Sodamentology 101

Were you looking for sedimentology?

Still here, eh? Well, if you haven't seen what happens when you drop a dozen Mentos into a two-liter bottle of soda, you might want to stop reading and go try it for yourself ... somewhere outside.

This experiment has been widely reported on the Internet. Follow the links to a variety of amazing videos and an equal variety of explanations, some of which are actually accurate.

My contribution to the rapidly expanding field of sodamentology (a.k.a. sodamento physics) is the design of the delivery system shown in the attached videos. Compared to the classic test tube technique, the method described here is a more reliable way of getting the Mentos into the soda.

Start with a sheet of flexible but relatively stiff transparent plastic. An overhead projector sheet is ideal. (Paper is also acceptable but can't be reused, and it's harder for the spectators to see what's happening.) Roll it around a tube of Mentos and tape it to keep it from unrolling. With the tube of Mentos still inside, but 2 or 3 cm from the end of the plastic tube, take a skewer (bamboo, wood, metal) and poke it across the diameter of the tube so that you have two holes, one on each side. Replace the skewer with a toothpick as in the pictures below if you wish, or leave the skewer in place. Remove the tube of Mentos, open it, and load about 12 to 14 Mentos into the open end of the plastic tube, so that they stack neatly and are supported by the toothpick or skewer.

Take your plastic tube of Mentos and a two-liter bottle of soda outside. The middle of a large lawn is an ideal location. This is not an indoor experiment -- you have been warned!

Now open the bottle of soda. Try not to shake it up; the experiment will work best if the soda has as much dissolved carbon dioxide as possible. Any soda will do (although plain seltzer doesn't seem to work quite as well as diet cola, in my experience). Sodamento physicists favor diet soda because it's less sticky than regular soda.

Holding the plastic tube upright, insert the lower end into the open soda bottle until the toothpick or skewer is resting across the lip of the bottle. If you are wearing anything that should not be covered with foaming diet soda, this would be a good time to change your clothes. Get the spectators to back up by a meter or two, let them give you a countdown, and in one quick motion, remove the toothpick or skewer (steadying the bottle with your other hand so it doesn't fall over) and let the Mentos drop into the soda bottle.

You will have about 50 milliseconds to decide what to do next. If you leave the tube in the mouth of the bottle, you should get impressive results and will probably stay mostly dry. If you remove the tube quickly, after all of the Mentos have dropped into the soda and without knocking the bottle over, you should get extremely impressive results and you will probably get at least a bit wet.

Sodamentologists have not yet established what combination of soda and Mentos produces the most satisfying results, but in both of the videos below, we used Diet Coke and fruit-flavored Mentos. It seems unlikely that the Mento flavor affects the results, but (for what it's worth) we have achieved better results with fruit-flavored Mentos than with mint Mentos. It is plausible that the choice of soda might affect the results, because of either the amount of dissolved CO2 (the fizzier, the better) or the surface tension.

It's possible that 12 to 14 Mentos may be more than what is needed to cause release of the CO2 in two liters of soda. Larger numbers of Mentos appear to have no additional effect. Other factors such as the temperature of the soda and the amount of time between opening the bottle and dropping the Mentos have not yet been investigated systematically. The field of sodamento physics is literally dripping with possibilities.


Download movie 1 (AVI, 3.2 MB). Click on the thumbnails:

Download movie 2 (AVI, 2.6 MB), or click on the thumbnails:

The top of the fountain was probably about 2.5 meters above the mouth of the bottle. Analysis of the second video shows about 1.8 meters of the soda jet visible above the 30 cm tall bottle.

George Moody
5 February 2006