The objective of this paper is to investigate water supercooling. Supercooling occurs when a liquid does not freeze although its temperature is below its freezing point. In general, supercooling is an unstable condition and occurs under special conditions. The parameters that influence supercooling stability and probability of occurrence include freezer temperature and water’s initial temperature. In this paper, it is shown that with a freezer temperature range of -3℃ to -8℃, supercooling is most likely to happen and is independent of the water’s initial temperature. Furthermore, as the freezer temperature decreases, the probability of nucleation increases, causing instant freezing. Finally, it is concluded that the Mpemba effect, in which initially hot water freezes faster than initially cold water, is due to the supercooling instability in initially hot water in which nucleation agents are more active.
By observing the water’s phase diagram, it seems impossible to have liquid water several degrees below its freezing point. When water temperature reaches 0˚C, (at atmospheric pressure) its temperature remains constant and phase transition process occurs. However, there are circumstances in which water temperature drops below its freezing point, but no phase transition happens while water remains in liquid phase. This condition is called supercooling.
Supercooling phenomenon is of particular interest in food industry. While in transport, foods like fruits and vegetables require storage in a cold environment where the temperature is several degrees below freezing point. However, freezing the food would reduce its quality by damaging food cells and changing its color. However, if one can store food in a subfreezing temperature without freezing, there would be no loss of quality. For instance, unpeeled garlic was stored in −6˚C for a week, without freezing [
Supercooling was first introduced by Brown in 1916 to explain why hot water pipes burst more often than cold water pipes [
In 1955, Mossop investigated freezing of supercooled water of high purity. In an experiment, water was supercooled down to −34.5˚C [
In 1977, Gilpin carried out numerous experiments on water supercooling and confirmed that hot tap water supercools more than cold water [
In 1995, Auerbach carried out extensive experiments in which he varied freezer temperature and water’s initial temperature before placing it in the freezer [
Despite many experimental studies on water supercooling, there is still uncertainty on the parameters that influence this phenomenon. The objective of the present study is to first confirm the existence of this phenomenon and then study the parameters influencing its occurrence and stability. Finally, we have explained how Mpemba phenomenon is related to supercooling.
Melting Point (MP)/Freezing Point (FP): Melting point of a solid is the temperature at which all solid crystals have disappeared while being heated slowly. Since it was believed that a solution would freeze when cooled to its MP, the freezing point of a solution was thought to be the same as its MP. However, there are a number of situations in which a solution does not freeze at its freezing point. For a solution to freeze, not only should the liquid’s temperature reach FP, but also there should be nucleation sites to help the molecular structure change from liquid to solid. If any of these are absent freezing will not occur [
Nucleation Temperature (NT): Nucleation temperature is the temperature at which the first ice crystals appear in a solution. It is also referred to as supercooling point (SCP) or crystallization temperature [
Nucleation Agent: In order for water to freeze, its molecular structure needs to change from liquid into ice. This change is possible only if water molecules find nucleation sites. Aggregates found in water or dissolved gasses in it can serve as these nucleation sites, which are referred to as nucleation agents.
Homogenous/Heterogeneous Nucleation: There are two types of nucleation: homogeneous and heterogeneous. Nucleation caused by electrostatic attraction between water polar molecules is referred to as homogenous nucleation. Since such attractions are weak, a large number of molecules need to be present to initiate nucleation. If nucleation happens with the aid of an extrinsic nucleator, it is referred to as heterogeneous nucleation [
Supercooling Capacity: Supercooling capacity is the difference between MP temperature and NT. It shows how much a solution has been supercooled.
To study water freezing, one needs to possess a cold chamber. Some researchers have used cold air in a commercial freezer. A major problem with this mechanism is that heat transfer rate from the sides of the water beaker (convection cooling) is different from its bottom (conduction heat transfer), which can affect supercooling.
In the present study, a cold liquid bath is used as the freezing compartment in order to solve the above problem. There were 30 liters of ethylene glycol (liquid) in the bath, which was cooled by evaporator coils of a vapor/compression refrigerator. To have an isotherm cold liquid, a circulator was placed in the ethylene glycol (
Since small distractions including compressor vibration would influence water supercooling, the compressor was turned off during the test period. However, the bath temperature did not change considerably due to the fact that the test period was short (about 10 minutes) and the size of bath was large (30 liters of ethylene glycol).
To study the effect of water’s initial temperature on supercooling, water with different initial temperatures was to be prepared. The initial temperature of water is defined as the temperature of water at the start of the experiment before placing it into the freezer. The heating mechanism must be such that other parameters of water, such as dissolved gases, remain almost constant for different initial temperatures to minimize their influence on supercooling. If the samples were heated directly to the desired temperature, then the amount of dissolved gases, specifically CO2, would change. The amount of dissolved gases is believed to influence water supercooling [7,10].
To overcome this problem, water was first heated to 95˚C, out of which 50 ml was cooled in the ambient temperature of 22˚C in a beaker. When water temperature reached the desired value, the beaker was placed in the freezer.
