Probing the Mechanism of Bubble Nucleation in and the Effect of Atmospheric Pressure on the Candy–Cola Soda Geyser

Abstract

The so-called Diet Coke and Mentos experiment is initiated by dropping Mentos candies into a bottle of Diet Coke or other carbonated beverage. This causes the beverage to rapidly degas, causing foam to stream out of the bottle. Simple application of the gas laws leads to the straightforward prediction that ejection of greater foam volume is expected at lower atmospheric pressure. This hypothesis is bolstered when principles of bubble physics are taken into account. This hypothesis was tested and confirmed by monitoring the foam produced during the Diet Coke and Mentos experiment at various altitudes above sea level. Upon further application of the aforementioned principles, a relationship between degassing kinetics, beverage CO2 concentration, and the size of pores on the candy surface that serve as nucleation sites can be derived. Using this relationship and experimental measurements of degassing kinetics, students estimated that the nucleation sites on Mentos candies are on the order of 2–7 μm in dimension. Students in Physical Chemistry, General Chemistry, and nonmajors’ courses have found these experiments to be of great interest.