chemical engineering Professor Howard A. Stone of Princeton University, who specialises in fluid mechanics.
"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."
A team led by Professor Stone used a combination of video microscopy and a combination of water and ethanol in whisky-like proportions to examine the effect. They found that the patterns were caused by a combination of surface tension and the different evaporation rates of water and ethanol -- the ingredients in whisky -- caused the effect, a process known as the Marangoni effect.
When the last few remnants of whisky are left in the bottom of the glass, it evaporates in a particular way. First, the ethanol evaporates, because compared to water, it has lower vapour pressure. As the ethanol concentration increases, thanks to the evaporating liquid, the receding contact line increases. At high concentrations of ethanol, such as what you might find in whisky, this contact line recedes, drawing particles along with it, which are deposited in ring-shaped patterns.
"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," said postdoctoral fellow Hyoungsoo Kim, who helped Professor Stone research the phenomenon. "Deposits in the actual whisky come from a small amount of inherent raw materials present from the preparation process."
The next step is to try to replicate the effect. Although it may seem a frivolous exercise, there are some potential applications in manufacturing -- where the ability to deposit a thin film of particles is desirable.
"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," Kim said.
The team presented their research, "Painting Pictures with Whisky", at the 67th Annual Meeting of the American Physical Society Division of Fluid Dynamics.