Tanaka Hajime
Faraday Discuss. 2013;167:9-76. doi: 10.1039/c3fd00110e.
Liquids are often assumed to be homogeneous and isotropic at any lengthscale and translationally invariant. The standard liquid-state theory is constructed on the basis of this picture and thus basically described in terms of the two-body density correlation. This picture is certainly valid at rather high temperatures, where a liquid is in a highly disordered state. However, it may not necessarily be valid at low temperatures or for a system which has strong directional bonding. Indeed, there remain fundamental unsolved problems in liquid science, which are difficult to explain by such a theory. They include water's thermodynamic and kinetic anomalies, liquid-liquid transitions, liquid-glass transitions, and liquid-solid transitions. We argue that for the physical description of these phenomena it is crucial to take into account many-body (orientational) correlations, which have been overlooked in the conventional liquid-state theory. It is essential to recognise that a liquid can lower its free energy by local or mesoscopic ordering without breaking global symmetry. Since such ordering must involve at least a central particle and its neighbours, which are more than two particles, it is intrinsically a consequence of many-body correlations. Particularly important ordering is associated with local breakdown of rotational symmetry, i.e., bond orientational ordering. We emphasize that translational ordering is global whereas orientational ordering can be local. Because of the strong first-order nature of translational ordering, its growth in a liquid state is modest. Thus any structural ordering in a liquid should be associated primarily with orientational ordering and not with translational ordering. We show that bond orientational ordering indeed plays a significant role in all the above-mentioned phenomena at least for (quasi-)single-component liquids. In this Introductory Lecture, we discuss how these phenomena can be explained by such local or mesoscopic ordering in a unified manner.
通常认为液体在任何长度尺度下都是均匀且各向同性的，并且具有平移不变性。标准的液态理论是基于这一图景构建的，因此基本上是用两体密度关联来描述的。在相当高的温度下，液体处于高度无序状态，这一图景当然是有效的。然而，在低温下或对于具有强方向性键合的体系，它不一定有效。事实上，液体科学中仍然存在一些基本的未解决问题，很难用这样的理论来解释。这些问题包括水的热力学和动力学异常、液 - 液转变、液 - 玻璃转变以及液 - 固转变。我们认为，对于这些现象的物理描述，考虑多体（取向）关联至关重要，而这在传统液态理论中被忽视了。必须认识到，液体可以通过局部或介观有序化来降低其自由能，而不破坏全局对称性。由于这种有序化至少必须涉及一个中心粒子及其邻居，而邻居粒子不止两个，所以它本质上是多体关联的结果。特别重要的有序化与旋转对称性的局部破坏有关，即键取向有序化。我们强调，平移有序化是全局性的，而取向有序化可以是局部的。由于平移有序化具有很强的一级性质，它在液态中的增长是适度的。因此，液体中的任何结构有序化都应主要与取向有序化相关，而不是与平移有序化相关。我们表明，至少对于（准）单组分液体，键取向有序化在上述所有现象中确实起着重要作用。在这篇 introductory Lecture 中，我们将讨论如何用这种局部或介观有序化以统一的方式来解释这些现象。 （注：这里“Introductory Lecture”直译为“入门讲座”，放在语境中不太好理解，可能是特定课程或系列讲座的名称，保留英文未翻译更合适。）
