Diet Coke and Mentos: Soda-Candy Explosions

Tonya Coffey and her students reported "an experimental study of the Diet Coke and Mentos reaction (made famous in a 2006 Mythbusters episode), and consider(ed) many aspects of the reaction, including the ingredients in the candy and soda, the roughness of the candy, the temperature of the soda, and the duration of the reaction." (Am. J. Phys., 2008, 76(6), 551 - 557)

Basically,

1. The Mythbusters correctly identified potassium benzoate and aspartame as key ingredients in the Diet Coke–Mentos reaction. (Static water) Contact angle measurements were used to show that these ingredients reduce the work required for bubble formation, allowing carbon dioxide to rapidly escape from the soda. The small amount of caffeine in Diet Coke is unlikely to contribute significantly to the reaction.

2. The Mythbusters also correctly identified the roughness of the samples as one of the main causes of the reaction. The importance of sample roughness was shown by comparing SEM and AFM images of the samples to the explosive power of the reaction. Increased surface roughness implies a higher surface area to volume ratio, meaning that more growth sites should be present on per unit volume.

3. Samples which encounter less viscous drag and hence fall more quickly through the soda will cause a more explosive reaction. Also, for the same amount of mass lost, the eruption will be more dramatic when the reaction takes place over a shorter time.

4. Finally, hotter beverages result in a more explosive reaction. (A direct illustration of Henry's law and Le Chatelier's Principle)

The kinds of things researchers do...like popping candy into sodas and watching them fizzle and frazzle.