A Mystery for Many Years
This short video contains a demonstration of the startling reaction of Diet Coke and Mentos sweets. For many years, several mysteries about the cause of the reaction have circulated with the secret being eventually unveiled in 2008.
The rough, dimply surfaces of the Mentos drops promote CO2 bubble growth because they efficiently disrupt the polar attractions between water molecules, creating bubble growth sites.
The velocity of CO2 bubble growth is additionally influenced by low surface tension. Since the surface tension in water containing the sweetener aspartame is lower than in sugary water, Diet Coke creates more dramatic fountains than sugary Coke.
The surface tension in the liquid is further reduced by the gum arabic coating of Mentos.
More on the experiment and an instruction to use this experiment with students: