Explorers

On October 2, 2008, the Explorers investigated the properties of “nucleation.” Sounds innocent enough, right? The club members found out otherwise!

So what is nucleation? Put simply, it is the process of a phase change occuring rapidly due to a seeding opportunity. Okay, maybe that wasn’t so simple, so let’s investigate a little further, using the same materials the Explorers used: 2-liter bottles of pop and mint-flavored Mentos candies.

Most people know that carbonated water is what gives “pop” its fizz—and the “carbonated” part just means that carbon dioxide gas is present in the liquid. The gas gets there through a process where it is dissolved, or forced under pressure, into the liquid when the soda is bottled.

Of course, a material in a gas phase takes up far more room than one that is liquid, and as soon as the pop bottle is opened and the pressure released, the gas slowly starts to form bubbles, and the pop loses its fizz. But—and this is the important part!—it happens much slower than we might otherwise expect, because the molecules of carbon dioxide can only combine with each other and form bubbles when they are given some sort of surface to cling to. This is why if you pour a glass of soda, you will find nearly all of the bubbles clinging to the inside of the glass, not floating around loosely in the drink. Try it for yourself if you’re not convinced.

The first step the Explorers took was to verify this observation for themselves. Next, they let a glass of pop settle down for a few minutes and then stuck a finger gently into the drink—and they all observed that a fresh set of bubbles quickly formed on the surface of their fingers. Then they took a wadded up paper towel and stuck it into the drink, and they were impressed at how the drink just absolutely turned to foam.

What was happening? Nucleation, that’s what. The fingers provided a slightly rough surface for the carbon dioxide molecules to cling to and meet up to form bubbles, and the much rougher surface of the paper towel allowed for an even more dramatic demonstration of nucleation. They weren’t done, though.

Finally, the club moved outside (do NOT try this indoors, unless you have plenty of time—and towels—available for clean-up!). Mr. Ramsey showed the innocent-looking candy known as Mentos—these small ovals look and feel fairly smooth, but in fact their surfaces are absolutely riddled with microscopic pits that are like the Promised Land to carbon dioxide molecules. (Note: there are several flavors of Mentos available—the mint ones are the only ones with sufficiently rough surfaces to get a good reaction.)

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The students took turns in pairs dropping several of the candies into 2-liter bottles of pop. Any club members who were expecting a bit of a splash or fizzle quickly found out just how powerful nucleation can be. We didn’t call this experiment “Pop Bottle Geysers” for nothing! Some of our eruptions easily reached higher than 10 feet.

We used “diet” varieties of pop, simply because they are much less sticky than the regular type. We also tried a range of flavors and colors, all of which seemed to work just fine—we ended up with brown geysers, purple geysers, red geysers, orange geysers, and yellow geysers.