Supercooled water is liquid water that remains unfrozen below its normal freezing point of 0 °C (32 °F). It stays liquid because freezing requires more than just cold temperatures; water molecules need a trigger to organize into the rigid crystal lattice of ice. Without that trigger, water can stay liquid far below zero, down to roughly −38 °C in laboratory conditions before it freezes spontaneously. The phenomenon is surprisingly common in nature, drives dangerous icing on aircraft, and sits at the center of one of the most debated questions in physics: whether deeply supercooled water can split into two distinct liquid forms.
Why Water Doesn’t Always Freeze at Zero
Freezing is not an automatic switch that flips the instant the thermometer hits 0 °C. It is a two-step process. First, a tiny cluster of water molecules has to arrange themselves into an ice-like structure large enough to be stable. This initial seed is called a nucleus, and the process of forming it is called nucleation. Second, that nucleus has to grow outward as neighboring molecules lock onto its surface. If the first step never happens, you never get ice, even if the water is well below freezing.
In everyday life, nucleation almost always happens quickly because tap water, rainwater, and puddles contain microscopic impurities: dust, mineral grains, scratches on the container wall, even bacteria. These impurities give water molecules a surface to cling to, making it much easier for an ice crystal to get started. Researchers call this heterogeneous nucleation, and it is the reason your ice-cube tray works on schedule.
Remove those impurities, though, and things change dramatically. In very clean water held in a smooth container, there is nothing to help the first crystal form. Water molecules are constantly jostling around, forming fleeting ice-like clusters that fall apart almost instantly. As the temperature drops further below zero, those clusters become more likely to survive long enough to grow, but it can take a surprisingly long time. This is the supercooled state: thermodynamically, the water “wants” to be ice, but it is kinetically stuck as a liquid because nucleation hasn’t happened yet.
The moment a nucleus does form, or you introduce a disturbance like a vibration, a scratch, or a speck of dust, the supercooled water freezes extremely fast. Videos of this instant crystallization are striking: you can watch a bottle of water turn solid in seconds once the process starts. The freezing releases latent heat, which temporarily warms the mixture back toward 0 °C as the first rush of ice forms, a phase called recalescence.
The Two-Liquid Mystery Inside Deeply Supercooled Water
Water is already famously weird. It expands when it freezes, it is densest at 4 °C rather than at its freezing point, and its heat capacity and compressibility behave in ways that set it apart from almost every other liquid. These anomalies become more extreme as water is supercooled, and for decades researchers have suspected that the explanation lies hidden in a region of the phase diagram that is almost impossible to study experimentally: the zone below about −38 °C where water ordinarily crystallizes too quickly to observe as a liquid.
The leading hypothesis is that supercooled water can actually separate into two distinct liquid phases: a low-density liquid (LDL) with an open, ice-like molecular arrangement, and a high-density liquid (HDL) where molecules pack more tightly. These two forms would be connected by a liquid-liquid phase transition, complete with its own critical point, in much the same way that liquid water and steam are connected by the familiar boiling-point phase transition. If this critical point exists, its influence on the surrounding thermodynamic landscape would explain why water’s anomalies sharpen as it cools.
For years this remained theoretical, because experiments in that deeply supercooled zone were almost impossible. Recently, though, several independent lines of evidence have converged. X-ray laser experiments, which can probe water on timescales faster than crystallization, have observed behaviors at atmospheric pressure consistent with a liquid-liquid critical point, including maxima in thermodynamic response functions around 230 K (about −43 °C).1Journal of Non-Crystalline Solids: X. Origin of the anomalous properties in supercooled water based on experimental probing inside “no-man’s land” Separate experiments that rapidly heat low-density amorphous ice have directly observed the coexistence of high-density and low-density liquid regions, with distinct scattering signatures for each phase, before crystallization takes over.2Nature Communications. Liquid-liquid phase separation in supercooled water from ultrafast heating of low-density amorphous ice
Computational work has reinforced these experimental hints. First-principles molecular simulations, using models trained directly on quantum-mechanical calculations rather than fitted approximations, have demonstrated a first-order liquid-liquid transition and located a critical point in their model of water.3PubMed. Liquid-Liquid Transition in Water from First Principles And simulations of supercooled water nanodroplets have shown that as droplet size shrinks, internal pressure can push the droplet from the low-density phase into the high-density phase, producing core regions that still behave like bulk water even at the nanoscale.4PubMed. Liquid-Liquid Phase Transition in Simulated Supercooled Water Nanodroplets
None of this is fully settled. Directly measuring properties of bulk liquid water at −50 °C or below, at equilibrium and without crystallization, is still beyond what experiments can cleanly achieve. But the convergence of ultrafast experiments, amorphous-ice studies, and first-principles simulations has shifted the consensus significantly: most researchers in the field now treat the liquid-liquid critical point as likely rather than speculative.
