What causes "legs" to form in the snifter when sipping neat?


Sign up or Log In to change notification settings.
Paul B avatar image
Paul B 🇺🇸 | 472 ratings Author Posted 18 Oct '21

Normally, I would first pose this question to Google, but I feel that fellow rummies are much more capable of answering this question.

I only had one college chemistry course, which I enjoyed very much. From those days, I know that the "legs" that show up in the snifter are caused by the viscosity of the liquid. On heavily sugared and viscous rums, the "legs" are guaranteed to show up. However, on recent purchases of rums with no added sugar, a few minor "legs" showed up on aged rums, but not for white rums. All I can figure is that the aging process introduces elements of viscosity.

Please help me to solve this mystery!

Thanks!!!!

KU
kudzey 🇵🇱 | 38 ratings Replied 18 Oct '21

It's more about physics than chemistry. When you swirl the rum in the glass, a thin film remains on the wall of the sniffter due to capillary action. Let's focus on the film. Gravitation wants to pull it down but the capillar forces hold it up. Rum is composed mostly of alcohol and water, and alcohol evaporates faster. The film on the sniffter wall has lower alcohol content, the higher distance from the rum reservoir on the bottom of the glass. The less alcohol, the thicker the mixture becomes (if sugar is added, even thicker), which means that the surface tension is increased. The higher the surface tension, the higher does the rum climb. Eventually, upper layers are just being pushed up by the lower ones but their surface tension is not enough to remain sticked to the glass, so they fall down. This is called Marangoni effect.

 

Ok but why do we see legs, instead of an uniform waterfall on the whole circumference of the glass? As I mentioned, the layer of rum begins it's way down after reaching certain height. It is unlikely that our swirl is so perfect that the height of the films on the sniffter walls is the same everywhere. This means that the process begins somewhere. A single droplet forms from excess rum and falls down, paving a way for a continuous downflow of all excess rum. Another droplet could form in the neighborhood, but, before it does, it done it's convenient way down through the formed leg. 

Paul B avatar image
Paul B 🇺🇸 | 472 ratings Author Replied 18 Oct '21

kudzey:

Thanks for your absolutely brilliant response!  It's physics and not chemistry.

Paul B avatar image
Paul B 🇺🇸 | 472 ratings Author Replied 18 Oct '21

Thanks Stefan! A picture is worth a thousand words.

So now rum is like women. Never judge either one by their legs.