When water gets too cold, the molecules can assemble into larger structures while still remaining liquid. This can happen in two ways, leading to the separation of liquids of very different densities. New research sheds light on the shapes of these supermolecules and what it says about the nature of each liquid.
The idea of two liquid phases for water well below freezing was proposed more than 30 years ago, but was unproven at the time. The enormous amount of computer time used two years ago allowed modeling to support the idea.
But the details of how water organizes itself during these phases (unlike water itself, which must be extremely pure to prevent freezing) were still not entirely clear. Now, a new article in Nature Physics provides some answers.
Considering that water consists of only two elements and its molecules consist of only three atoms, it may be surprising that the result is so complex. The fact that it is denser in the liquid phase than solid and allows ice to float is almost unbelievable since the behavior of water is so contradictory to most of the remaining matter if it were not for us to encounter it almost daily.
Things get even more complicated when you cool the water below freezing before it freezes. The fact that this can be done (unless the water has impurities or the roughness of the container) makes for great demonstration videos, but what’s really going on in “superchilled water” at the molecular level as temperatures get colder isn’t quite clear yet.
Scientists have observed that supercooled water can form high- and low-density liquid phases. These are different from heavy water, which is produced by varying the number of hydrogen atoms in the water, such as deuterium. The molecular composition of water remains the same, but its density depends on how the molecules arrange themselves.
According to the article, the high-density form of supercooled water can be seen as the molecules themselves entangling together like a clover knot or the linked loops — the Hopf connection — beloved by amateur magicians.
“By creating nodes and links, the system can simultaneously minimize its volume and maximize the number of links in the network,” the article says. Meanwhile, as previously suspected, the low-density liquid phase, the gap in the middle, contains rings of unentangled water molecules, which contributes to how light this phase can be.
“This understanding has given us a completely new perspective on what has now been a 30-year research problem and hopefully will be just the beginning,” said Andreas Neophytou, a PhD student at the University of Birmingham.
Unfortunately for now we only need to use computer models for these measurements and for now we seem far from a real experiment. Yet breakthroughs in this field are heading towards an exciting time when we can learn much more about the interesting and complex nature of water.
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