Frozen but not solid?

INTRODUCTION

One might be wondering how is that even possible. Well, in this article we are going to introduce the reader to a bewildering phenomenon which clears the confusion. The phenomenon is called supercooling. As the name suggests, it is cooling to a super-low temperature- a normal explanation that is surely going to pop up in the reader’s mind. However, supercooling involves something more than that; it is cooling of fluids below their normal freezing point without them solidifying. In this article we are going to explain how supercooling happens for water. If one looks at the phase diagram of water (see Fig.1), it seems almost thermodynamically unstable for such a process to occur. However, in specific circumstances the water reaches below it’s freezing pointwithout turning into ice.

This topic has never been given much importance in the high schools, however it has a lot of practical significance. This process is widely used in food industry in storing of items in a cold environment without any loss of their quality. An example includes storing of garlic at a very low temperature of -60 C without it getting frozen [1]. It is also observed in several natural processes like the slippery glare ice 1 formation; it is formed due to the freezing of the supercooled water [6] [8]. Supercooling also helps in the survival of some organisms. In regions of very low temperatures, plants have to somehow prevent nucleation2 of their essential fluids otherwise they might undergo lethal freezing. Some animals like the Polar Teletost fish also uses this phenomenon to survive in subfreezing temperature [2].

Supercooling was reported many decades ago first by F. Brown in 1916 to explain bursting of hot water pipes in household systems [3]. From then, scientists have been experimenting to precisely figure out on what parameters this phenomena depends on and how far can water be supercooled. There is another phenomenon closely related to supercooling of water named as Mpemba effect3 (described separately in the last section). Both of these phenomena can be studied by performing simple experiments at undergraduate laboratories.

HOW AND WHEN IT OCCURS ?

The occurrence of supercooling is quite uncommon and can be seen under certain unique conditions as already mentioned before. In case of water, supercooling can start under standard pressure and 00 C if proper conditions are aided until the temperature is less than a certain mark where homogeneous nucleation starts spontaneously.

What exactly is homogeneous nucleation? But before that let us see what nucleation is. In order to know a general explanation to supercooling, one has to know a simple fact that in any liquid, crystals form due to a process called nucleation. The term Nucleation is a process observed in case of liquids during phase change to solid in which either a foreign particle or a constituent particle of the liquid acts as a nucleus and the other constituent particles or molecules pile up around it to form crystals of the liquid. Nucleation is of two types [4]:

Homogeneous nucleation : Nucleation(in case of water) that is caused due to electrostatic attraction between polar water molecules is called homogeneous nucleation. In case of water, this is seen below -48.30 C.

Heterogeneous nucleation: Nucleation happens due to the presence of a nucleation site in this type of nucleation What are nucleation sites? Water or any fluid needs a central element (be it a dust particle or any sort of impurity present in the fluid) to help it in the process of crystallisation. The water molecules pile up around that to form crystals. This dust particle is precisely known as a nucleation site. The primary cause of supercooling of water is absence of nucleation sites in the fluid. Pure water, namely distilled water (or roughly, a water sample that has undergone reverse osmosis) is free of any nucleation site, thus can be easily supercooled. In the absence of nucleation sites, if thermodynamic conditions are maintained suitably and the medium is not much disturbed, then supercooling can be observed to quite an appreciable extent until homogeneous nucleation starts [4] [6]. In the next section, we would explain how supercooling can be performed in a simplified experiment. If an experiment is to be performed to observe supercooling then there are various factors that may affect the process. Some important factors among them are the initial temperature of water and type of the media where the fluid is kept in order to get cooled. How the initial temperature of water sample affects the process? It has been observed experimentally that cold water has more chances of showing supercooling as compared to hot water. This is because in hot water nucleating agents are more active as compared to cold water. Although this fact is not stated surely and needs a bold reasoning still now. [4] But this pretty much gives us a fair explanation to a strange effect observed in water called the Mpemba effect. However, if somehow the nucleating agents are eliminated due to high temperature in hot water, then the opposite might happen i.e. hot water may have more chances of getting supercooled as compared to cold water. [6]

Now comes the role of chilling medium. It is fairly obvious that the chilling medium should be in a very low temperature initially.

But, what exactly matters is the rate of change of temperature of the medium or in other words, how fast the medium comes to a normal temperature(same as that of its surroundings). This depends on few thermodynamic factors like the heat capacity of the constituent of the medium and on the latent heat if a phase change is involved i.e. if the constituent comes from solid to liquid state during the temperature change. Also, if an experiment is set up to observe supercooling then we may see that rate of cooling of the sample liquid also affects the supercooling to quite some extent. If the liquid is cooled slowly then there are more chances of observing supercooling as concluded experimentally. Rapid cooling involves a large change of temperature of the liquid in a short period of time that reduces the chances of supercooling.[6]

Computer simulation of a well studied water sample suggests that supercooled water under negative pressure does not exist in a single phase, rather it coexists in two distinct metastable liquidwater phases[5]. This opens up a plethora of topics for researchers to study on and consequently predict the behaviour patterns of water. However, explanation of the above mentioned fact is beyond the scope of this article.

