Amazing stuff! The many wonders of water!
"Despite being the most familiar liquid, water is weird. It breaks many of the usual rules that govern the liquid state. In 1992 a team of researchers suggested why that is. Perhaps, they said, there are two types of liquid water, which become distinct only at temperatures well below freezing point, where it’s all but impossible to keep water liquid. Researchers have sought evidence for this bold conjecture ever since – and now an international team claims to have found it.
Water is weird. It has over 60 properties that mark it as rather different to other liquids, such as high surface tension, high boiling point and low compressibility. Is it because liquid water is better thought of as two different liquids? ...
used ultrafast laser pulses to rapidly melt ice at temperatures and pressures close to those at which the two deeply supercooled liquid phases of water are thought to exist. They then used x-ray scattering to see a signature of the two liquids and the liquid–liquid phase transition between them – an abrupt (first-order) transition that, like the transition between a normal liquid and gas phase, ends in a critical point where the two phases become indistinguishable. ..."
"Using x-ray lasers, researchers at Stockholm University have been able to determine the existence of a critical point in supercooled water at around -63 °C and 1000 atmosphere. Ordinary water at higher temperatures and lower pressures is strongly affected by the presence of this critical point, causing the origin of its strange properties. ..."
From the editor's summary and abstract:
"Editor’s summary
Polyamorphism and the resulting liquid-liquid critical point (LLCP) in supercooled water are intriguing phenomena in condensed-matter science. Rapid spontaneous ice formation that the water can undergo when probed makes finding the LLCP extremely challenging experimentally.
Previously, evidence for polyamorphism relied on indirect signs such as extrapolating unusual physical properties, the presence of two amorphous ice forms, and liquid-liquid transitions in simulations.
Experiments presented by You et al. with isochoric heating of high- and low-density amorphous ices using infrared ultrafast laser pulses followed by x-ray scattering at time scales before ice formation have directly and convincingly demonstrated two liquid states near a critical point in supercooled water ...
Abstract
The search for the liquid-liquid critical point in supercooled water is challenging owing to rapid crystallization. We studied supercooled water at timescales before ice formation by heating high- and low-density amorphous ices using infrared ultrafast laser pulses, followed by x-ray scattering.
By varying the pump laser fluence, we accessed liquid states straddling the predicted critical point. We observed a crossover from a discontinuous to a continuous transition at which broad and slow structural variations occurred, consistent with critical fluctuations and slowing down.
We also observed a rapid increase in the heat capacity indicating a critical divergence at 210 ± 8 K coincident with enhanced density fluctuations. These results suggest that our experiments have directly probed the vicinity of a critical point in supercooled water."
Experimental Discovery of a New Critical Point in Water (original news release)
Experimental evidence of a liquid-liquid critical point in supercooled water (no public access)
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