Demonstration pubs.acs.org/jchemeduc
Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
Tribonucleation: A New Mechanism for Generating the Soda Geyser Thomas S. Kuntzleman,*,† Michael W. Nydegger,† Brooke Shadley,† Ninad Doctor,‡ and Dean J. Campbell§ †
Department of Chemistry, Spring Arbor University, Spring Arbor, Michigan 49283, United States Department of Chemistry, Eastern Carolina University, Greenville, North Carolina 27858, United States § Department of Chemistry, Bradley University, Peoria, Illinois 61625, United States ‡
S Supporting Information *
ABSTRACT: Observations of the rapid release of CO2 from carbonated beverages, also known as sodas, provide a rich assortment of experiments for chemical educators and their students to explore. For example, dropping Mentos candies into a freshly opened bottle of soda creates a fountain that can jet several meters into the air. The fountain is generated by rapid formation of CO2(g) bubbles on innumerable nucleation sites that exist on the rough surface of the candies. Interestingly, it is possible to create fountains of moderate height by dropping smooth objects into bottles of soda. Exploration of how smooth objects create such fountains provides new classroom demonstrations, laboratory experiments, and inquiry-based projects that can be tied to topics that include kinetics, density, and gas solubility.
KEYWORDS: General Public, Elementary, Middle School Science, High School, Introductory Chemistry, First Year Undergraduate, General, Demonstrations, Laboratory Instruction, Public Understanding, Outreach, Consumer Chemistry, Physical Properties, Surface Science
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INTRODUCTION Carbonated beverages, also called sodas, are sealed under high pressure of CO2, PCO2, to increase the amount of CO2 that is dissolved in the beverage. The solubility of CO2 in a soda, S, can be estimated using Henry’s Law: S = kHPCO2
made air bubbles into which into which CO2(aq) can escape into the gas phase with relative ease. More nucleation sites in a beverage produce faster degassing. Increased nucleation explains why a shaken soda degasses quickly upon opening, making a mess. When a bottle is shaken, air from the beverage headspace is forced into the liquid. The air thus introduced into the liquid takes the form of a myriad of tiny air bubbles, nucleation sites. If the beverage is opened soon after shaking, these nucleation sites provide innumerable routes for rapid CO2 escape and a wet, foamy mess results. The popular Diet Coke and Mentos experiment provides a striking example of bubble nucleation in carbonated beverages.7−14 A Mentos candy contains innumerable nucleation sites on its rough surface (Figure 1, left). When several of these candies are added to a freshly opened bottle of Diet Coke, rapid degassing occurs which produces a foamy fountain that jets out of the bottle. Other objects with significant surface roughness such as Lifesavers candy, salt, sand, chalk, and iron filings have also been observed to create impressive fountains when added to carbonated beverages.7,15 In addition, ultrasound has been used to create these fountains.16 Herein, the term surface nucleation will be used to describe degassing events occurring primarily on the rough surfaces of objects. Collectively,
(1) −1
where kH is the Henry’s law constant (kH = 0.0345 M bar at 25 °C).1 Therefore, a soda under 5 bar pressure is expected to contain about 0.17 M CO2. When a soda bottle is opened, the beverage becomes exposed to the atmosphere, in which case PCO2drops to 0.0004 bar. Under these conditions, the solubility of CO2 drops to essentially zero, causing the escape of CO2 to be thermodynamically favored: CO2 (aq) → CO2 (g)
(2)
However, the CO2 escapes slowly because a very high activation energy exists for the spontaneous formation of bubbles in a liquid.2 So while it is true that a carbonated beverage begins to discharge CO2 immediately upon opening, this occurs gradually. Slow degassing from sodas is usually preferred, so that one can enjoy a fizzy drink rather than having to clean up a wet, decarbonated mess. In fact, the CO2 predominantly escapes the beverage at nucleation sites, which are tiny gas pockets trapped at imperfections in beverage containers, in minute dust or fiber particles, and the like.3−6 These nucleation sites provide ready© XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: February 21, 2018 Revised: April 16, 2018
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DOI: 10.1021/acs.jchemed.8b00127 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
experiments presented herein describe classroom demonstrations and student laboratory experiments that illustrate how NISD can be initiated by the mechanism of tribonucleation. Explanations on how to carry out these experiments as lecture demonstrations and laboratory experiments are included in the Supporting Information.
