Physicists Solve Mystery of Scotch Whisky Rings [Slide Show]

How a photographer’s curiosity led to new scientific insight into the fluid dynamics of binary liquids

November 24, 2014 | |

Ernie Button was never a whisky drinker, but his wife loved the stuff. So, in an attempt to develop his palate the Phoenix-based began sampling everything from blended malts to single grains. Along his culinary journey he became intoxicated not only with sipping the amber liquid but with photographing it, too.

His subject, however, was not a two-fingers’ pour but the dried drops left over in the bottom of the glass, which—as he discovered one morning while loading the dishwasher—leave intricate, lacelike designs in their evaporative wake.

Photographic interest piqued, Button began experimenting with different Scotches, whiskeys and bourbons to see what kind of patterns he could coax out of those spirits’ departing drops. Rather than photoshop the images, he used different combinations of colored lights to highlight the undulations the evaporated liquid left behind. He discovered that any aged Scotch whisky or American or Irish whiskies will leave complexly patterned rings but moonshine and white whiskeys will not. Neither would other alcohols he tested, including cognac and wine.

As the discoveries poured in and his portfolio grew, a question began nagging at Button: Why did the rings appear at all? When Google failed to provide an explanation, he decided to try the experts. First, he reached out to a researcher who studied the so-called coffee-ring effect, in which evaporating liquids containing a single liquid component (such as water) deposit small fractions of dissolved solids (such as coffee grounds). But unlike coffee, whiskies are binary liquids, composed of about 60 percent water and 40 percent ethanol. The science behind the coffee-ring effect did not apply.

So Button contacted Howard Stone, a professor of mechanical and aerospace engineering at Princeton University whose name popped up in a search of “art” and “fluid mechanics.” To the photographer’s surprise, Stone quickly agreed to investigate. “It’s my personality: I don’t ignore e-mails very easily,” he says. “Plus, it sounded interesting.”

Stone recruited his postdocs, who began chipping away at the question in their free time. First, they created a whisky proxy composed of the liquor’s most basic ingredients, water and ethanol, plus some micron-size particles to reveal the final deposition pattern after evaporation. (A micron is one millionth of a meter, or about 40 millionths of an inch.) “If you want to understand the mechanism, you have to take it apart,” Stone says. “So we systematically took whisky apart.”

The investigators then used video microscopy to examine the water–ethanol solution’s evaporative process but they found that the intricate patterns did not appear. Next, they added a small amount of polymeric surfactant—a compound that lowers a liquid’s surface tension, such as a layer of soap or oil that sits between the interface of water and air. Surfactants contribute to a phenomenon called the Marangoni effect, in which a liquid’s evaporative flows are driven by variation in surface tension. Still the patterns did not appear. So they added one more polymer. That last addition, it turned out, was key: the elusive whisky ring–like patterns finally formed. “We think that particle deposits are regulated by small polymeric components added in the manufacturing process,” Stone says. “Wood materials from barrels, for example, could serve as those components.” Without similarly rigorous studies, he adds, it’s impossible to know why the other alcohols that Button tested do not leave their own intricate deposition patterns.

“Hopefully we have added a little to the beautiful story that Ernie started,” Stone remarks. In addition to illuminating what drives the rings’ formation, Stone also thinks his group’s results—presented November 24 at the American Physical Society’s Division of Fluid Dynamics Meeting in San Francisco—will contribute to scientific understanding of binary-liquid particle deposits.

As for Button, he says his curiosity has been satisfied: “I’ve never even talked to Dr. Stone on the phone or met him in person but he’s been so nice in entertaining my questions. His team’s findings have definitely helped to provide some clarity.”

>>View a slide show of Button’s otherworldly photos of Scotch whisky rings.