各向异性系统的计算机模拟研究。十八。圆柱形和球形颗粒致线虫混合物的再入相分离

R. Hashim, G. R. Luckhurst, S. Romano
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引用次数: 8

摘要

在向列相液晶中加入由准球形分子组成的溶质可以降低向列相各向同性转变温度。在过渡阶段还形成了由共存的向列相和各向同性相组成的双相体制。此外,汉弗莱斯和卢克赫斯特提出的这种混合物的分子场理论预测,在一个狭窄的组合物范围内,向列相之后会出现一个重新进入的双相区域。这种不寻常的再入相分离在真正的线虫源混合物中没有观察到,可能是因为系统在再入相形成之前就冻结了。本文报道了用计算机模拟蒙特卡罗技术研究了由圆柱形和球形颗粒形成的模型线虫混合物的双相结构的观察结果。粒子被限制在一个简单的立方晶格的位置,但仍然保持它们的旋转自由;因此,混合物不能在传统意义上冻结。热容的温度变化和溶质-溶质径向分布函数揭示了预测的向再入双相体系的转变。并确定了内能和二级取向参数随温度的变化规律。汉弗莱斯-卢克赫斯特理论的预测结果与模拟结果在定性上是一致的。
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Computer simulation studies of anisotropic systems. XVIII. Re-entrant phase separation in nematogenic mixtures of cylindrical and spherical particles
Addition of a solute composed of quasi-spherical molecules to a nematic liquid crystal is known to depress the nematic–isotropic transition temperature. A biphasic régime, consisting of coexisting nematic and isotropic phases, is also created at the transition. A molecular field theory of such mixtures, developed by Humphries and Luckhurst, predicts, in addition, the appearance of a re-entrant biphasic region following the nematic phase for a narrow range of compositions. This unusual re-entrant phase separation has not been observed for real nematogenic mixtures, presumably because the system freezes before the re-entrant phases can be formed. Here we report the observation of this biphasic régime for a model nematogenic mixture, formed from cylindrical and spherical particles, which was studied using the Monte Carlo technique of computer simulation. The particles are confined to the sites of a simple cubic lattice but still retain their rotational freedom; in consequence the mixture is unable to freeze in the conventional sense. The temperature variation of the heat capacity and the solute–solute radial distribution function reveal the predicted transition to the re-entrant biphasic régime. The internal energy and the second-rank orientational order parameter were also determined as a function of temperature. The predictions of the Humphries–Luckhurst theory are found to be in good qualitative accord with the results of the simulation.
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