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MATERIALS AND METHODS
Measurement of Fountain Height
A geyser tube20 was used to add seven 1/2 in. (12.7 mm) diameter spheres of varied composition (Table 1) to bottles of
Figure 1. Scanning electron microscope (SEM) images of the surface of (left) a Mentos candy, (right) a nonrusted steel sphere, and (inset) a rusted steel sphere. Length of red bar = 50 μm in all images.
Table 1. Experimentally Determined Densities of 1/2 in. Diameter Spheres Used in Experiments Reported Herein
nucleation induced soda degassing (NISD) has been used to describe these experiments.9,14 Given the remarkable heights of the fountains produced in these experiments, it is no surprise that chemical educators and other science teachers have taken advantage of NISD experiments to create lessons and demonstrations.7−11,13−16 These impressive fountains and similar experiments have been demonstrated to students of all ages and levels of sophistication to introduce topics involving surface chemistry, chemical kinetics, and solubility. Experiments on the effects of variables such as temperature and surface area of bubbles produced on fountain height have demonstrated that fountain heights are primarily driven by kinetic factors.8,9,13,14,16 We have observed that rusted steel spheres work about as well as Mentos to produce fountains when dropped into carbonated beverages (Figure 2). This is expected, given the
sphere composition
density/g mL−1
nylon delrin aluminum aluminum oxide zirconium oxide steel brass copper lead tungsten carbide
1.09 1.38 2.66 3.98 6.14 7.77 8.43 9.08 11.81 14.34
Diet Coca-Cola (either 500 mL or 2 L) at room temperature (20−23 °C) within less than 20 s of first opening the beverage. The resulting fountain was filmed using a home video or smartphone camera. An object of known height was also filmed in the frame view. The videos were played back on a computer with a flat top screen and paused at the maximum fountain height achieved. The height of the fountain as it appeared on the computer screen was measured. The height was taken as the distance from the top of the beverage liquid to maximum height achieved. After measuring the height of the known object as it appeared on the screen, the actual fountain height was determined by ratio. Measurement of Foam Volume
An empty 1 L beverage bottle had 25 mL of water added to it. The bottle was then capped, inverted, and the level of liquid was marked with a permanent marker. The bottle was opened and this process was repeated several times until the entire bottle was lined with graduated marks in 25 mL increments. Once this was done, a drill fitted with a 1−1/8 in. boring drill bit was used create a circular opening in the bottom of the bottle. Next, a 7/8 in. boring drill bit was used to widen the opening of a tornado tube21 to allow the 1/2 in. diameter spheres or Mentos candy to freely fall through it. The tornado tube was securely attached to the graduated beverage bottle. It was important to directly drop spheres into the beverage as uniformly as possible between experiments. This was achieved by dropping spheres through a 3/4 in. PVC pipe (or 1 in. PVC pipe for Mentos) that was inserted through the bored hole in the graduated bottle and positioned directly above the opening in the tornado tube. To carry out a foam volume experiment, a bottle of Diet Coca-Cola (500 mL, 1.25 L, or 2 L) at room temperature (20−23 °C) was opened and the plastic ring on the neck of the bottle was removed. The opened bottle was attached to the end of the tornado tube opposite the graduated beverage bottle. The attached assembly was then placed on a
Figure 2. Fountain heights achieved upon addition of seven Mentos candies (left) or seven 1/2 in. diameter spheres (center, right) to 500 mL of Diet Coca-Cola through a geyser tube. At least 15 trials were conducted in each case. Error bars throughout the text represent one standard deviation.
