The coffee ring effect is a phenomenon familiar to coffee lovers and those who share homes or offices with them. Although coffee may seem to have magical properties, this dark elixir is really just tiny particles of coffee beans suspended in water. The dark ring is caused by the high concentration of particles around the outer edge of the spill.
Like the brew itself, the coffee ring effect has fascinated scientists for decades. Now, researchers have turned their attention to another liquid with, let’s say, intriguing physical and chemical properties: whiskey.
Stuart Williams, Associate Professor at the Department of Mechanical Engineering at the University of Louisville, studied the mechanisms behind the formation of web-like patterns of dried whiskey droplets. The results of the study suggest that different kinds of American whiskey form distinct patterns, which could help companies and consumers alike identify counterfeit spirits.
“This study was initiated from a simple question – can we tell the difference between whiskey types (age, brand, etc.) from looking at the pattern after its droplet evaporated? There have been a plethora of studies that have looked at such patterns from coffee, to blood, to polymers. More notably, there was a 2016 paper by Howard Stone’s group, where they observed that whiskey left a thin uniform film at high proof,” Williams told ZME Science.
“I had a case of bourbon whiskey (I was conducting a different study for Brown Forman) when I was on my sabbatical visiting Dr. Orlin Velev at North Carolina State University. We also observed thin films at high proofs, but we unexpectedly came across these fascinating patterns when we diluted them further, an unusual phenomenon that we wanted to pursue further,” the researcher added.
Williams and colleagues noticed that drops of diluted American whiskeys formed distinct patterns when dried on a glass surface. This was not the case for their Scotch or Canadian counterparts.
“We stumbled upon these structures by accident – we were expecting thin films or ‘coffee rings’ or some hybrid of the two, but these were unexpected. I remember sitting in a meeting with Dr. Velev where we showed him these web-like structures for the first time and he was fascinated, encouraging us to look into this further. When a world-renowned scientist (with years of experience) is fascinated by something you pay attention! We focused on trying to determine what fundamental mechanism was occurring and why these structures formed, which is the foundation for this paper,” Williams said.
The team employed time-lapse microscopy in order to examine the droplets of diluted American whiskey as they evaporated.
This analysis showed that non-volatile compounds (phenols, aromatics, and esters) clumped together and were expelled to the surface of the droplet, forming monolayers.
As the surface area of the droplet decreased due to evaporation, the monolayers collapsed, leaving behind web-like strands.
These web patterns are distinct to each type of American whiskey, which is supposed to have a unique combination of solutes. In tests that compared their lab samples to unknown samples, the researchers could identify the brand of whiskey 90% of the time.
The results, which were published this week in the journal ACS Nano, show that whiskey webs can be a reliable indicator for distinguishing genuine bourbon from counterfeits.
“We are currently assessing methods of doing this at home; in other words, we want to find a (very) robust testing procedure that is portable and repeatable. We have found out that temperature, humidity, and other environmental factors influence the result. Although the sensitivity of this technique provides its own challenges, it also inherently enables sensitivity within the testing procedure itself. Also, I’m mostly working from home nowadays (due to COVID-19) and I am using various smartphone lenses and USB digital microscopes to conduct some ‘at home’ trials,” Williams said.
The whiskey webs are not only fascinating from a scientific perspective, they’re also quite visually stunning. Williams and colleagues have set up a digital art gallery where you can see how various brands of bourbon form webs under a microscope. These were also displayed at various whiskey themed events but also art galleries like the ACCelerate Festival at the Smithsonian.
In the future, the researchers would like to continue their work, extending the fundamental principles of monolayer collapse to other fields of science.
“We want to apply this work towards fundamental investigations of monolayer collapse, which is applicable to many fields beyond bourbon. For example, monolayer collapse is of interest in studies that investigate biological lung surfactant mixtures. Conducting fundamental studies using microliter droplets enables rapid, parallel testing of heterogeneous surfactant mixtures, streamlining such investigations,” Williams said.
