The Phenomenon of Nucleation by Krzysztof Bahrynowski

The Phenomenon of Nucleation

1)   Introduction:

Nucleation is the first process that occurs in the formation of a crystal from a vapour, a liquid or a solution. During a crystallisation, a small number of atoms, molecules, or ions are arranged in a particular pattern of a crystalline solid upon which additional particles are deposited during the crystal growth.

But crystallisation is not the only kind of nucleation. Nucleation in liquids makes gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in vapour is also characterized by nucleation.

Most nucleation processes are physical rather than chemical (the famous example is that of Diet Coke® and Mentos®, which will be further explained).

2)   Explanation of the phenomenon:

We can distinguish two types of nucleation processes, heterogeneous or homogeneous. We will explain here the process of crystallisation.

a.       Homogeneous nucleation:

Here, a few particles come into correct juxtaposition while they randomly move through the bulk of the medium. There is no preferential site for the nucleation, and the nucleation occurs spontaneously and randomly. This process becomes more likely while the degree of supersaturation or supercooling increases. Glycerol for example, is well-known for a tendency towards to supercooling.

When the molecules are in solution, more often there are only solvent molecules around them. But sometimes, they can be in contact with other solute molecules. There exist attractive forces between them, and most of the time, when they meet they stay linked for a little while until other forces pull them apart. However, two molecules can stay together long enough to meet up with a third, and then a fourth, etc… solute molecule. At this point, if there are only a few molecules joined together, they break apart, but when there are a certain threshold number of solute molecules, the attracting force between the molecules becomes stronger than the other ones in the solution likely to separate them. We obtain then a pre-crystal, which forms a site of nucleation. Other solute molecules are attracted by this pre-crystal and join it and the crystal begins to grow.

b.   Heterogeneous nucleation:

It is the more common nucleation. During this process, the surface of a substance, such as a dust particle or a defect on a material or at the surface of a container, represents the centre on which the first atoms, molecules or ions of the crystal are properly oriented (that's why bubbles in soda pop or in boiling water usually form on the sides and bottom of the container). That is a nucleation site.

Almost the same thing happens as in homogeneous nucleation. A solute molecule encounters a surface, it adsorbs to it, and stays on it a certain time before other forces of the solution fall it out. Some solute molecules tend to adsorb and aggregate on the surface. This is at this moment that the pre-crystal is created, and the same process as described above happens.

3)   One famous example of nucleation: Diet Coke® and Mentos® eruption

a.       Introduction

The Diet Coke® and Mentos® experiment is all over Internet. You can see a lot of videos on YouTube® about this funny experiment. And if you type “mentos soda” on Google®, you will have more than 1.730.000 results (on the 14th of June 2012). At first glance, one might think that this is a chemical reaction between one ingredient of the candy and another of the soda. But this is not a chemical reaction, but a physical one.

b. Experimental procedure

Here, we will describe a very serious and developed experiment. In this experiment you will test if the number of dimples (nucleation sites) added to a Mentos® candy will change the height of exploding soda.

To realise it, you will need:

  • 3 rolls of Mentos® mint flavoured candies

  • 3x2 Litre bottles of Diet Coke®

  • Sharp, pointy metal skewer or nail

  • 2 index cards  

  • Video camera with tripod

  • Blue painter's tape

  • Tape measure

  • Eye protection (goggles or glasses)

Ø  First, you will need to organize your Mentos® candies for the experiment. Each pack of Mentos® comes with 14 candies inside, so if you eat 2 candies you will have enough left for three trials with four candies each (12 total).

Ø  Each package of Mentos® will be given a different number of dimples. The first package will have no dimples added. The second package will have one small dimple added to each side. Using a sharp, metal skewer or nail, dig into the center of the Mentos® candy to make a small dimple. Flip the candy over and put one on the other side. Repeat with all of the Mentos® in the second package. The third package will have five small dimples added to each side.