rough surface of rusted spheres (Figure 1, inset). Curiously, we have observed that spheres comprised of ceramics or metals with very smooth surfaces such as nonrusted steel spheres can also produce fountains that are substantially high (Figure 2, right). This result is counterintuitive, given the lack of surface roughness on the nonrusted steel (Figure 1, right). However, it is known that increased nucleation in fluids can occur when solids are rubbed together in a liquid in a process known as tribonucleation.17−19 It is therefore hypothesized that steel spheres induce fountains during NISD experiments primarily by tribonucleation as the nonrusted spheres strike the bottom of the beverage container. In this paper, experiments are presented that are consistent with this proposal. In addition, B
DOI: 10.1021/acs.jchemed.8b00127 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
Figure 3. Video frames of rusted steel spheres (top panel) and smooth steel spheres (bottom panel) dropped into cream soda. Time elapsed since first sphere first broke the beverage surface is indicated.
firm surface. The PVC pipe was inserted through the bored hole in the graduated botte and held as firmly as possible against the neck of the bottle, near the tornado tube. Spheres were dropped into the beverage through a 3/4 in. PVC pipe that was inserted through the bored hole in the graduated bottle. After dropping spheres into the beverage, the maximum foam volume achieved was determined by observing the highest mark on the graduated bottle the foam reached. The headspace volume of the Diet Coca-Cola bottle was included in this maximum foam volume measurement. For kinetic experiments, a video of a running stopwatch placed next to the Diet Coke/ graduated bottle assembly was recorded. The video playback was analyzed for the foam volume at corresponding times after the spheres were dropped into the Diet Coke. So that spheres could be dropped from various heights, the 3/4 in. PVC pipe was held vertically and two small holes were drilled through the pipe in the horizontal direction at 10 cm increments up the pipe. A toothpick placed across the holes allowed the spheres to be held in position prior to being dropped into the beverage; removal of the toothpick initiated the drop of spheres into the beverage.
higher density were expected to strike the bottom of the soda bottle with greater force, causing increasing tribonucleation. Indeed, sphere density correlated quite well with fountain height, with higher fountains resulting when more dense spheres were dropped into sodas (Figure 4). Consistent with these results, kinetic measurements of foam volume showed that spheres with greater density produced more foam more quickly when dropped into soda (Figure 5).
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RESULTS AND DISCUSSION To gain insight into why smooth steel spheres create fountains when dropped into sodas, 1/2 in. diameter rusted and smooth steel spheres were dropped into 2 L bottles of a colorless carbonated beverage (cream soda) and filmed with a Phantom Miro EX-2 high speed camera at 1000 frames per second. A colorless soda was chosen to enhance visibility of the spheres as they traveled through the soda. The rusted steel spheres were observed to create a trail of bubbles as they traveled through the beverage (Figure 3, top panel) indicating that rusted spheres generate fountains primarily through surface nucleation. In contrast, no train of bubbles was observed to trail the smooth steel spheres. However, a large “cloud” of bubbles was observed to form around the smooth steel spheres after collision with the bottom of the container. This observation is consistent with the hypothesis that the smooth steel spheres create NISD by tribonucleation. To further probe the possibility that spheres impacting the bottom of the bottle caused NISD, smooth spheres of varied density were dropped into bottles of Diet Coke using a geyser tube as described in the Materials and Methods. Spheres with
Figure 4. Fountain heights achieved upon dropping 7 spheres into 2 L (closed circles) or 500 mL (open circles) bottles of Diet Coke through a geyser tube. Data represented by open circles were collected by students in a general science course and four sections of General Chemistry Laboratory. Lines are second order polynomial fits to the data.
