Water is a fascinating material. Its composition is simple—one oxygen and two hydrogen atoms—but its chemistry and physics are extremely complex and exhibit 75 documented anomalies. Although these anomalies and their molecular origin are not completely understood, we know that hydrogen bonds play key roles in all of the phases of water. Moreover, there is experimental evidence that the polymorphism of the ice structure extends into the liquid phase and is associated with a liquid-liquid coexistence line. This is currently a topic of great interest in water research because there are indications that the end point of the coexistence line corresponds to a second critical point inside the supercooled liquid regime. We examine the recent progress in understanding water anomalies and the liquid-liquid phase transition hypothesis, including the results of recent experimental work and molecular simulations of both bulk and confined water. We examine experimental results that test whether the behavior of liquid water is consistent with the “liquid polymorphism” hypothesis that liquid water can exist in two distinct phases of differing densities. We also examine recent research on the anomalies of nanoconfined water and, in particular, on water in biological environments. We find that the concept of liquid polymorphism can also describe the properties of other liquids that have two characteristic length scales.

水是一种迷人的物质。它的组成很简单-一个氧和两个氢原子-但它的化学和物理极为复杂，并显示了75个已记录的异常。尽管这些异常现象及其分子起源尚不完全清楚，但我们知道氢键在水的所有相中都起着关键作用。而且，有实验证据表明，冰结构的多态性延伸到液相中并且与液-液共存线有关。当前，这是水研究中的一个热门话题，因为有迹象表明，共存管线的终点对应于过冷液体状态下的第二个临界点。我们研究了在了解水异常和液-液相转变假设方面的最新进展，包括近期实验工作的结果以及散装和承压水的分子模拟。我们检查了测试结果，这些结果测试了液态水的行为是否与“液态多态性”假说相符，假说液态水可以存在于密度不同的两个不同阶段。我们还将检查有关纳米受限水异常的最新研究，尤其是有关生物环境中水的异常的研究。我们发现，液体多态性的概念还可以描述具有两个特征长度尺度的其他液体的特性。我们检查了测试结果，这些结果测试了液态水的行为是否与“液态多态性”假说相符，假说液态水可以存在于密度不同的两个不同阶段。我们还将检查有关纳米受限水异常的最新研究，尤其是有关生物环境中水的异常的研究。我们发现，液体多态性的概念还可以描述具有两个特征长度尺度的其他液体的特性。我们检查了测试结果，这些结果测试了液态水的行为是否与“液态多态性”假说相符，假说液态水可以存在于密度不同的两个不同阶段。我们还将检查有关纳米受限水异常的最新研究，尤其是有关生物环境中水的异常的研究。我们发现，液体多态性的概念还可以描述具有两个特征长度尺度的其他液体的特性。