Supercooled Water in the Atmosphere
If you have ever looked at a cloud on a cold day, you were probably looking at supercooled water. Cloud droplets are tiny, often just ten to twenty micrometers across, and their small size and relative purity mean they can stay liquid well below 0 °C. Mixed-phase clouds containing both supercooled liquid droplets and ice crystals are common at altitudes where temperatures range from 0 °C down to about −38 °C. Observations of persistent single-layer mixed-phase clouds have shown that supercooled droplets can coexist with ice for more than a day, even as ice crystals continuously nucleate and fall out of the cloud as precipitation.5Quarterly Journal of the Royal Meteorological Society. The formation of ice in a long‐lived supercooled layer cloud
The persistence of these supercooled droplets matters because it influences how much sunlight clouds reflect, how precipitation forms, and how weather models handle cloud microphysics. Below about −38 °C, homogeneous nucleation becomes essentially unavoidable, and cloud droplets freeze on their own. Surface crystallization of the droplets themselves may explain why very little supercooled water is observed in the atmosphere near that threshold.6PubMed Central. Surface crystallization of supercooled water in clouds
Supercooled cloud droplets also pose a serious hazard for aviation. When an aircraft flies through a cloud full of supercooled water, the droplets impact the fuselage and wings and freeze on contact. Smaller droplets tend to freeze into a thin, rough coating called rime ice. Larger supercooled droplets, called supercooled large droplets (SLDs), are more dangerous: they can spread across the surface before freezing completely, forming unusual ice shapes like ridges and runback ice in places the aircraft’s de-icing systems were not designed to protect.7International Journal of Heat and Mass Transfer. Impact freezing modes of supercooled droplets determined by both nucleation and icing evolution Understanding exactly how supercooled droplets freeze upon impact is an active area of research that feeds directly into anti-icing engineering.
How Living Things Exploit Supercooling
Nature has been dealing with supercooled water for a lot longer than physicists have. Organisms that live through harsh winters have evolved remarkably sophisticated strategies that either exploit or prevent supercooling, depending on whether their survival plan is to freeze in a controlled way or to avoid freezing entirely.
Many overwintering insects take the avoidance route. They purge their bodies of anything that could serve as an ice nucleus: food particles, dust, certain gut bacteria. With those nucleation sites removed, their body fluids can supercool to roughly −20 °C. On top of that, they accumulate polyols and sugars that depress the freezing point further, and some produce antifreeze proteins that stabilize the supercooled state even if a tiny ice crystal does manage to form.8PubMed. Physiology of cold tolerance in insects The result is that the insect’s tissues stay liquid at temperatures that would freeze most water solid.
Antifreeze proteins (AFPs) are worth a closer look because they work in a fundamentally different way from chemical antifreezes like road salt. Rather than simply lowering the freezing point of the solution, AFPs bind directly to the surface of any ice crystal that begins to form. Once bound, they block additional water molecules from attaching, effectively preventing the crystal from growing. Experiments using microfluidic devices have shown that this binding is practically irreversible and that surface-bound AFPs are sufficient to stop ice growth even after all the free protein is washed away from the surrounding solution.9PubMed Central. Microfluidic experiments reveal that antifreeze proteins bound to ice crystals suffice to prevent their growth Different classes of AFP achieve this through different strategies: some block growth on the fast-growing faces of an ice crystal, while others target the slower-growing basal plane.10PubMed Central. Blocking rapid ice crystal growth through nonbasal plane adsorption of antifreeze proteins AFPs have evolved independently in fish, insects, plants, fungi, and bacteria, making them one of the best examples of convergent evolution in biochemistry.11PubMed Central. New insights into ice growth and melting modifications by antifreeze proteins
Trees use a version of supercooling too. Bark, cambium, and leaf cells typically tolerate winter by allowing ice to form outside the cell, which draws water out and dehydrates the cell in a controlled way. But the xylem parenchyma cells deep inside woody tissue take a different approach: they supercool, keeping their intracellular water liquid even as temperatures drop well below zero.12PubMed. Mechanism of Overwintering in Trees Studies of woody xylem have shown that when this deep supercooled water does eventually freeze, it does so in many small independent events, cell by cell or in tiny groups of cells, over a temperature span of as much as 20 °C. Ice grows very slowly out of these events, and water moves sluggishly between frozen and unfrozen cells, which limits the damage.13PubMed Central. Units of freezing of deep supercooled water in woody xylem
On the other side of the equation, some organisms actively promote ice formation. The bacterium Pseudomonas syringae produces ice-nucleating proteins (INPs) on its outer membrane that are among the most efficient ice nucleators known, triggering freezing at temperatures near 0 °C.14PubMed Central. The influence of Pseudomonas syringae on water freezing and ice melting This is not accidental. When these bacteria live on plant leaves, triggering ice formation damages the leaf tissue and gives the bacteria access to the nutrients inside. INPs from P. syringae are also used commercially in snow-making machines and have been proposed as tools in atmospheric research because of their ability to nucleate ice so efficiently.
Preserving Organs and Food
The ability to keep biological tissue in a liquid state below zero, slowing metabolism without the destructive effects of ice crystal formation, has obvious medical appeal. Standard organ preservation relies on cold storage at about 4 °C, and the clock starts ticking as soon as an organ leaves the donor. For human livers, an improved supercooling protocol has demonstrated that storage at −4 °C can extend the viable preservation window by about 27 hours compared to conventional methods. The technique works by minimizing sites where ice could nucleate and preconditioning the organ with protective agents during machine perfusion.15PubMed Central. Supercooling extends preservation time of human livers Viability before and after supercooling was unchanged, and the livers tolerated the stress of simulated transplantation.
