We call everything “ice cold” but the exact meaning of this definition is actually questionable. The freezing point of water is one of the first things we learn about science in school: 0°C. Yet like so many things we learn in school, this subject is more complex than we were taught at the beginning. In fact, water has many freezing points and often requires temperatures well below 0°C to freeze. Now, a new record has been set for how cold the water can be while remaining liquid. The knowledge gained in this research process can have many applications, from organ transplantation to aviation.
Below 0°C, water still needs a nucleation site where it can freeze. Very pure, still water lacks nucleation sites and can be cooled to temperatures below 0°C without freezing. This may have been a popular example before for those who want to prove that the Earth is a complex place. However, even purity cannot prevent water from freezing if you cool it enough.
University of Houston’s Dr. Hadi Ghasemi explores the factors that determine when supercooled water will freeze. In the process, Ghasemi and his colleagues announced in Nature Communications that they had broken the record for coldest liquid water. This temperature was surprisingly recorded as -44°C. That is, 6°C below the previous record was reached.
The drops of water Gahsemi used to set the new record were too small to be seen and only a few nanometers in diameter. “Experimental investigation of the freezing temperature of a few nanometer-sized water droplet has been an unresolved challenge. Here, thanks to newly developed metrology, we were able to investigate the freezing of water droplets from the micron scale to the 2 nanometer scale,” says Gashemi. The problem, according to the article, is that tiny water droplets will either evaporate or clump together, making it difficult to study them individually.
“We found that if a water droplet is in contact with a soft interface, the freezing temperature can be significantly lower than on hard surfaces,” says Ghasemi.
The authors put droplets of various sizes in the oil-enclosed aluminum oxide membrane pores and reported large differences in freezing point between 10 and 2 nanometers.
Without the oil, the membrane walls acted as nucleation sites and the droplets were frozen at or near 0°C. However, the pressure from the oil and the lack of a hard surface kept the drops liquid.
Previous research has shown that it takes at least 275 molecules to form crystal ice from liquid water. Droplets significantly smaller than 2 nanometers in diameter lack the molecules to form ice crystals, and these droplets have been found to require temperatures of -183 to -158°C to nucleate, but it’s also controversial to consider something this small as a liquid.
Successful imitation could allow hospitals to freeze organs or control ice buildup on airplane wings instead of transplanting under time pressure. Even if we ourselves don’t cool water to such low temperatures, understanding how this occurs could also improve modeling of cloud formation and therefore weather and climate predictions. . .
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