Next, the beaker containing water was drowned in the freezer such that its top surface was out of ethylene glycol making it lose heat homogenously (
An Omega HH147 data logger was used to record water temperature. An Omega5SRTC K type thermocouple was immersed in water and connected to the data logger.
Distilled water was used so that dissolved materials would not influence water supercooling. Each test was performed with 50 ml of distilled water in a beaker. Moreover, the experiments were conducted in an environment with an ambient temperature of 22˚C and a pressure of 663 mmHg. The experiments were performed with initial water temperature in the range of 20˚C to 90˚C and freezer temperature range of −4˚C to −12˚C.
The first objective of the experiments was to see whether supercooling occurs or not. In one of the experiments 500 ml of distilled water with initial temperature of 65˚C was placed in a freezer temperature of −8˚C. Water temperature “History” is shown in
To illustrate how nucleation can end supercooling state and initiate phase change, 1 ml of frost is added to the supercooled water in the previous experiment,
(a) (b) (c) (d)
Five different regions can be identified in
1): Cooling is Newtonian [
2): Temperature fluctuations are seen in this region. This is primarily due to density changes near 4˚C, which would cause side wall boundary layer to collapse [
3): At 0˚C the phase change process does not happen and water supercools until its temperature reaches −7˚C.
4): Nucleation is forced by dropping some frost in water beaker. Due to release of the latent heat of crystallization, water temperature rises.
5): Water becomes a mixture of ice and liquid.
Freezer temperature was varied in the range of −4˚C to −12˚C to study its effect on supercooling. The results are shown in
- For freezer temperature range of −4˚C, −6˚C and −8˚C, nucleation was not observed and water remained in supercooled condition for as long as five hours in one of the tests. This implies that there were no nucleation agents present to initiate freezing.
- For freezer temperature of −10˚C nucleation was random, but it was higher in the initially hot water, indicating higher activity of nucleation agents in initially hot water.
- For freezer temperature of −12˚C nucleation always happened and supercooling was rarely seen, indicating
that nucleation agents were active in both initially hot water and initially cold water.
Therefore, it can be deduced that in general, initially hot water has lower supercooling capacity than initially cold water. This behavior is also observed in a study of strawberry supercooling [
As mentioned before, for freezer temperature range of −4˚C to −8˚C supercooling occurs for a long time and is independent of the water’s initial temperature. This is due to the lack of nucleation sites, as discussed before. For colder freezer temperatures such as −10˚C and −12˚C, supercooling becomes unstable and nucleation happens. At −10˚C, the nucleation agents are more active in initially hot water than initially cold water. But at a freezer temperature of −12˚C, they are so active that they cause nucleation in both hot and cold water. Therefore, it can be deduced that in general, nucleation sites increase as freezer temperature is decreased and initial temperature of water is increased.
There are several possibilities to explain this behavior, specifically the difference between supercooling of initially hot water and initially cold water. Some believe that water’s cooling history play a role, specifically dissolved gasses [12,13]. Initially hot water has less dissolved gasses compared to initially cold water. However, this cannot explain this phenomenon since less dissolved gasses implies less nucleation sites. An acceptable explanation of this behavior is yet to be found.
The fact that in some specific conditions initially hot water freezes faster than initially cold water is referred to as the Mpemba effect. This phenomenon was first introduced to modern science in 1969 by Erasto B. Mpemba, a Tanzanian high school student [
Many theories have been proposed to explain this phenomenon. In 1969, Kell suggested that surface evaporation could explain the Mpemba effect [
In 1971, Deeson suggested that convection currents cause initially hot water to have a higher rate of heat transfer from its top surface [
Dissolved gases in water is also believed to have a role in the Mpemba effect, since initially hot water has less dissolved gases than initially cold water [12,13,17]. However, Auerbach reported that dissolved gases in water do not influence Mpemba effect [
If these theories are correct, they are still unable to explain the experiments in which initially hot water freezes much faster than initially cold water. In one of our experiments, initially cold water froze five hours after initially hot water froze. Noting that because initially hot water tends to nucleate sooner than initially cold water, suggests that supercooling instability of initially hot water plays an important role in this phenomenon. Specifically, when the nucleation starts in initially hot water, the initially cold water is still in the supercooling phase. The visual examination of the supercooled water may give the wrong impression that its temperature is above 0˚C since water is still in the liquid phase while the initially hot water appears completely frozen. This may result in a conclusion that initially hot water freezes faster than initially cold water.
Water supercooling was studied by varying water’s initial temperature and freezer temperature through numerous
experiments. It is concluded that supercooling is unstable at low freezer temperatures. Moreover, it is observed that initially hot water tends to nucleate and freeze sooner compared to initially cold water.
Furthermore, it is concluded that the Mpemba effect, in which initially hot water freezes faster than initially cold water, is due to the supercooling instability in initially hot water. Specifically, initially hot water nucleates faster than initially cold water and as a result, starts to freeze, while the initially cold water is still in the supercooled liquid phase.
We are in debt to Mohammad Samadi who helped us in conducting the experiments. We have to specially thank Behnood Gholami who proofread the manuscript and gave us valuable feedback. All of the experiments have been performed in Thermodynamics Lab of Mechanical Engineering Department of Amirkabir University of Technology.