Research is now pushing into more complex tissues. Preliminary work on rat and pig limbs has explored supercooling for vascularized composite allografts (things like hands and arms for transplantation), where the mix of muscle, bone, skin, and blood vessels makes preservation far more challenging than for a single organ. Rat hindlimbs supercooled at −1 °C for 72 hours showed better bone viability and vascular preservation compared to standard cold storage, though bone marrow cells did not survive beyond 12 hours in either method.16PubMed Central. Supercooling: a promising technique for prolonged preservation in solid organ transplantation, and early perspectives in vascularized composite allografts Scaling up to pig limbs with longer supercooling times and protective agent cocktails has shown mild post-reperfusion injury, better than standard cold storage but still far from clinical readiness. Muscle tissue, which is highly sensitive to both oxygen deprivation and osmotic shock, remains the main bottleneck.
Food preservation is a simpler but commercially promising application. By storing produce at subfreezing temperatures in sealed, liquid-filled chambers (a technique called isochoric supercooling), researchers have kept whole pomegranates and fresh-cut pomegranate arils in good condition for over a month, maintaining both texture and chemical quality better than conventional refrigeration.17Postharvest Biology and Technology. Isochoric freezing and isochoric supercooling as innovative postharvest technologies for pomegranate preservation The key is that the food stays in the liquid phase, so there is no ice damage to cell walls and no mushy texture upon thawing. The approach is being tested on other fruits and vegetables, with the constant-volume (isochoric) chamber design preventing the pressure drops that would normally trigger nucleation.
Engineering Surfaces That Shed Supercooled Drops
Aircraft icing, wind turbine icing, and frozen power lines all start the same way: supercooled water droplets hit a surface and freeze on contact. Engineering surfaces that can prevent this freezing or shed ice once it forms is one of the most active practical applications of supercooling research.
Superhydrophobic surfaces, which repel water using a combination of chemical coatings and microscale texture, were early candidates for anti-icing duty. But supercooled water is more treacherous than room-temperature water. When a supercooled droplet hits a textured surface at speed, the impact forces the liquid into the surface’s tiny grooves and pits, dramatically increasing the contact area. That larger contact area raises the probability that ice will nucleate somewhere, and once nucleation starts, the droplet pins in place instead of bouncing off. The adhesion at low temperatures comes from this ice nucleation process, not from the water simply becoming more viscous.18International Journal of Heat and Mass Transfer. Supercooled water droplet impact on superhydrophobic surfaces with various roughness and temperature Counterintuitively, smooth superhydrophobic surfaces with nanoscale roughness perform better against high-speed supercooled droplets than heavily textured ones, because the finer features give the liquid less room to infiltrate.
Newer work has explored gradient polymer coatings deposited using chemical vapor techniques. These coatings showed that the combination of low wettability and moderate roughness can cause supercooled droplets to rebound and roll off even under high wind speeds, without increasing the freezing probability upon impact.19Advanced Materials Interfaces. Study of Supercooled Water Drop Impact on Icephobic Gradient Polymer Coatings Studies of what happens during the freezing process itself have identified two mechanisms by which surface repellency fails and have demonstrated that rationally designed textures can promote ice self-expulsion, where the expansion of water as it freezes actually pops the ice droplet off the surface.20PubMed Central. Freezing-induced wetting transitions on superhydrophobic surfaces Getting surfaces that work reliably in real conditions, with dirt, wear, and varying impact angles, remains the main engineering challenge.
Supercooled and Glassy Water on Other Worlds
Water’s tendency to supercool or vitrify (form a glass rather than crystallize) is not confined to Earth. On Mars, perchlorate salts in the soil dissolve into water to form brines with extremely low freezing points. Under Martian surface conditions, these brines may not crystallize at all, instead passing through a supercooled state into a glassy solid as temperatures drop. Laboratory measurements of perchlorate brines relevant to Mars suggest that vitrification, the formation of a disordered solid without ice crystals, may be a common outcome at Martian cooling rates.21PubMed Central. Effect of Concentration, Cooling, and Warming Rates on Glass Transition Temperatures for NaClO4, Ca(ClO4)2, and Mg(ClO4)2 Brines with Relevance to Mars and Other Cold Bodies
The icy moons of the outer solar system also appear to host water in metastable states. Observations indicate that Europa’s surface is primarily amorphous ice, Ganymede has a mix of amorphous and crystalline ice, and Enceladus shows both forms as well. Several mechanisms could produce these glassy states: the salt content of subsurface oceans, rapid cooling during cryovolcanic eruptions, or bombardment by high-energy particles that disrupt crystal structure. The implication is that supercooling and vitrification are not laboratory curiosities but widespread planetary processes. For astrobiologists, this matters because the physical state of water, whether it is crystalline ice, a glassy solid, or a metastable supercooled liquid, affects whether chemical reactions and biological processes could persist in cold environments beyond Earth.