Wayne Schmidt's Mentos and Diet Coke Geyser Page ...Experiments, with pictures, to discover how to make the tallest eruption.

 
While experts argue about who first discovered that dropping Mentos mints into diet coke creates an explosive eruption of foam, many references claim that the effect was initially popularized by Lee Marek on the David Letterman show. I myself never knew about it until I saw a Mythbusters television show in 2007 in which they set a record by producing an geyser that reached 29 feet. (They also claim to have hit 34 feet by sustituting rock salt for Mentos.) Intrigued, I began searching the Internet for information about the relationship between the number of Mentos dropped into a 2-liter bottle of diet Coke and the height of the ensuing eruption. I found nothing in dozens of searches. Equally frustrating, I couldn't find any quantitative information on whether the Mentos work better in cold or warm soda, if the Mentos should be frozen, or how well substituting table salt for Mentos, which some people recommend, really works.

I created this page to answer these questions through a series of experiments whose results are verified by photographs of the actual tests. I hope you find it interesting and entertaining. But first... a little theory:

 
What causes a mentos and diet coke geyser?

Look at the inside of a glass of soda and you'll see streams of bubbles appearing as if by magic from certain spots on the inside of the glass. Swirl the soda and the streams continue to form at these spots. This is because there is a sharp speck of dust or a nick in the side of the glass that acts as a seed for carbon dioxide gas dissolved in the soda to be released. In a new glass there may be fewer than a dozen such locations and the rate of carbon dioxide coming out of the soda will be slow. Consequently, the glass will remain fizzy for a long time. In a dirty glass or an old one with many nicks there will be many more sites where bubbles can form and escape. In this glass the bubbling will be faster and the soda will loose its fizz faster.

If you looked at the surface of a mint mentos under an electron microscope, you'd see that the candy shell covering it was very rough. This structure creates tens of thousands of sharp sites on which bubbles can form. This then is the reason mentos cause sodas to foam so violently: they provide a multitude of sites to release the carbonation. This is a predominantly physical effect caused by the structure of the shell rather than one dominated by a chemical effects.

The image above shows the surface of a mentos magnified 173 times.
(The distance between the two small arrows is 0.001-inches.
The smallest details are less than 1/10,000-inch in diameter,
40 times smaller than the thickness of a human hair.)
It looks like the surface of the moon rather than the
porous surface hinted at by most descriptions.

Because the image above didn't present the pockmarked surface I expected, I began wondering if the surface quickly dissolves and each of the tiny particles released creates a site for bubbles to form. Such a process would greatly increase the number of site produced. I studied mentos held against the inside surface of a glass of diet Coke and saw that this "dissolving" theory isn't observed. The bubbles only form on the surface of the mentos. If the dissolving theory was correct, then there would be a diffuse cloud of bubbles forming all around the mentos.

While some references claim a single mentos mint provides "thousands" of bubble-forming sites while others claim "millions," the image above enables a more accurate estimate. Assuming that each of the visible points in the image is a nucleation site, then based on the surface area of a mentos (1.3-square inches) and the scale of the image it's possible to count the number of points, multiply them by the area scale factor and come up with the number of sites as approximately 833,644. I've attempted to photographically verify this by taking pictures of the fizzing process, but issues of focus and interfering bubbles prevented it. By observing the process with magnifying lenses I estimate the closest bubble source sites are on the order of 0.001-inches apart. This suggests a nucleation site count closer to 500,000 per mentos.

 
While mentos geysers can be created using any carbonated liquid, diet colas work best because their unique chemical make-up creates the most violent reaction and a fine, stable foam that is easy to see. In the Mythbusters show about Mentos geysers they discovered that the potassium benzoate (preservative), aspartame (artificial sweetener) and caffeine in diet Coke all contributed to creating an explosive release of foam. The remaining ingredients had little or no effect. They also claimed that the gum Arabic and gelatin in the Mentos also contribute to the foaming action. However, they determined this by pouring each of these ingredients into soda and noting that some foaming resulted. Since these two ingredients are contained in the chewy center of the Mentos they concluded that these also contributed to the geyser. In this I believe they were mistaken.