AN EXPERIMENTAL SETUP TO STUDY SUPERCOOLING

Supercooling can be easily studied by using a simple apparatus as suggested by Kah-Chye Tan et al, 2015 .[6] Their apparatus is called an IFM (Investigate Freezing and Melting) as shown in Fig.2. The inner container is a thick aluminium test tube which is attached to the base of the outer container at the middle. The outer container is made up of plastic. The water sample to be experimented, is placed in the inner container and the chilling medium is placed in the outer container. Aluminium being a good conductor of heat will allow the flow of heat from water sample to the chilling medium. However, plastic will prevent any heat flow between the chilling medium and the environment. There are two short tubes in the plastic cover to prevent mixing of the water sample and chilling medium. Additionally, a temperature sensor is fixed to the system by using an extension rod. At first, the chilling medium is put into the IFM and then put into freezer for 24 hours. After the temperature of the chilling medium has cooled to that of the freezer, the apparatus is taken out and then the water sample is poured into it. Now, the apparatus can be used to cool water sample without a freezer. This simple experiment can be carried out using different water samples and also different chilling media to observe how supercooling depends on these factors. The initial temperature of the freezer and water sample can also be changed for different observations.

THE RELATION OF SUPERCOOLING TO MPEMBA EFFECT

In 1963, Erasto Bartholomeo Mpemba during his schooling times observed that an ice-cream mixture freezes faster when heated initially as compared to a cold mixture. Later in 1969, he along with one of his professors experimentally proved his observation of ice cream mixtures and published the results[7]. Therefore, this peculiar effect is named after him. The fact that hot water freezes faster than cold water may seem so unreal to any general person but it actually happens, not always but if provided with certain special conditions.

After this effect caught the attention of scientists, many came up with various possible explanations but none of them till now has been fixed as an exact solution to elucidate the effect. However, some explanations have influenced people like evaporation, hydrogen bonding in water and yes the subject of our article, supercooling. Here, only the relation of the two effects is mentioned and we will not go into the details of other reasons for Mpemba effect. Supercooling is one of the prominent explanations to the Mpemba effect. It gives a validation to this effect in a way that for a given temperature(under the freezing point of the liquid), the initially hot water might have turned into solid whereas cold water might actually be in a supercooled state, i.e. it is still in its liquid state. For any normal observer who is unaware of the temperatures of these two liquids can say that he found Mpemba effect true. This pretty much does justice to the explanation with respect to supercooling as we have already mentioned that there are more chances of observing supercooling in cold water as compared to hot water. However, this is not the only plausible reason for the effect to happen or rather not a complete justification as supercooling itself occurs under restricted surroundings and Mpemba effect has been observed in the absence of these surroundings. So for now, it will be safe to say that both the effects go hand in hand. Two of the effects are one single phenomenon looked from two different perspectives. In case of Mpemba effect we see which one among hot and cold water freezes faster, whereas in supercooling our focus is not on freezing but on it’s opposite i.e. to see which one doesn’t freeze.

Anyways, the more we dig deep in studying this queer fluid named water, the more it astounds us.

REFERENCES

[1] C. James, V. Seignemartin and S. J. James, The Freezing and Supercooling of Garlic (Allium sativum L.)," International Journal of Refrigeration, Vol. 32, No. 2, 2009, pp.253-260. http://dx.doi.org/10.1016/j.ijrefrig.2008.05.012

[2] A. DeVries, Biological Antifreeze Agents in Cold-waterFishes," Comparative Biochemistry and Physiology PartA: Physiology, Vol. 73, No. 4, 1982, pp. 627-640.dx.doi.org/10.1016/0300-9629%2882%2990270-5

[3] F. Brown, The Frequent Bursting of Hot Water Pipesin Household Plumbing Systems," Physical Review, Vol.8No.5, 1916, pp. 500-503. http://dx.doi.org/10.2307%2F1005602

[4] Amir Gholaminejad et al, "A Study of Water Supercooling", Journal of Electronics Cooling and Thermal Control,2013, 3, 1-6. http://dx.doi.org/10.4236/jectc.2013.31001

[5] C. Austen Angell, "Supercooled water two phases ?",News and Views, Nature Materials, Vol. 13, 2014.

[6] Kah-Chye Tan et al, "A study of the occurrence of supercooling of water", American Journal of Physics, 84, (2016). dx.doi.org/10.1119/1.4939792

[7] "Can hot water freeze faster than cold water?",Nov, 1998, Monwhea Jeng, Department of Physics, University of California https://math.ucr.edu/home/baez/physics/General/hot water.html#History

[8] V. F. Petrenko and R. W. Whitworth, "Physics of Ice",Oxford U. P., Oxford UK, 1999