We call things "cold as ice", but how cold is that really? The freezing point of water is one of the things we all learn before we have left school: 0 ºC, or 32 ºF if you’re an American. Yet like so many things we learn in school, it’s more complex than we are initially taught. In fact, water has many freezing points, often requiring temperatures far below 0 ºC (32 ºF) to freeze. Now the record for how cold water can be while remaining liquid has been broken. The knowledge gained in the process could have applications from organ transplants to aeronautics. 

Below 0 ºC (32 ºF), water still needs a nucleation site around which it can freeze. Very pure, still water lacks nucleation sites and can be cooled to temperatures substantially below 0 ºC (32 ºF) without it freezing, a popular demonstration for those keen to prove the world is a complex place. Get things cold enough, however, and even purity won’t stop water freezing. 

Dr Hadi Ghasemi of the University of Houston has been exploring the factors that determine when supercooled water freezes. In the process, Ghasemi and colleagues have announced in Nature Communications that they have broken the record for the coldest liquid water – an astonishing -44 ºC (-47 ºF). This is 6 ºC (11 ºF) below the previous record. 

The drops of water Gahsemi used to set the new record were far too small to see, just a few nanometers across.  

“Experimental probing of freezing temperature of few nanometer water droplets has been an unresolved challenge. Here, through newly developed metrologies, we have been able to probe freezing of water droplets from micron scale down to 2-nanometer scale,” Ghasemi said in a statement. The problem, the paper notes, is that tiny water droplets are likely to either evaporate or clump together, making them hard to study individually.  

“We found that if a water droplet is in contact with a soft interface, freezing temperature could be significantly lower than hard surfaces,” Ghasemi said. 

The authors put droplets of various sizes in aluminum oxide membrane pores, surrounded by oil, and report major differences in freezing point between 10 and 2 nanometers (0.00000004-0.0000004 inches). 

Without the oil, the membrane walls acted as nucleation sites, and the droplets froze at 0 ºC (32 ºF) or close to it. However, the combination of pressure from the oil, and the lack of a hard surface kept the drops liquid. 

Previous research has shown it takes at least 275 molecules to form crystalline ice from liquid water. Droplets substantially smaller than 2 nanometers in diameter lack the molecules to form ice crystals and have been found to need temperatures of -183 to -158 ºC (-297 to -252 ºF) to nucleate, but something that small isn’t necessarily considered liquid. 

There are real-world consequences to the flexibility of water’s freezing point. Faced with the challenge of surviving cold winters some animals make their own anti-freeze chemicals, or burn food to keep warm. Others, however, allow themselves to freeze but do it on their own terms so their cells don’t rupture, as would normally occur when ice crystals form inside. Wood frogs are masters of this art, but the way they do this is not fully understood.  

Successful mimicry might allow hospitals to freeze organs, instead of having to perform transplants under time pressure, or control ice formation on aircraft wings. Even when we are not cooling water to such low temperatures ourselves, understanding how it occurs could improve modeling of cloud formation, and therefore weather and climate predictions.