Perhaps one of the more interesting YouTube videos are those that involved the combination of Mentos® and Diet Coke®. All you need to do is to go to YouTube and search for those foodstuffs, and you’ll find hundreds of them. For my money, this is the best one.
Although I suspect that there have been some experimentation in Physics classrooms, the science behind this phenomenon has never been rigorously explored. It was thought that perhaps it was the interaction of the covering of the candy with the acidity of the liquid that produced the gushers. The notion of enucleation sites—basically the bumpy surface of the Mento—produced a large number of carbon dioxide bubbles that therefore led to the explosion of the Diet Coke.
A recent paper in the Journal of American Physics now explores this issue directly.
Here’s what they did:
We examined the reaction between Diet Coke and samples of Mint Mentos, Fruit Mentos, a mixture of Dawn Dishwashing detergent and water, playground sand, table salt, rock salt, Wint-o-Green Lifesavers, a mixture of baking soda and water, liquid gum arabic, and molecular sieve beads (typically found in sorption pumps). We also examined the reaction between Mint Mentos and Diet Coke, Caffeine Free Diet Coke, Coca-Cola Classic, Caffeine Free Coca-Cola Classic, seltzer water, seltzer water with potassium benzoate added, seltzer water with aspartame added, tonic water, and diet tonic water. All of the samples were at room temperature unless otherwise indicated.
We constructed a bottle stand (roughly 10° off vertical) to prevent the bottles from tipping over and the liquid from falling back into the bottle. To maintain consistency we also constructed a tube to fit over the mouth of the bottle and a delivery mechanism for the solid materials. The liquid samples, including the gum arabic, the baking soda–water mixture, and the Dawn–water mixture, were administered by injection using a 10 ml syringe with an 18-gauge needle. The seltzer water and tonic water trials were 1 l bottles with 16 g of Mint Mentos added; all other trials were 30 g of solid material added to a 2 l bottle of liquid. The intensity of the reaction was determined by measuring the mass of the bottle using a double pan balance before and after the reaction to determine the mass lost in the reaction and by measuring the horizontal distance traveled by the soda’s spray. To ensure accurate distance measurements and to extract other useful information, a video was made of the reactions, and marker flags were placed every half foot on the level ground, up to a distance of 25 ft away from the bottle stand. For the Mint Mentos and baking soda trials, the pH of the Diet Coke before and after the reaction was measured by a pH meter with a two point calibration.
Sample morphology was determined by imaging the samples in an environmental scanning electron microscope (SEM).4 The uncoated samples were imaged in low vacuum mode. Quantitative surface roughness measurements were made with a Digital Instruments contact mode atomic force microscope (AFM) with Nanoscope III control electronics and a J type scanner with a 24 µm z range. For each of the samples a (10 µm)2 image was acquired, and the root-mean-square (rms) roughness in the image was reported. This size image was chosen for comparison between samples because the samples imaged were quite rough and had significant curvature, and images larger than 100 square µm often resulted in a z range larger than 24 µm.
For the temperature dependent trials one of the Diet Coke 2 l bottles was refrigerated for several hours prior to the experiment. The other bottles were heated in a water bath on a hot plate for approximately 10–20 min. Prior to heating, the bottle was opened to release some of the internal pressure, and then closed again. This procedure prevented the explosion of the bottle during heating, but the early release of some of the carbon dioxide gas may have caused these reactions to be less explosive than the cold or room temperature trials.
They basically found that it is indeed the rough surface of the Mentos, combined with the speed with which it falls to the bottom of the bottle, that causes the eruption of brown yuckiness. What is not to love about this study? This is just a perfect example of why science matters.
