Super Cool Supercooled Water

 by Bruce Mao

I am always obsessed by the phenomenon below:

Figure 1 Supercooled water. Source: https://j.gifs.com/KjbNd9.gif. 

You might have figured out what happens in the picture: When the person bangs the bottle, the liquid inside it solidifies. It’s interesting, isn’t it? Because we don’t normally see this phenomenon in daily life. And it might be attempting to suggest that the liquid inside the bottle is very unusual to most of us, or that this is ‘magic’ or ‘unreal’. But in fact, the liquid is just WATER and hence the solid is just ice.

When we bang a bottle of water of room temperature, it never becomes ice. Then, what on earth is going on with the water there? Well, people call it supercooled water. As its name suggests, the water is ‘super cool’—Its temperature is below 0 ℃. What people usually know is that water turns into ice at 0 ℃, and we call this temperature the ‘freezing point’. However, sometimes, liquid doesn’t freeze below its freezing point.

To explain this, we need to think about how the freezing happens. Molecules making liquid roll over each other and move randomly inside the container. (Figure 2) When freezing happens, those molecules are no longer able to move randomly or freely. Each of them can only vibrate about a fixed point. (Figure 3)

Figure 2 Molecules of a liquid. Source: https://i.gifer.com/7SEb.gif. 

Figure 3 Molecules of a solid. Source: https://i.gifer.com/6JJ.gif. 

For the molecules to ‘calm down’ and be vibrating about fixed points instead of moving randomly around, they need a place to settle down i.e., a nucleus onto which the solid ‘grow’ is required. When water is about to turn into ice, some water molecules first form small solid lumps. (nucleation) From there, a large volume of ice can grow. We call a small lump a nucleation site. In the example of the supercooled water above, the temperature of water is only a few Celsius degrees below zero, at which nucleation sites are hard to form. However, if we give it a bang, some nucleation sites can form and therefore the whole bottle of water turns into ice. [1] [2]

This also happens in aircraft contrails. (Figure 4) Contrails are the white trails we see produced by aircraft. They are primarily ice crystals. [3] The engine exhaust is mainly composed of water vapour and other products of combustion. When it is expelled out of the engine, they meet the very cold environment around them. Also, the concentration of water vapour near the engine is high. Therefore, those molecules tend to come together and form liquid or solid, i.e., water or ice. Other molecules, such as sulfur compounds, act as nucleation sites, helping the growth of water or ice. [3]

Figure 4 Contrails. Source: https://www.britannica.com/science/vapor-trail#/media/1/623212/116787.

Interesting demonstration

https://www.youtube.com/watch?v=ph8xusY3GTM&t=145s

References

[1] Wikipedia (2022), Nucleation [online] Last accessed 19 October 2022: https://en.wikipedia.org/wiki/Nucleation

[2] Veritasium (2011), Supercooled Water – Explained! [online] Last accessed 19 October 2022: https://www.youtube.com/watch?v=ph8xusY3GTM&t=145s

[3] Wikipedia (2022), Contrail [online] Last accessed 19 October 2022: https://en.wikipedia.org/wiki/Contrail