As of today the "Diet Coke + Mentos" video is at almost at 12 million hits and "The Coke Zero & Mentos Rocket Car" video has 2 million hits on YouTube (above). Not to mention a host of other videos on the same topic. So what's this all about, scientifically speaking?
Well, Coke and Mentos have a physical reaction. The nucleation sites in the pitted surface of the Mentos candy allows for the rapid formation of bubbles. The gelatin and gum arabic found in Mentos help to break the surface tension, allowing the rapidly forming bubbles to more rapidly form. When the candy sinks to the bottom of the Coke bottle the pressure from the forming bubbles/gas pushes the liquid up at a high rate.
A couple of years ago a paper was published in the American Journal of Physics looking at this physical reaction. They took two varieties of Mentos - fruit and mint - and scanned them with an electron microscope to look at the nucleation. They tested nucleation surfaces using the fruit Mentos, mint Mentos, liquid gum arabic, a mixture of Dawn Dishwashing liquid and water, table salt, rock salt, playground sand, Wint-o-Green Lifesavers, a mixture of baking soda and water, and molecular sieve beads. They observed these candy reactions in Coco-Cola Classic, Diet Coke, Caffeine Free Diet Coke Diet Coke, Caffeine Free Coca-Cola Classic, seltzer water, seltzer water with potassium benzoate added, seltzer water with aspartame added, tonic water, and diet tonic water. There were also temperature dependent trials where bottles were either refrigerated, heated, or kept at room temperature.
The results showed that drinks containing aspartame (a sweetener) and potassium benzoate (a preservative) to be more explosive than unsweetened drinks and drinks sweetened with sugar or corn syrup. This is probably due to a reduction in the work required for bubble formation. They found that the best nucleation sites were on the Mentos (both kinds) and that Mentos had a quick fall time which increased the number of bubbles coming from the bottom of the bottle, making it more explosive. Also, warmer Coke was found to have a larger explosion (due to Henry's Law).
So you want a big explosion? Place a Mentos with a lot of surface roughness in a diet Coke that has been heated.
Here's the article:
Coffey, T. (2008). Diet Coke and Mentos: What is really behind this physical reaction? American Journal of Physics: 76(6), 551-557. (DOI: 10.1119/1.2888546)
"Episode 57: Mentos and Soda" of Mythbusters on the Discovery Channel (first aired August 9, 2006)
Eichler, Jack F., Heather Patrick, Brenda Harmon, and Janet Coonce. (2007) Mentos and the scientific method: A sweet combination. Journal of Chemical Education: 84(7), 1120–1123. (DOI: 10.1021/ed084p1120))