Science of Mentos-Diet Coke Explosions Explained

The startling reaction between Diet Coke and Mentos sweets, made famous in thousands of YouTube videos, finally has a scientific explanation. A study in the US has identified the prime factors that drive the fizzy plumes from Coke bottles: the roughness of the sweet and how fast it plummets to the bottle's base. "If you drop a pack of Mentos into a bottle of Diet Coke, you get this huge fountain of spray and Diet Coke foam coming out," says Tonya Coffey, a physicist at Appalachian State University in Boone, North Carolina. "This was a good project for my students to study because there was still some mystery to it."

When mint or fruit Mentos are dropped into a fresh bottle of Diet Coke, a jet of Coke whooshes out of the bottle's mouth and can reach a height of 10 metres. Theories abound as to why this happens, with some bloggers speculating that it is an acid-base reaction because Coke is acidic.

Experiments in a 2006 edition of the Discovery Channel programme Mythbusters suggested the chemicals responsible for the reaction are gum arabic and gelatine in the sweets, and caffeine, potassium benzoate and aspartame in the Coke. But there have been no rigorous scientific studies of the reaction until now....

As it turns out the "rough, dimply surfaces of Mentos encourage bubble growth" and also factors which lower surface tension help cause the explosion.

... Measurements showed that the surface tension in water containing the sweetener aspartame is lower than in sugary water, explaining why Diet Coke creates more dramatic fountains than sugary Coke.

Another factor is that the coatings of Mentos contain gum arabic, a surfactant that further reduces surface tension in the liquid. Rough-surfaced mints without the surfactant did not create such large fountains.  - ns

From a related story:

With a bottle of Coca-Cola raised above his head, Ukec surprised reporters when he threatened to cut off imports of gum arabic, an emulsifier made from the acacia tree that adds to the fiziness of sodas.

"I want you to know that the gum arabic, which runs all the soft drinks all over the world, including the United States, mainly 80 percent is imported from my country," Ukec said.

"I can stop that gum arabic and all of us will have lost this," he said. - abc