Advanced #8

Liquid water is easily supercooled but difficult to vitrify

Water can remain liquid far below 0 degrees Celsius, yet forming amorphous ice is difficult.

Scientific Explanation

Water does not necessarily freeze at 0 degrees Celsius. In the absence of nucleation sites — tiny impurities or surface defects where the first ice crystals can form — liquid water can be cooled well below its freezing point without crystallizing. This state is known as supercooling.

In the laboratory, pure water in small droplets can be supercooled to approximately minus 41 degrees Celsius before spontaneous nucleation (homogeneous nucleation) sets in and the water freezes almost instantaneously. This temperature is called the homogeneous nucleation temperature and represents a practical lower limit for supercooling.

What makes water particularly unusual is the contrast between easy supercooling and difficult vitrification (glass formation). Many substances can be relatively easily brought into a glassy state by cooling them fast enough. For water, this is extraordinarily difficult: the cooling rate would need to reach approximately 10 to the power of 6 kelvins per second — a million times faster than for many other liquids. The reason lies in water’s strong tendency to form crystalline structures, driven by the tetrahedral arrangement of hydrogen bonds.

Amorphous ice — glassy, non-crystalline ice — can nevertheless be produced in the laboratory, for example by vapor deposition onto extremely cold surfaces or through high-pressure methods. The different forms of amorphous ice (LDA, HDA, VHDA) are of great interest to water research because they may represent the frozen counterparts of the hypothetical two liquid phases.

Supercooling Curve of Water Cooling curve showing how water temperature drops below 0 degrees Celsius (supercooling) before rapid crystallization occurs, at which point the temperature jumps back to 0 degrees C and remains constant during freezing. Time Temperature 20 °C 0 °C -20 °C Liquid cooling Supercooled! Nucleation Freezing plateau Solid Supercooling and Crystallization of Water
Cooling curve with supercooling: the temperature drops below the freezing point before crystallization drives it back to 0 degrees Celsius.

Everyday Relevance

Supercooling is more common in daily life than one might think. In the atmosphere, supercooled water droplets exist in clouds at temperatures down to minus 38 degrees Celsius. When these droplets strike cold surfaces, they freeze instantly — a phenomenon known as freezing rain or glaze ice, one of the most hazardous winter road conditions.

In medicine, supercooling plays an important role as well. Cryopreservation of cells, tissues, and organs aims to freeze water without crystal formation, since ice crystals destroy cell membranes. The difficulty of vitrifying water remains one of the greatest obstacles to the long-term preservation of whole organs.