John Bettina S, Juhlin Carol, Escobedo Fernando A
School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853-5201, USA.
J Chem Phys. 2008 Jan 28;128(4):044909. doi: 10.1063/1.2819091.
The phase behavior of suspensions of colloidal hard tetragonal parallelepipeds ("TPs") (also known as rectangular nanorods or nanobars) was studied by using Monte Carlo simulations to gain a detailed understanding of the effect of flat-faceted particles on inducing regular local packing and long range structural order. A TP particle has orthogonal sides with lengths a, b, and c, such that a=b and its aspect ratio is r=c/a. The phase diagram for such perfect TPs was mapped out for particle aspect ratios ranging from 0.125 to 5.0. Equation of state curves, order parameters, particle distribution functions, and snapshots were used to analyze the resulting phases. Given the athermal nature of the systems studied, it is the interplay of purely entropic forces that drives phase transitions amongst the structures observed that include crystal, columnar, smectic, parquet, and isotropic phases. In the parquet phase that occurs for 0.54<r< or =3.2, for example, the particles possess some translational entropy (mobility) but reduced orientational entropy; particles arrange in stacks oriented perpendicular to one another, so that all particle axes are aligned along three common directors. Multicanonical-type simulations were used to study in more detail the isotropic-parquet phase transition. Both similarities and differences were identified between the results for these perfect TPs and those unveiled in our previous study of approximate (polybead) TPs.
通过蒙特卡罗模拟研究了胶体硬四方平行六面体（“TPs”，也称为矩形纳米棒或纳米条）悬浮液的相行为，以深入了解扁平颗粒对诱导规则局部堆积和长程结构有序的影响。一个TP颗粒具有长度为a、b和c的正交边，使得a = b，其纵横比为r = c/a。针对纵横比范围从0.125到5.0的此类完美TPs绘制了相图。使用状态方程曲线、序参量、粒子分布函数和快照来分析所得相。鉴于所研究系统的无热性质，正是纯熵力的相互作用驱动了所观察到的结构之间的相变，这些结构包括晶体相、柱状相、近晶相、拼花相和各向同性相。例如，在纵横比0.54 < r ≤ 3.2时出现的拼花相中，粒子具有一些平移熵（迁移率）但取向熵降低；粒子排列成相互垂直的堆叠，使得所有粒子轴沿着三个公共方向排列。使用多正则型模拟更详细地研究了各向同性 - 拼花相转变。确定了这些完美TPs的结果与我们之前对近似（多珠）TPs研究中揭示的结果之间的异同。
