Mentos react with Diet Coke because their rough surfaces provide suitable sites for rapid growth of carbon dioxide bubbles, according to Mental Floss magazine. The bubbles’ buoyancy coupled with their rapid growth, causes them to rise to the surface, resulting in an eruption of foamy diet soda.
Low surface tension also encourages rapid growth of bubbles. Diet Coke has a lower surface tension than sugary Coke because it contains aspartame; and Mentos candies are coated with gum arabic, a surfactant that further lowers the surface tension of the soda. Aspartame is the reason why Mentos react more vigorously with Diet Coke than with sugary Coke. Lastly, Mentos are quite dense, and they sink fast, encouraging even faster bubble growth.