The reaction happens when the rough surface of the candy causes bubbles to "seed" out of the liquid. The imperfections on the candy surface cause microscopic bubbles of dissolved gas to gather together into bigger bubbles. These rise to the surface, eventually erupting out of the bottle opening.
Why Does This Happen?
Why do they always use Mentos and diet cola? Wouldn't any rough surface work with any fizzy drink?
Physicists studied that very question. They experimentally tested the reaction using different seeding objects, such as different candies, sand, rock salt and dish soap.
They also tested different fizzy drinks, including regular, diet and caffeine‑free colas, and soda water-‑measuring the height and volume of the resulting geysers for each combination.
Mentos And Diet Cola
It turns out Mentos were the best seeders for the reaction. One reason is their large surface‑to‑volume ratio, which allows millions of bubbles to form from each piece. They are dense and sink quickly. As they sink the bubbles formed seed even more bubbles as they rise through the cola.
Plus, the Mentos shell contains gum arabic, a surfactant. This allows bubbles to form more easily.
Diet cola--with or without caffeine--produced the biggest reactions, because soda sweetened with aspartame has lower surface tension than sugary sodas or plain water. Lower surface tension contributes to rapid bubble formation. Warm cola also resulted in more dramatic explosions than cold.