Why doesn’t whiskey leave rings like coffee?

25 Mar 2016

What have coffee, whiskey and future coating technologies got in common? Annoying rings, or lack thereof.

One of the more bizarre pieces of research we’ve come across in recent years is the fantastic effort from researchers at Princeton University in the US who, with the help of a photographer, investigated why coffee rings are rarely the domain of whiskey.

Think about it, both coffee and whiskey are often left with sips remaining in glasses and mugs, yet only one leaves a distinct, infamous ring. Why?

Firstly, it turns out whiskey does, but only if you buy the right stuff. After a chance moment when Ernie Button, a photographer living in Arizona, noticed a dramatic lighting effect when a glass of whiskey was left to dry out at the bottom of a clear glass. Various rings were formed when drops dried out.

Each whiskey reacted differently though, so after getting in touch with Princeton, he and Howard Stone investigated why.

The Macallan 150, part of Ennie Button’s ‘Vanishing Spirits: The Dried Remains of Single-Malt Scotch’

The Macallan 150, part of Ennie Button’s ‘Vanishing Spirits: The Dried Remains of Single-Malt Scotch’

Different coloured lighting revealed the various ways whiskey left its mark on glass, with aged Scotch of Irish whiskey leaving distinct, complex sets of rings. Clear whiskey, though, didn’t.

Looking at the specific evaporation properties of moisture, the team established that coffee (or certain aged whiskeys) don’t disappear in a uniform fashion. As the moisture evaporates from the outside in, moisture rushes out to fill the vacated spaces.

With this, minuscule amounts of residue get dragged out and left to fend for itself in what soon becomes a coffee ring.

The whiskeys that don’t leave rings have a unique make-up, featuring specific molecules described as ‘fat-like’ that lowered the surface tension. This means the pulling out to fill vacated, evaporated space was levelled out, leaving a more uniform, even evaporation process.

As Phys.org explains, the second feature was plant-derived polymers that caused a sticking effect, which in turn helped to channel particles in the liquid to the base material (the drinking glass) where they stayed stuck.

“We think that particle deposits are regulated by small polymeric components added in the manufacturing process,” Stone said. “Wood materials from barrels, for example, could serve as those components.”

Why is this important? Well the properties behind the clear whiskey, leaving uniform residual remains, could play a key role in future coating technologies, for obvious reasons.

So the next time your whiskey leaves a stain on a table, be safe in the knowledge you’ve stocked up on the good stuff. As for coffee rings? They’re just filthy.

Main whiskey image via Shutterstock

Gordon Hunt was a journalist with Silicon Republic