How different are whisky and coffee? Yes, one gets you drunk, and the other wakes you up. But they also linger in different ways.

A spilled drop of coffee leaves a stain with dark, sharply defined edges. When whisky dries, it leaves a more uniform, often beautiful film. An unlikely research team of five scientists from Princeton and a photographer from Phoenix described the complex dynamics of evaporating whisky in a paper published this year in Physical Review Letters.

The Phoenix photographer, Ernie Button, first noticed the whisky dregs at the bottom of his glass a decade ago and started photographing them under colored lights to accentuate the patterns. 

Mr. Button was curious about the underlying science and, through a Google search, found Howard A. Stone, a professor of mechanical and aerospace engineering at Princeton, who does not drink much whisky but is fascinated by how fluids flow.

The Princeton scientists performed a series of experiments, revealing that the key difference between whisky and coffee is that whisky is a mix of two liquids — alcohol and water — while coffee is just water, with brown bits mixed in. As the alcohol in whisky evaporates, the concentration of water increases, and that creates flows that generate the patterns.

Other molecules in whisky also prove important: a surfactant, a chemical that reduces the surface tension of the droplets, and long, stringlike molecules known as polymers, which attach to the glass, providing a template.

Hyoungsoo Kim, a research scientist working with Dr. Stone, first reported the findings at a conference in 2014. The mystery has not been completely solved. The scientists have not identified the particular surfactant and polymers in whisky or where they come from. “For now, we are not sure,” Dr. Kim said. “To prove that kind of detail, we need to do a careful chemical study.”

A likely source for the surfactants and polymers are the wood barrels used to age whisky. Mr. Button has found that most spirits not aged in wood, like vodka, do not create patterns like those left by whisky.

That supposition is supported by experimentation that took place last month. Mr. Button tracked down a bottle of Oak by Absolut, a vodka steeped in wood chips. Whiskylike patterns appeared. “But it’s not as strong,” Mr. Button said of the patterns, “because it’s only resting in the wood a short amount of time.”

Source – The New York Times

http://www.nytimes.com/2016/08/20/science/at-the-bottom-of-a-whisky-glass-a-beautiful-experiment.html

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