Flash freezing

In physics and chemistry, Flash freezing is a phenomenon occurring naturally in nature, but also used by people in everyday life in the food industry and forecasting in the field of meteorology.

The results are also of importance in atmospheric science, as they may improve the climate model of the formation of ice clouds in upper troposphere, which effectively scatter incoming solar radiation and prevent earth from becoming overheated by the sun.

Flash freezing is closely related to classical nucleation theory, which gives good results for many materials, phenomena and theories.

Flash freezing refers to the process in various industries whereby objects are frozen in a few hours by subjecting them to cryogenic temperatures, or in direct contact with liquid nitrogen at 196 degrees below zero Celsius.

Now, it is shown how cold water can get before it absolutely must freeze: 48 degrees below zero Celsius.

Freezing water is a central issue for climate, geology and life. On earth, ice and snow cover 10 percent of the land and up to half of the northern hemisphere in winter. Polar ice caps reflect up to 90 percent of the sun’s incoming radiation. But you need to know how water droplets freeze, how much water in the atmosphere is in the liquid or crystal state, at what temperature it freezes, whether it crystallizes from within or from the surface.

The freezing of nanoscale water or silicon liquid drops is initiated at a number of different distances from the centre of the droplet, providing new insights on a long-standing dispute in the field of material and chemical physics.

If you put water in the freezer to make ice, you trigger a dynamic phase transition. It matters how fast you cool the system: If you cool water below its freezing point slowly, you’ll get a big, pretty-looking ice crystal. If you flash freeze it, you’ll get a poly-crystalline solid.

There are phenomena like supercooling, in which you cool the water below its freezing point, but the water remains liquid, if there are too few defects to seed crystallization. You therefore observe a delay until the water adjust to the new, below-freezing temperature. Supercooled liquid water must become ice at minus 48 C (minus 55 F) not just because of the extreme cold, but because the molecular structure of water changes physically to form tetrahedron shapes, with each water molecule loosely bonded to four others. This suggest the structural change from liquid to "intermediate ice". The crystallization of ice from supercooled water is generally initiated by a process called nucleation. Because of the speed and size of nucleation, which occurs within nanoseconds and nanometers.

The surface environment does not play a decisive role in the formation of ice and snow. The density fluctuations inside drops result in that the possible freezing regions cover the middle and the surface regions. The freezing from the surface or from within may be random. However, in the strange world of water, tiny amounts of liquid water theoretically still are present, even as temperatures go below minus 48 C (minus 55 F) and almost all the water has turned solid, either into crystalline ice or amorphous water. Below minus 48 C (minus 55 F), ice is crystallizing too fast for any property of the remaining liquid to be measured.

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Wikipedia