Whisky Versus Coffee: Dueling Droplets—Speaking of Chemistry

Whisky Versus Coffee: Dueling Droplets—Speaking of Chemistry


Whisky–love it or hate it–has a rich and
complex chemistry. And this chemistry gives whisky some fascinating properties that could
help a multi-billion-dollar industry you wouldn’t think has anything to do with booze. Our whisky adventure begins with a photographer
named Ernie Button who noticed that when a drop of whisky dries, it leaves residue pretty
much everywhere the droplet was. Compare that with the world-famous coffee ring, where the residue forms, well, a ring. Usually on your table. Coasters, people. Ernie wanted to know why whisky dries so differently,
so he started reaching out to scientists to find out. He got in touch with Howard Stone,
a chemical engineer at Princeton. Although Howard doesn’t drink much whisky, he’s
very interested in fluid dynamics. He also knows uniform drying is a big deal
for the paint and coatings industry, which is worth billions of dollars in the US alone,
so a better understanding of how drying works would be a good thing. And Howard’s office
is right across the street from a liquor store and if that’s not a sign from the universe,
I don’t know what is. Now, whisky and coffee are obviously different.
For starters, whisky has alcohol in it. But there’s more to it because mixtures of water
and ethanol–the alcohol in alcoholic beverages–don’t dry uniformly, like whisky does. Howard’s team created several different
mixtures to mimic whisky, and found two key ingredients that are very important to whisky’s special drying properties: surfactants and polymers. Whiskey gets these naturally from plants used to make it: corn, wheat, barley, etc. Whisky’s surfactants are phospholipids,
and its polymers include lignin and polysaccharides. So the Princeton team created a whisky mimic
by mixing ethanol and water together with a synthetic surfactant , sodium dodecyl sulfate,
and a polymer, polyethylene oxide. The team also added some fluorescent beads
so they could actually image the drying process without affecting it. The Princeton mixture was pretty spot on in terms of drying like whisky does. In whisky–and in the Princeton mimic–ethanol
evaporates faster than water, changing its concentration and setting up a concentration
gradient. This gradient creates a type of fluid motion known as Marangoni flow that
essentially stirs a drying droplet, which is good for uniformity. In coffee, there’s
no ethanol gradient to set up this stirring. Once all the ethanol evaporates, the whisky
surfactants drive a new flow to pull particles away from the droplet edge. Again, you don’t
get this in coffee, so stuff can and does settle at the edge. The last critical whisky ingredient, the polymer,
helps anchor other particles in the solution to the surface, keeping things in place instead
of getting pulled around by the droplet. Coffee doesn’t have this anchoring system so its
contents go to where the fluid flows. Guess where that is? A ring around the edge of the
droplet. Stone says these whisky experiments are simpler
than what companies do to develop commercial coatings. But his team’s simplified model
revealed the important roles of a few key ingredients in uniform coatings. And it made us wonder if adding whiskey to
your cup of joe could keep it from forming Leave your guess in the comments. Then check out this video to find out if you were right. Thanks to Ernie Button, Howard Stone, and postdoctoral
researcher Hyoungsoo Kim of Princeton for talking to us about their work. And also for
giving us an excuse to do whisky experiments.

10 thoughts on “Whisky Versus Coffee: Dueling Droplets—Speaking of Chemistry

  1. Cool science. It would be interesting to analyze different surfactants on their ability to make drying more uniform in other types of substances.

  2. very interesting phenomenon. however, i am still bit not clear. the rings seems to me are because of different compounds. or they have the same composition as the experiment showing? and why surfactants draw particles (solvents or solutions)? why coffee leave a ring like stain?

  3. Add whiskey to coffee — and there would be no drying problem, because there would be no fluid left! 😀

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