Supercooling is a physical phenomenon where a liquid cools below its typical freezing point without solidifying. This allows substances, such as water, to remain liquid at temperatures where they would ordinarily freeze. Supercooled liquids demonstrate that freezing depends not only on temperature but also on other specific conditions.
The Science of Unfrozen Liquids
Freezing requires a starting point for ice crystals to form, a process known as nucleation. These “nucleation sites” are often microscopic impurities, dust particles, or minute irregularities on a container’s surface. Without these sites, liquid molecules struggle to arrange into a solid’s ordered structure, even below the normal freezing point.
When a liquid cools below its freezing point without solidifying, it enters a “metastable state.” In this state, the liquid is thermodynamically unstable but persists because it lacks the necessary trigger for crystallization. For instance, highly purified, undisturbed water can be supercooled to approximately -42°C before spontaneously freezing. This happens because, without external catalysts, water molecules must spontaneously align to form the initial crystal structure, which is less likely at higher sub-zero temperatures.
Supercooling in Nature and Technology
Supercooling occurs in various natural settings. Supercooled water droplets are common in clouds, particularly at higher altitudes, remaining liquid well below 0°C. When these droplets encounter a surface or an ice crystal, they freeze instantly, leading to phenomena like freezing rain or aircraft icing. This poses hazards, as sudden ice accumulation can affect infrastructure and aircraft safety.
Some organisms use supercooling as a survival mechanism in cold environments. Certain insects, fish, and plants produce natural antifreeze compounds, such as proteins or polyols, that inhibit ice crystal formation within their tissues. This allows their bodily fluids to remain liquid even when external temperatures are below freezing, preventing cellular damage from ice formation.
Supercooling also has practical applications. In food preservation, it extends the shelf life of perishable items like meat and produce by keeping them chilled below their freezing point without forming damaging ice crystals. This maintains the fresh quality and texture of food more effectively than traditional freezing. Supercooling is also explored in cryopreservation, a technique used to cool and store biological materials like organs, tissues, and cells at sub-zero temperatures without ice formation, which could otherwise cause structural damage.
Making Supercooled Liquids Freeze
A supercooled liquid can be induced to freeze with a small disturbance, triggering rapid solidification. One common way to initiate freezing is through agitation, such as shaking or tapping the container sharply. This physical disturbance provides the energy for liquid molecules to overcome the barrier to forming initial ice crystals.
Another method involves introducing a “seed crystal,” a small piece of ice, into the supercooled liquid. This seed acts as a template, providing an existing crystalline structure around which other liquid molecules can readily align and attach, initiating a chain reaction of freezing. Even a tiny impurity or a scratch on the container’s inner surface can serve as a nucleation site, providing the initial point for solidification. Once triggered, the supercooled liquid often freezes quickly, with a wave of ice spreading throughout. This rapid solidification releases latent heat, the energy stored in the liquid state, causing a temporary rise in the temperature of the newly formed ice to its normal freezing point.