Mentos geysers only last a very few seconds, much less if heated soda (please see below) is used. This time scale is much too short for the insides of the Mentos to react with the soda. This is confirmed by an experiment (below) where cut-up Mentos were used instead of whole Mentos. If the gum Arabic and gelatin increased the foaming, the broken up ones, which expose much more of these two chemicals to the soda, would produce higher geysers. They didn't. This is also verified by another experiment where the outside of a Mentos is removed. The result: little or no foaming.

The foaming reaction in Mythbusters from these two compounds was most likely the result of how they were added to the soda, which was different than just dropping a mentos core in.

Mentos were first released in the 1950s by the Perfetti Van Melle Corporation in Holland. They consist of a thin, hard shell covering a chewy interior. Only the mint flavored mentos create diet Coke geysers. Fruit flavored mentos have an additional coating that prevents their shells from producing the required explosion of bubbles.

Tic Tac mints also work, but their small size makes them difficult to handle.

 
The experiments:

In each experiment the bottle of diet Pepsi (it was on sale for half the price of diet Coke) was gently and slowly opened just before dropping the mentos into it to insure that it contained the maximum amount of carbonation. The mentos were placed in a tube so they could be quickly dropped into the bottle.

I tried 3/4-inch PVC pipe and discovered that it was too wide and allowed the Mentos to turn sideways and jam. One-half inch tubing was too small. By rolling poster board around the 1/2-inch diameter tube I ended up with a stiff paper tube that was the perfect diameter. The mentos have to be loaded by pushing them in from the bottom. Dropping them in from the top ends up causing two of them to wedge in sideways and they won't drop cleanly into the bottle. Once loaded, the bottom of the tube was covered with a small piece of poster board, the unit placed over the open end of the diet Coke bottle, and the paper slip pulled away to let the Mentos drop into the bottle in a solid line.

 
What is the relationship between the number of Mentos dropped into a 2-liter bottle of diet cola and the height of the resulting geyser?

The first experiment consisted of dropping 1, 5, 10, 15 and 20 Mentos into bottles of diet Pepsi and recording the height of each geyser.

One mentos created a geyser that was 2-feet high, 5 created a 9-foot geyser, 10 a 12-foot fountain, 15 a 14-footer and 20 mentos produced a 15-foot geyser. Both the 15 and 20-mentos eruptions were so violent that the last mentos in each case was blown out of the bottle before it could react with the soda. This, and the diminishing returns from increasing the number of mentos used, suggests that the optimum number of mentos for the highest diet coke geyser is 20. More may produce a slightly higher fountain but it's just as likely that the extra mentos will be ejected and wasted.

The price paid for mentos eruption height is duration. The 1-mentos eruption lasted 10 seconds. The 20-mentos eruption only lasted two.

 
How consistent are mentos geysers?

Repeating the 5-mentos geyser three times I discovered that they were all within 1-foot of each other.

 
Does breaking the mentos up increase geyser height?

No. Breaking 5 mentos into 4 pieces each produced a geyser that was the same as geysers from 5 whole mentos.

Looking at the cross section of a mentos:

shows that it consists of a thin, white, hard outer shell and a softer core. Sanding off the outer shell from one mentos and washing it off a second then dropping each in a bottle of diet Pepsi produced the following two eruptions compared to a normal 1-mentos geyser:


From Left to right: 1 normal mentos, 1 mentos with the outside
coating sanded off and one mentos with the outside coating
washed off.

This experiment proves that it's only the thin outer shell that causes the fountaining effect. Breaking mentos up only increases the amount of non-reactive inside material so it has no effect.

 
Does freezing the mentos increase geyser height?

It didn't for me. A 5-mentos geyser using frozen mentos still only reached 9 feet. This was surprising because I expected the frozen shell to crack off the mentos from thermal shock exposing both sides as it fell into the comparably warm soda, thereby greatly increasing the reacting surface area.

