Why Diet Coke Loves Mentos

Step 1: Take a bottle of Diet Coke, ideally a 2-liter one, and open it.

Step 2: Add Mentos.

Step 3: Duck.

If you haven’t tried it, you’ve probably seen a video of it.  If you haven’t seen a video — or, if you just want to watch it again, here you go:

It’s a sight to be seen and, if you’re not prepared for it, a big surprise. The Mentos turn the Diet Coke into a geyser, shooting minty cola foam straight up to the sky. (The people in the video above make the fountains even larger by putting a pinhole in the cap and then putting the cap back as quickly as possible.) It’s very cool and — as I can say from experience — an experiment your kids will really appreciate. (Just do it outside or in a shower, don’t try to put the cap back on after adding the Mentos, and wear eye goggles just to be extra safe.)

But why do Diet Coke and Mentos “react” in the first place?

A team led by Dr. Tonya Coffey, a professor of physics at Appalachian State University in Boone, North Carolina, set out to determine just that.

At first, they wondered if this reaction was similar to what happens when one mixes vinegar with baking soda, but they quickly ruled that out, as the pH of the resulting stream of Diet Coke hadn’t changed. Rather, they concluded that this wasn’t a chemical reaction at all. Rather, the researchers concluded that the culprit is just the disruption of water molecules by a rough object with a high surface area.  Coffey explained to New Scientist:

“Water molecules like to be next to other water molecules, so basically anything that you drop into the soda that disrupts the network of water molecules can act as a growth site for bubbles. And if you have rough candy with a high ratio of surface area to volume, then there’s more places for the bubbles to go.”

And when a lot of bubbles get together, there’s nowhere to go but up.  Way up.

So in theory, the Diet Coke part of the equation could be substituted with any carbonated beverage, and the Mentos could be swapped out with, say, a rock with the right type of surface, right? Well, not quite. While Caffeine Free-Diet Coke worked just as well as the more common, caffeinated version, regular Coke didn’t do as well. And Mentos, it turns out, are the near-perfect mint option. Why? Two reasons.

First, the surface tension of the liquid matters — the lower the surface tension, the more incredible the geyzer.  And that makes diet drinks a better option. Aspartame — the sweetener in Diet Coke — lowers surface tension more than the sugar found in regular Coke, making the diet variety more effective. And the choice of the into also impacts the surface tension;  Mentos have a coating made of gum arabic, which lowers the surface tension of the soda when it is introduced.  (Coffey doesn’t seem to have tested non-cola drinks, like Fresca, which is understandable because that would be a waste of Fresca.)

Second, the density of the mint matters: Mentos fall quickly to the bottom of the bottle, collecting bubbles on the way down. Less dense mints which float or bob do not have as dramatic an effect.

So if you try this at home — and you probably should, but again, don’t put the cap back on and wear eye protection — grab a diet cola, and don’t bother mixing in a pack of Tic Tacs.

Bonus fact: From 2000 to 2011, the Coca-Cola Company introduced nine new varieties of Diet Coke. Of the nine, only six were introduced in the United States — Diet Raspberry Coke, Coca-Cola Light Sango (blood orange flavored), and Diet Coke with Citrus Zest were not offered for retail sale in domestic markets.

From the Archives: Cola Enforcement Agency: About the cocaine inside your Coke.

Related: Diet Coke and Mentos.