Ø  Next, you need to make your Mentos® cartridge to hold the Mentos® for you before you drop them into the soda bottle. Take one of the index cards and roll it into a tube, slightly larger than the diameter of a Mentos® candy. Tape the tube together on the side, and now you have a cartridge for holding your stacks of Mentos®. You will use the other index card to place beneath the tube of Mentos®, to keep the Mentos® from dropping into the soda bottle until you are ready.

Ø  Now, prepare your test site. At the base of an exterior wall with no windows, set one bottle of Diet Coke®. On the exterior wall, use a tape measure and the blue painter's tape to mark off the height from the top of the soda bottle in 10 centimetres.

Ø  Set up the video camera with a tripod, and make sure that the soda bottle and marked measurements are in view.

Ø  Remove the cap from the soda bottle and place the flat index card on top, covering up the hole.

Ø  Starting with the first package of Mentos® (the one with no dimples) add four Mentos® to your cartridge and put on your safety goggles.

Ø  Before each trial, speak into the camera and say, "This is trial number (1, 2, 3...) using Mentos® with (0, 1, or 5) dimples." This will help when you go back to analyze your results.

Ø  Place your full cartridge on top of the flat index card. When you are ready, quickly remove the flat index card by pulling it, releasing the Mentos® into the bottle. Step back without tipping the bottle over or disturbing the reaction. When the bottle stops spouting, stop recording.

Ø  Repeat last step two more times, for a total of three trials using four Mentos® candies each time. Repeat with the other two packages of Mentos®. Go in order so that you can keep track on your video recorder.

Ø  Now you are ready to watch your tape and analyze your data. As you watch the tape for each trial, use slow motion and pause the recording when the spout is at its maximum height. Then using the tape marks in the background, do your best to estimate the height of the spout. Do this for each trial and keep your data in a data table:

Number of Dimples

No Dimples

8 Dimples

40 Dimples

Trial 1 Height (m)

Trial 2 Height (m)

Trial 3 Height (m)

Total Height (m)

Average Height (m)

c. Explanation of the physical reaction:

The first thing which contributes to the reaction is something called a saturated solution. In this case, this is the soda. All the bubbles in a soda come from carbon dioxide gas that is dissolved into the soda solution. When the soda is inside the bottle, the gas is kept in solution by the pressurised conditions in the bottle. Diet Coke and Coke Zero tend to go a bit higher than regular soda, because they have a little more carbonation and the sweeteners help make the reaction a little bigger.

The second element which is involved in the reaction is the phenomenon of nucleation. If you look at a piece of Mentos® candy under a microscope, you will see tiny bumps coating the entire surface of the candy. Each one of them acts like a nucleation site, a place where nucleation can get started. The surface of a Mentos® is sprayed with over 40 microscopic layers of liquid sugar (gum arabic and gelatine). That makes it not only sweet but also covered with lots and lots of nucleation sites. The Mint kind is the best choice, because they have a matte finish (better for reaction), whereas the finish on the fruit Mentos® is a gloss finish. At each nucleation site, a bubble of carbon dioxide gas can form and escape the solution. Multiplying that by all the tiny bumps on a Mentos® and by the number of candy, you have a geyser. Indeed, since the Mentos® are also heavy enough to sink, they react with the soda all the way to the bottom, and as the sweet layers are dissolved under the action of the soda. The escaping bubbles quickly turn into raging foam, and the pressure builds up dramatically. All that pressure has got to go somewhere, and before you know it, you've got a big geyser happening.

In this experiment, you will see that, if you add some extra sites of nucleation to the candy (dimples), you will increase the height of the eruption. Another way to improve height of reaction is to freeze the candies. Indeed, gum arabic, like water, expands when frozen, which allows the reaction to happen at a quicker rate, because the candies become more porous.


Post scriptum:

The fountain produced will depend on the diameter of the neck of the container. Do this in a wide glass and the effervescence produced raises the soda only a or two  centimetres. But when you observe your video you will see that the soda bubbles collect below the curve of the bottleneck and when the CO2 bubbles reach a critical mass, the fountain erupts. So there are more than two variables effecting the phenomenon.

What is also interesting is that the carbon dioxide released could be used as a force as movement energy.