 
Does soda temperature effect geyser height?

Yes! Greatly! As the following pictures demonstrate:

The image on the left is a 5-mentos geyser using room temperature (70-degrees F.) soda, the middle picture was of refrigerator-cold soda (38 degrees F.) and the one on the far right was from soda left in the sun until it was 110-degrees. This last mentos eruption easily hit 16-feet. What's remarkable about this test is that it proves that increasing soda temperature is a much more powerful technique for creating the tallest fountains than increasing the number of mentos.

WARNING!!!
Heating a bottle of carbonated beverage is extremely dangerous.
Never attempt it. Doing so is extremely hazardous and
could cause serious injury or death.

 
Does drilling holes in mentos and stringing them on a wire effect geyser height?

Yes. Five mentos strung on a wire:

and dropped into a 2-liter diet cola produced the following geyser:

Five loose mentos typically produces a geyser that's 9-feet tall. Stringing them on a a wire reduced this to 7 feet. This may have been caused by the holes drilled reducing the amount of shell that could react with the soda or by the mentos no being free to separate and expose all of their surface to the soda. Too bad. This would be an easy way to load large numbers of mentos into a bottle.

 
Does salt work as good as mentos?

Not for me. Using the same weight of table salt as 5 mentos produced the following weak geyser:

 
After rewatching the Mythbusters show about Mentos/diet Coke geysers I found out that they used rock salt, rather than table salt, in their test that reportedly created a 34-foot geyser. I repeated the above experiment using 5 mentos (15 grams) versus 15 grams of rock salt. The following images show the results:

The left image is a Mentos geyser and the right is the rock salt geyser. The Mentos win by a small margin. I suspect that what happened with the Mythbusters experiment is that the soda used with the rock salt was a few degrees warmer than the one that got Mentos.

(Note: the backgrounds in these images look different than in the rest of the images on this page because this experiment was conducted in a different location.)

 
Does diet Coke work better than diet Pepsi?

Yes. Diet Coke geysers averaged 10-percent higher than diet Pepsi. I also tried Albertson's diet MAX Cola and found it produced smaller mentos geysers than either diet Pepsi or diet Coke. (From this point on I only used diet Coke for the experiments.)

 
Conclusions:

The highest mentos and diet coke geysers are created using 20 mentos dropped into hot diet Coke. Putting all this together produced the following eruption that hit 25 feet:

After the geyser subsided, I counted 9 mentos that had been ejected by the exploding column of foam pushing upward, again indicating that diet Coke's high temperature was more important for fountain height than trying to push a large number of mentos into the bottle.

 
Does fitting the end of a diet Coke bottle with a nozzle create higher geysers?

Yes. I made a nozzle by lining a 7/8-inch diameter dowel with aluminum foil tape with the adhesive facing out. Then I wrapped it with a 6 x 11-inch long piece of poster board and covered the outside with more foil tape. I then pulled the tube off the dowel, cut it to 6-inches long, and attached it to the cap of a Coke bottle with a 7/8-inch diameter hole cut into it with still more foil tape. Finally, I cut two "V" notches in the end of the tube to close it down to 3/4-inch diameter opening. This provided a 20-percent area reduction, which should provide 20-percent greater exhaust velocity.

Twelve mentos were loaded into the nozzle and held in place by a sewing pin pushed all the way through it.

Pulling the pin releases the mentos, which fall very quickly and cleanly into the bottle of diet Coke. Here's the result using diet Coke warmed to 130-degrees F.:

As far as I can determine this mentos fountain hit 27 feet. The highest part of the column is hard to see because the eruption was so violent that the stream was blown into a fine spray.

The speed with which the nozzle feeds mentos into the bottle is considerably faster than the tube-feed system used earlier, so more mentos can be dropped in before foam rising from the first mentos prevents the last mentos from entering the tube. With nozzle delivery as many as 15 mentos might be safely dropped into the bottle.