It must be noted that spheres comprised of aluminum oxide (D = 3.98 g mL−1) often produced fountain heights and foam volumes much higher than expected based on its density (data not shown). These anomalies were thought to occur as a result of surface nucleation occurring on the aluminum oxide spheres. To deal with this discrepancy, the aluminum oxide spheres were coated with clear coat, after which the aluminum oxide spheres behaved as indicated in the figures reported in this paper. In a final set of tests, one 1/2 in. sphere comprised of either aluminum or brass was dropped through a PVC pipe from successively higher heights into 500 mL of Diet Coke and maximum foam volumes achieved were recorded. This was C
DOI: 10.1021/acs.jchemed.8b00127 J. Chem. Educ. XXXX, XXX, XXX−XXX
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have assigned students the task of determining the density of 1/ 2 in. diameter spheres of assorted composition. Once this was done, students determined the relationship between the density of spheres dropped into sodas and fountain height (see Figures 4 and 5, for examples, of student data). The Supporting Information provides more details on experiments carried out in laboratory and lecture. The graduated bottle/tornado tube assembly provides a unique solution for demonstrating experiments involving NISD. Experiments involving NISD tend to create unwanted messiness, especially when carried out indoors. The assembly provides a means for students and teachers to carry out NISD experiments and demonstrations in a tidy manner. The assembly allows for several experimental runs to be carried out in a short period of time with little to no mess, and the graduated bottle allows for facile observations and collection of quantitative data. Further specifics regarding this experimental setup and associated demonstrations can be found in the Supporting Information. Finally, students find it interesting that tribonucleation may be involved in common experiences. For example, when a prankster taps a freshly opened bottle of beer with the bottom of another bottle, the tapped bottle foams over, making a mess. This process is initiated by effects similar to tribonucleation.6 Furthermore, the “popping” sound heard when joints crack has been hypothesized to originate from the collapse of bubbles that are formed via tribonucleation.22,23
Figure 5. Kinetics of foam volume produced upon dropping one 1/2 in. sphere of varied composition from a height of 40 cm into a 500 mL Diet coke at room temperature. Spheres comprised of aluminum (crosses), zirconium oxide (open diamonds), steel (closed diamonds), copper (open circles), lead (closed circles). Data for aluminum, zirconium oxide, and copper collected by students in a nonmajors science course.
done to control for possible variations in surface roughness between spheres of different composition. Both aluminum (Figure 6, closed circles) and brass (Figure 6, open circles)
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CONCLUSION It is well-known that objects with rough surfaces, such as Mentos candies, can cause rapid degassing when dropped into carbonated beverages.7−16 The experiments presented here demonstrate that under certain conditions smooth objects can also generate considerable degassing when dropped into sodas. Smooth objects likely generate NISD through a process that involves tribonucleation, which is the mechanical rubbing together of solids in a liquid. Explorations of NISD produced by tribonucleation have the potential to provide opportunities for students of chemistry to investigate topics such as density and kinetics in unexpected ways. Several extensions of the experiments presented herein, especially use of the graduated bottle/tornado tube assembly, should prove useful in providing inquiry-based studies for chemical educators and their students to enjoy.
Figure 6. Effect of drop height on maximum foam volume achieved upon addition of one aluminum sphere (closed circles) or one brass sphere (open circles) into 500 mL of Diet Coke. Drop height was taken from the distance above the fluid level of freshly opened sodas. Lines are second order polynomial fits to the data.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00127. Notes on acquisition and use of 1/2 in. diameter spheres, student hand-out sheets, directions for making a graduated bottle/tornado tube assembly, and suggested demonstrations (PDF, DOCX)
spheres produced increasingly more foam as dropped from higher heights. Because dropping spheres from increased height causes the spheres to strike the bottom of the beverage container with greater force, these results further suggest that smooth spheres cause NISD through a process that involves tribonucleation.
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CONNECTIONS TO THE CLASSROOM The experiments described in this paper provide opportunities for several uses in chemistry classrooms. Thus, we have used these experiments in General Chemistry, Introductory Chemistry, a chemistry course for nursing majors, and a science course for nonmajors. Because of its strong connection to fountain heights, density is a topic that can naturally be explored using experiments similar to those reported here. We
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Thomas S. Kuntzleman: 0000-0002-2691-288X Dean J. Campbell: 0000-0002-2216-4642 D
DOI: 10.1021/acs.jchemed.8b00127 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
Notes
(22) Kawchuk, G. N.; Fryer, J.; Jaremko, J. L.; Zeng, H.; Rowe, L.; Thompson, R. Real-Time Visualization of Joint Cavitation. PLoS One 2015, 10, e0119470. (23) Chandran Suja, V.; Barakat, A. I. A Mathematical Model for the Sounds Produced by Knuckle Cracking. Sci. Rep. 2018, 8 (4600), 1−8.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank Dennis Downing who assisted and provided us with the use of an SEM to acquire images and also the many students who participated in the experiments described herein.
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REFERENCES
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DOI: 10.1021/acs.jchemed.8b00127 J. Chem. Educ. XXXX, XXX, XXX−XXX
