Appalachian State University has recently published the results of their study on the physics behind the Mentos + Diet Coke = Crazy Fizz phenomenom. This is some serious science that I can get behind! If you ask me, more research dollars need to go towards the study of Mentos. According to the study, here is the reason the Diet Coke fizzes up when Mentos are added.
So why does the reaction occur? In an opened container of soda, carbon dioxide gas bubbles out over the course of minutes or hours until the concentration of carbon dioxide left in the soda is proportional to the carbon dioxide in the surrounding air. This de-fizzing reaction is slow because the surface tension of the liquid is very high, which keeps the gas bubbles trapped.
But when a Mentos is dropped in the beverage, it breaks the surface tension and as it falls the candy’s surfactant coating further reduces the surface tension of the liquid. The candy’s rough surface also provides growth sites for the gas, making it easier for carbonation to escape as a foam geyser.
The geyser also occurs when sand, salt or lifesavers were added to the Diet Coke, but the mass lost and volume traveled is much less spectacular.
If you wan to see the full ariticle in its full complex physics detaiils, you can access it from The American Journal of Physics Online.