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upstart writes in with an IRC submission for SoyCow8192:

We now know the effect of altitude on classic "Diet Coke and Mentos" fountain:

Back in 2006, Fritz Grobe and Stephen Voltz—the self-described mad scientists behind Eepybird—ignited an Internet sensation with their viral video of an elaborate version of the Diet Coke and Mentos fountain experiment, recreating the choreography of the Bellagio's world-famous fountain display in Las Vegas. The underlying physics and chemistry of the fountain effect is well-known.

But an intrepid pair of scientists at Spring Arbor University in Michigan wondered whether altitude, and associated changes in atmospheric pressure, would have any measurable impact on the intensity of the foaming fountain and performed a series of experiments to find out. They reported their results in a recent paper in the Journal of Chemical Education. The upshot: If you really want to get the most foaming action for your buck, conduct the Diet Coke and Mentos experiment at high altitudes.

[...] Co-author Thomas S. Kuntzleman first encountered the experiment at an elementary school science fair in 2005, and he has been interested in the underlying physics and chemistry ever since. For instance, he has found that various beverage additives such as sugars, citric acid, and natural flavors can enhance fountain heights. Most recently, he decided to test the hypothesis that the intensity of the fountain effect would be greater at higher altitudes, since one would expect atmospheric pressure to play a significant role in bubble nucleation.

Kuntzleman and his co-author, Ryan Johnson, purchased bottles of Diet Coke from the same store and made sure the bottles all had the same expiration date. For each experiment, they dropped a single Mentos candy into a bottle via a one-inch PVC pipe and then watched the reaction work its magic. They used a graduated bottle/tornado tube to measure the mass lost from the liquid over time. The measurements were all taken within one week of each other, and the team made sure to keep all the bottles at roughly the same temperature.

One set of experiments was performed at different altitudes during a climb of Pikes Peak; another set took place during a drive across North Carolina; and a third set of experiments was performed during Kuntzleman's family vacation, driving through multiple national parks in California, Nevada, and Utah. All told, they performed the experiment at altitudes ranging from below sea level (Death Valley, California) to more than 14,000 feet (Pikes Peak). As expected, they found that more foam was produced at higher elevations.

[...] For those chafing at extended sheltering-in-place orders, it also provides a possible fun DIY group experiment. Identify people in your social circles who live at different altitudes. Each person can do the Mentos and Diet Coke demo—outdoors highly recommended—wherever they are, taking note of the relative heights of the foaming effect, so everyone can then compare their respective results. (Scientific American has helpful instructions, as does Eepybird.)

If you're really feeling ambitious, everyone can do the experiment live simultaneously via Zoom or similar conferencing tool. Just protect your electronic equipment. And be sure to use Diet Coke or Coke Zero, not just because they make the biggest reaction—the aspartame lowers the surface tension of the water—but because they are less sugary and sticky. Trust us, you're going to get soaked.

DOI: Journal of Chemical Education, 2020. 10.1021/acs.jchemed.9b01177  (About DOIs).

Original Submission