Tribonucleation: A New Mechanism for Generating the Soda Geyser

Abstract

Observations of the rapid release of CO 2 from carbonated beverages, also known as sodas, provide a rich assortment of experiments for chemical educators and their students to explore. For example, dropping Mentos candies into a freshly opened bottle of soda creates a fountain that can jet several meters into the air. The fountain is generated by rapid formation of CO 2 (g) bubbles on innumerable nucleation sites that exist on the rough surface of the candies. Interestingly, it is possible to create fountains of moderate height by dropping smooth objects into bottles of soda. Exploration of how smooth objects create such fountains provides new classroom demonstrations, laboratory experiments, and inquiry-based projects that can be tied to topics that include kinetics, density, and gas solubility.

Ethanol as a Probe for the Mechanism of Bubble Nucleation in the Diet Coke and Mentos Experiment

“…This is substantially lower than the estimate of roughly 5–60 million pores outlined in the introduction. This discrepancy could partially be resolved by noting that the surface of the Mentos candy is not uniform, but rather contains both rough and smooth areas [ 8 ]. If the smooth areas on the surface do not contain nucleation sites, then the estimate of millions of sites could be 2–4 times too high.…”

“…Dropping Mentos candies into a freshly opened bottle of Diet Coke (or other carbonated beverage) is a popular experiment among science teachers and the general public [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. The wide-spread fascination with this experiment stems from the fact that it produces impressive results and is extremely easy to carry out.…”

“…The wide-spread fascination with this experiment stems from the fact that it produces impressive results and is extremely easy to carry out. When the candies enter the beverage, they catalyze a sudden degassing of dissolved CO 2 [ 3 , 8 , 9 , 10 ]:…”

“…The rapid escape of gas forces a jet of bubbly liquid out of the bottle that can reach up to a few meters high. Insight into the physical chemistry involved in fountain formation has been uncovered by studies on the kinetics of degassing and bubble growth in this experiment [ 6 , 7 , 8 , 9 , 10 ]. Further understanding of the relevant processes has been guided by studies on nucleation and bubble growth in Champagne and other carbonated beverages [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ].…”

“…This translates to about 40 million such sites over the entire 8 cm 2 surface of the candy. These estimates represent a maximum number of sites, given that SEM images display alternating smooth and rough regions on the Mentos surface [ 8 ]. Furthermore, the estimate assumes that all such sites are capable of actively nucleating bubbles, but the number of sites active sites likely decreases as the candy dissolves into the surrounding soda.…”