Kim Jeongmin, Sung Bong June
Department of Chemistry and Research Institute for Basic Science, Sogang University, Seoul 121-742, South Korea.
J Chem Phys. 2014 Jul 7;141(1):014502. doi: 10.1063/1.4885035.
Two dimensional (2D) colloids show interesting phase and dynamic behaviors. In 2D, there is another intermediate phase, called hexatic, between isotropic liquid and solid phases. 2D colloids also show strongly correlated dynamic behaviors in hexatic and solid phases. We perform molecular dynamics simulations for 2D colloids and illustrate how the local structure and dynamics of colloids near phase transitions are reflected in the spatial correlations and dynamics of voids. Colloids are modeled as hard discs and a void is defined as a tangent circle (a pore) to three nearest hard discs. The variation in pore diameters represents the degree of disorder in voids and decreases sharply with the area fraction (ϕ) of colloids after a hexagonal structural motif of colloids becomes significant and the freezing transition begins at ϕ ≈ 0.7. The growth of ordered domains of colloids near the phase transition is captured in the spatial correlation functions of pores. We also investigate the topological hopping probability and the topological lifetime of colloids in different topological states, and find that the stability of different topological states should be related to the size variation of local pores: colloids in six-fold states are surrounded by the most ordered and smallest pores with the longest topological lifetime. The topological lifetime of six-fold states increases by about 50 times as ϕ increases from liquid to hexatic to solid phases. We also compare four characteristic times in order to understand the slow and unique dynamics of two dimensional colloids: a caging time (τ(c)), a topological lifetime (τ(top)), a pore lifetime (τ(p)), and a translational relaxation time (τ(α)).
二维（2D）胶体展现出有趣的相行为和动力学行为。在二维体系中，在各向同性液相和固相之间存在另一种中间相，称为六方相。二维胶体在六方相和固相中也表现出强关联的动力学行为。我们对二维胶体进行了分子动力学模拟，并阐明了相变附近胶体的局部结构和动力学如何反映在空隙的空间相关性和动力学中。胶体被建模为硬圆盘，空隙被定义为与三个最近邻硬圆盘相切的圆（一个孔隙）。孔隙直径的变化代表空隙中的无序程度，在胶体的六边形结构图案变得显著且在体积分数（ϕ）约为0.7时开始发生冻结转变后，孔隙直径随胶体的面积分数（ϕ）急剧减小。相变附近胶体有序域的生长在孔隙的空间关联函数中得以体现。我们还研究了不同拓扑状态下胶体的拓扑跳跃概率和拓扑寿命，发现不同拓扑状态的稳定性应与局部孔隙的尺寸变化有关：处于六重态的胶体被最有序且最小的孔隙所包围，具有最长的拓扑寿命。随着ϕ从液相增加到六方相再到固相，六重态的拓扑寿命增加约50倍。我们还比较了四个特征时间，以便理解二维胶体缓慢且独特的动力学：一个囚禁时间（τ(c)）、一个拓扑寿命（τ(top)）、一个孔隙寿命（τ(p)）和一个平移弛豫时间（τ(α)）。
