Nature2014-06-23 4:35 PM

真空中迅速冷却的水滴 Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature

论文摘要 

在真空中迅速冷却的水滴在变成冰之前用来自一个飞秒X-射线激光器的强脉冲对其进行探测,这为Jonas Sellberg等人提供了关于在所谓“无人地带”中的深度超冷液体相(在其中水的异常性质大大增强,但冰的形成速度太快,无法用正常测定方法来测定)的独特结构信息。在第二篇论文中,Jeremy Palmer等人通过先进模拟方法来研究这一体系中的水,发现了在两种结构不同的液体之间的一个相变,这有可能解释水的很多异常。

Abstract 

Water has a number of anomalous physical properties, and some of these become drastically enhanced on supercooling below the freezing point. Particular interest has focused on thermodynamic response functions that can be described using a normal component and an anomalous component that seems to diverge at about 228 kelvin (refs 1,2,3 ). This has prompted debate about conflicting theories that aim to explain many of the anomalous thermodynamic properties of water. One popular theory attributes the divergence to a phase transition between two forms of liquid water occurring in the ‘no man’s land’ that lies below the homogeneous ice nucleation temperature (TH) at approximately 232 kelvin13 and above about 160 kelvin, and where rapid ice crystallization has prevented any measurements of the bulk liquid phase. In fact, the reliable determination of the structure of liquid water typically requires temperatures above about 250 kelvin. Water crystallization has been inhibited by using nanoconfinement, nanodroplets and association with biomolecules to give liquid samples at temperatures below TH, but such measurements rely on nanoscopic volumes of water where the interaction with the confining surfaces makes the relevance to bulk water unclear. Here we demonstrate that femtosecond X-ray laser pulses can be used to probe the structure of liquid water in micrometre-sized droplets that have been evaporatively cooled below TH. We find experimental evidence for the existence of metastable bulk liquid water down to temperatures of 227(-1,+2) kelvin in the previously largely unexplored no man’s land. We observe a continuous and accelerating increase in structural ordering on supercooling to approximately 229 kelvin, where the number of droplets containing ice crystals increases rapidly. But a few droplets remain liquid for about a millisecond even at this temperature. The hope now is that these observations and our detailed structural data will help identify those theories that best describe and explain the behaviour of water.

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