June 15, 2008 — -- 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.
To find out more, Coffey and a team of students tested the reactions between Diet Coke and fruit Mentos, mint Mentos, and various ingredients such as other mints, dish-washing detergent, table salt and sand. They also compared reactions using other fizzy liquids such as caffeine-free and sugary colas, as well as soda water and tonic water.
All the reactions took place in a bottle angled at 10° off vertical and the fountain trajectories were recorded on video. The team also investigated the total mass lost in the fountain and the influence of the sweet's surface roughness.
The results showed that Diet Coke created the most spectacular explosions with either fruit or mint Mentos, the fountains travelling a horizontal distance of up to 7 metres.
But caffeine-free Diet Coke did just as well, suggesting that caffeine does not accelerate the reaction, at least at the normal levels in the drink.