This nozzle had two problems. First, there were several sharp edges that undoubtedly created turbulance, thereby decreasing exhaust velocity. Second, the strength of the eruption was so great that it ripped the inner core of the nozzle loose and threw it into the air. (You can see it as a small white object indicated by the black arrow in the above photo.) This had to have disrupted the flow and reduced the height of the eruption.

Nonetheless, the nozzle worked so well I built several others:

This time a plastic cutoff riser was used. All total I made 4 with different diameter exhaust orifices made from epoxy putty: .75-inches, .68-inches, .60 and .50-inches in diameter. The straight tube on the left (.75-inch orifice) worked the best. The back pressure created by the more constricting nozzles reduced the height more than the increased exhaust speed increased it. Here's how it worked using 13 mentos and diet Coke heated in the sun to 133-degrees F.:

My best estimate is that this stream hit 30 feet. It's doubtfull that more than 12 mentos could make it into the bottle. The reaction of the first mentos with the hot soda is so violent that I was surprised the following 11 made it in.

I've repeated this test on windless days by firing the mentos geyser next to a string with helium balloons attached to it every ten feet for accurate height measurement and found the 30-foot height to be accurate. Increasing the number of mentos from 12 to 15 did not increase the fountain's height because the last three were ejected before they reacted with the diet Coke. The variance in height from shot to shot was one and one-half feet, depending on temperature, size of the mentos (they aren't all the same), speed with which I was able to attach the nozzle to the Coke bottle, differences in the amount of carbonation in the bottle and so on.

 
Getting a little weird:

While creating the tallest mentos and diet Coke geyser is entertaining, it by no means exhausts the entertainment possibilities of this phenomena. Two things I've tried are cutting the top off a bottle of diet Coke to make a 3-inch diameter opening and dumping 75 mentos in it.

The idea was to get a short but fat column of foam. This one ended up a little too short. Next time I'll cut the hole smaller.

Next I tried building the world's next terror weapon by taping an elbow and short length of PVC pipe to the top of the nozzle used in the earlier experiments to make a foam-shooting gun.

Looks like this one needs some more work too. The turbulence caused by the sharp turn broke the stream up. Perhaps using a longer exhaust nozzle would form it up better.

 
Mentos and Diet Coke Rockets?

Sorry, folks. It won't work. Using 75-degree diet coke, my best nozzle and 12 mentos the highest thrust I recorded was 1.6-pounds. I assume that this could be increased to perhaps as high as 2-pounds by optimizing the nozzle design but that's about it. Considering that a 2-liter bottle of diet Coke weighs over 4-pounds, twice the maximum possible thrust, I have to conclude that this idea isn't going to work.

 
Final thoughts:

Mentos and diet Coke geysers explode out of the bottle with unbelievable speed. Measurements of the geyser's height from a sequence of pictures taken every 0.2-seconds discloses that the intial exhaust speed is 22.5-feet per second. That sounds fast but converted to miles per hour it's only 15.3 mph. Still, it accelerates to that velocity in only 0.2 seconds. That's 3.5-Gs, about what the space shuttle experiences. For people who are more interested in motor sports that equates to a dragster doing the quarter mile in 4.8 seconds; a respectable time though far from world-class.

Creating mentos and diet Coke fountains is fun... but it isn't cheap. At $1.00 to $1.50 per geyser, which only lasts 3 seconds, means this show can cost up to $1800.00 per hour if done continuously.

 
NEW!!!

While this geyser tube toy looked like it was going to work well, in practice I had a lot of trouble with it.

First, the tube is a little small so that some Mentos jam in it. This can be avoided by testing each Mentos to be used ahead of time to make sure it falls through cleanly, but that's a hassle.

Second, the highest geyser I got was 16-feet, far short of the 25-feet claimed on the packaging.

Last, the foam rocket was blown off to the side as soon as it cleared the tube. I doubt if it got any higher than 4-feet off the ground.

This toy was very disappointing and I can't recommend it.

 
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