Thursday, May 29, 2008

More insight on the Mentos eruption

The June issue of the American Journal of Physics (the AAPT's flagship publication) contains a report by Tonya Shea Coffey of Appalachian State University on an undergraduate class project concerning one of the Youtube-era Internet's favorite kids' pranks: the Mentos eruption.

Fruit Mentos and Mint Mentos were found to cause nearly equal emptyings of Coke bottles. Playground sand, among other things, was an adequate substitute. Diet Coke and Coca-Cola were nearly equally emptied by the Mint Mentos. Differences between Diet Coke and Caffeine Free Diet Coke were not clearly statistically significant.

The Diet Coke-Fruit Mentos combination, however, produced the longest jet, followed by the classic Caffeine Free Diet Coke-Mint Mentos pairing. Further investigation by Coffey and her students showed from photographic evidence that aspartame reduces the work needed to form a critical bubble compared to plain seltzer water and that less work is needed to form a bubble in Diet Coke than in plain Coca-Cola. SEM study found that Fruit Mentos and Mint Mentos are nearly equally rough. Moreover, unlike a wax coating previously applied by the "Mythbusters", Fruit Mentos' coating dissolves rapidly in soda water. The researchers speculate about Mentos having a surfactant component but take no measurements.

Fairly useless research, yes, but it gets students thinking about questions more subtle than those found in introductory textbooks, provides a means of introduction to surface science intersting to the common eighteen-year-old, and it's more practical than quite a bit of what ends up getting funded by DARPA.

HT: Chronicle of Higher Education