José Manuel NievesJosé Manuel Nieves



At very cold temperatures, the Water shows very strange behavior. To begin with, and against all logic, when the expands instead of contracting (that’s why the ice floats). Cold water also turns out harder to compress than hotter. And to top it off, by freezing, its molecules can be organized in various ways. different ways.

It is difficult to find explanations for all this, and those that exist are subject to controversial and bitter scientific controversies. One of them, proposed almost three decades ago, is the idea that very cold water can exist in two different liquid forms, one less dense and structured than the other. In other words, there could be two kinds of water, and each of them would be a different liquid. Testing it in the laboratory is difficult, but a team of Italian and American researchers has just found strong evidence that it really could be. The work has just been published in the journal Science.

In his study, Pablo Debenedetti and Gül H. Zerze, from Princeton University, and Francesco Sciortico, from La Sapienza, in Rome, suggest that the “second critical water point” takes place at temperatures between -83 and -100 degrees and at an atmospheric pressure almost 2,000 times higher than there is at sea level. A critical point is a single temperature and pressure value at which two phases of matter become indistinguishable, and it occurs just before matter passes from one phase to another. Water, for example, has a well-known critical point when going from steam liquid.

“You can imagine our joy when we started to see the critical fluctuations behaving exactly as they were supposed to,” Sciortino explains. “Now I can sleep peacefully, because after 25 years, my original idea has been confirmed.”

So far, experiments using real water molecules to test for a second critical point of “supercooling“They have failed to provide unequivocal evidence of its existence. According to Debenedetti this is due in large part to the tendency of supercooled water to become ice.

For this reason, this time the researchers decided to use computer models. We can get an idea of ​​the difficulty of simulating the process if we think that, despite the tremendous power of today’s supercomputers, it took 18 full months of calculations to carry out the simulation.

In the simulations, and as temperatures dropped well beyond freezing, the density of the water began to fluctuate greatly. In the end, the scientists managed to detect the critical point they were looking for in two different computer models of water. In each model they subjected the water molecules to two different computational approaches, and both succeeded in finding the much desired second critical point for water.

As in the transition from the liquid phase to the vapor phase, the two phases of supercooled water occur because the shape of the water molecules can induce two different ways to join, or pack. Thus, in the liquid of lower densityFour molecules are grouped around a fifth central molecule in a geometric shape called a tetrahedron. In the higher-density liquid, however, a sixth molecule comes into play, which has the effect of increasing density.

In their article, the researchers write that “by probing the limits of what is currently computationally possible in this area, we provide clear evidence of the presence of a metastable critical point in the deeply supercooled region of the two water molecules.” .

Naturally, the finding will now have to be confirmed by other experiments “using even more precise and computationally more expensive means.”