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Reaction Dynamics


Taking the Diet Coke-and-Mentos demo to new heights, and testing a kombucha-based water filter

by Corinna Wu
April 12, 2020 | A version of this story appeared in Volume 98, Issue 14


Putting some fizz into a family vacation

Credit: Thomas Kuntzleman
Conducting the Diet Coke and Mentos experiment at different altitudes—40 ft (12.2 m) below sea level in Death Valley; 6,000 ft (1,829 m) in Zion National Park; and 10,900 ft (3,322 m) in Yosemite National Park—produces different volumes of foam.


Dropping a bunch of Mentos candies into a bottle of Diet Coke is a favorite science demonstration, as the action results in an impressive geyser of foam to rival Old Faithful. Thomas S. Kuntzleman, a chemistry professor at Spring Arbor University, says he first saw this demo in elementary school and “that was it—I was hooked.” Now, he has taken the famous Diet Coke fountain demo to new heights by measuring the amount of foam produced at different altitudes (J. Chem. Educ. 2020, DOI: 10.1021/acs.jchemed.9b01177).

Photo of Thomas Kuntzleman standing in deep snow holding a bottle of Diet Coke and a foam guide with blue sky and mountains in the background.
Credit: Courtesy of Thomas Kuntzleman
Mountain high: Thomas Kuntzleman starts up a Diet Coke-and-Mentos fountain near Tioga Pass in California.

The rough surface of Mentos candies provides nucleation sites for bubbles to form and grow from the carbon dioxide dissolved in the Diet Coke. Kuntzleman hypothesized that atmospheric pressure should affect the frequency of bubble formation and was inspired to put this to the test.

He recruited a friend, chemistry teacher Ryan Johnson, to measure the foam produced during a climb up Pikes Peak in Colorado (4,300 m elevation). Kuntzleman also did the experiment himself during a summer vacation as he and his family drove through a range of altitudes, from Death Valley, California (13 m below sea level), to a snowy spot near Tioga Pass in Yosemite National Park (3,260 m elevation). He used 500 mL bottles of Diet Coke, making sure they all came from the same manufacturing lot because carbon dioxide leaks out over time. He also made sure the bottles were at a consistent 27 °C using the heat and air-conditioning from the car.

To his delight, he saw that the amount of foam did increase at the high altitudes, and he was able to model the bubble nucleation. “I’m tickled that we figured out how to do that,” he says.


Brewing up a water filter

Photo of a thin round living membrane draped over the back of a gloved hand.
Credit: Environ. Sci. Technol. Lett.
Super-SCOBY: A bacteria-and-yeast culture used to make kombucha can filter particles out of water.

Some people partial to carbonated beverages enjoy imbibing kombucha, a fizzy, fermented drink made from tea, sugar, and a mushroom-like “mother” known as a SCOBY—symbiotic culture of bacteria and yeast. Katherine R. Zodrow, an assistant professor and head of the Environmental Engineering Department at Montana Technological University, saw a micrograph of the network of thin cellulose fibers in a SCOBY and thought that it looked a lot like a membrane. “I’ve wanted to make membranes that are alive for a long time, and this was the perfect place to start,” she says.

So Zodrow and her colleagues grew kombucha SCOBYs about 1–1.5 mm thick and tested their ability to filter particles from water. They found that the living membranes could remove more than 98% of 50 nm particles and 99% of 100 nm particles, suggesting that a SCOBY filter could remove most bacteria and protozoa. What’s more, the membranes heal themselves when punctured (Environ. Sci. Technol. Lett. 2020, DOI: 10.1021/acs.estlett.0c00019). With more optimization, perhaps SCOBYs will one day serve a dual purpose—filtering water as well as making a bubbly drink.

Corinna Wu wrote this week’s column. Please send comments and suggestions to


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