On the points of melting
On the points of melting
复制标题
关于熔点
作者：
D. Grier
虽然熔化和冻结是日常经验中最常见的两种结构相变，但令人惊讶的是，人们对它们的微观机制知之甚少。要完全理解这种基本的转变，部分困难在于，对铜或水等常规材料的实验，几乎不能告诉我们晶体转变为流体时在原子尺度上发生了什么。原子不仅体积小、运动快，而且在任何时候参与这一过程的原子也相对较少。即使是计算机模拟也很难适应将一块冰转化为一滩水所涉及的巨大样本大小和时间尺度范围。托马斯和莫尔描述的等离子体晶体ll 1]是一种新的模型系统，其中结构相变的一些奥秘可以在科学家可以用标准摄像机观看的舞台上展现出来。等离子体晶体的工作原理相当简单，即带相同电荷的粒子相互排斥。在托马斯和莫尔的实验中，粒子是微米级的聚合物球体，它们通过浸入被称为等离子体的电离气体中获得电荷。每个球体大约是人类头发直径的十分之一。因为它们的容器阻止它们独立地移动，所以球体采取了一种简化，在给定它们的温度和密度的情况下，使它们的总能量最小化。如果球之间的电荷介导的相互作用足够大，以克服周围气体的随机化，那么它们就会形成规则间隔的阵列，类似于晶体中原子的有序排列。如果气体的热能获胜，等离子体晶体就会熔化成一种动态的无序状态，让人联想到流体力学。在这个意义上，“尘埃等离子体”中的球体系综充当了经历相变的简单材料中原子的模型。然而，与原子不同的是，等离子体晶体球足够大，可以用肉眼看到，它们的运动可以通过计算机图像处理来跟踪。我们希望对这样一个实验上可访问的模型系统的详细观察将提供与最广泛的一类凝聚态系统密切相关的见解。近几十年来，通过对其他模型系统的研究，也有类似的希望。胶体悬浮在液体溶剂中的微观球体
Although melting and freezing are two of the most common structural phase transitions in everyday experience, surprisingly little is known regarding their microscopic mechanisms. Part of the di culty in crafting a complete understanding of such fundamental transformations is that experiments on conventional materials such as copper or water can tell us very little about what happens on the atomic scale as a crystal transforms itself into a uid. Not only are atoms small and rapidly moving, but comparatively few of them take part in the process at any time. Even computer simulations are hard-pressed to accomodate the enormous sample sizes and range of time scales involved in converting a chunk of ice into a puddle of water. The plasma crystal described by Thomas and Mor ll 1] is a new model system in which some of the mysteries of structural phase transitions can be played out on a stage which scientists can watch with standard video cameras.The plasma crystal works on the fairly straightforward principle that like-charged particles repel each other. The particles in Thomas and Mor ll's experiments are micrometer scale polymer spheres which acquire their charges through immersion in an ionized gas known as a plasma. Each sphere is roughly a tenth of the diameter of a human hair. Because their container prevents them from moving apart inde nitely, the spheres adopt a conguration which minimizes their total energy given their temperature and density. If the charge-mediated interaction among the spheres is great enough to overcome the randomizing bu eting of the surrounding gas, then they form into regularly spaced arrays which are analogous to the orderly ranks of atoms in crystals. If the thermal energy of the gas wins, the plasma crystal melts to a dynamic and disordered state reminiscent of a uid. This is the sense in which the ensemble of spheres in the\dusty plasma" serves as a model for atoms in simple materials undergoing phase transitions. Unlike atoms, however, plasma crystal spheres are large enough to see with the unaided eye and their motions can be tracked through computerized image processing. The hope is that detailed observations on such an experimentally accessible model system will provide insights germane to the widest class of condensed matter systems. Similar hopes have been pursued through investigations on other model systems in recent decades 2]. Microscopic spheres colloidally suspended in a uid solvent