Kil Kye Hyoung, Yethiraj Arun, Kim Jun Soo
Department of Chemistry and Nanoscience, Ewha Womans University, Seoul 03760, Republic of Korea.
Department of Chemistry, University of Wisconsin-Madison, Wisconsin 53706, USA.
Phys Rev E. 2020 Mar;101(3-1):032705. doi: 10.1103/PhysRevE.101.032705.
The effect of confinement on the behavior of liquid crystals is interesting from a fundamental and practical standpoint. In this work, we report Monte Carlo simulations of hard rods in an array of hard nanoposts, where the surface-to-surface separations between nanoposts are comparable to or less than the length of hard rods. This particular system shows promise as a means of generating large-scale organization of the nematic liquid by introducing an entropic external field set by the alignment of nanoposts. The simulations show that nematic ordering of hard rods is enhanced in the nanopost arrays compared with that in bulk, in the sense that the nematic order is significant even at low concentrations at which hard rods remain isotropic in bulk, and the enhancement becomes more significant as the passage width between two nearest nanoposts decreases. An analysis of local distribution of hard-rod orientations at low concentrations with weak nematic ordering reveals that hard rods are preferentially aligned along nanoposts in the narrowing regions between two curved surfaces of nearest nanoposts; hard rods are less ordered in the passages and in the centers of interpost spaces. It is concluded that at low concentrations the confinement in a dense array of nanoposts induces the localized nematic order first in the narrowing regions and, as the concentration further increases, the nematic order spreads over the whole region. The formation of a well-ordered phase at low concentrations of hard rods in a dense array of nanoposts can provide a new route to the low-concentration preparation of nematic liquid crystals that can be used as anisotropic dispersion media.
从基础和实际的角度来看，受限环境对液晶行为的影响很有趣。在这项工作中，我们报告了硬棒在硬纳米柱阵列中的蒙特卡罗模拟，其中纳米柱之间的表面间距与硬棒长度相当或小于硬棒长度。通过引入由纳米柱排列设定的熵外场，这个特殊的系统有望成为一种生成向列相液晶大规模有序排列的方法。模拟结果表明，与本体相比，硬棒在纳米柱阵列中的向列有序性增强，也就是说，即使在低浓度下，硬棒在本体中仍保持各向同性，但向列序依然显著，并且随着两个相邻纳米柱之间通道宽度的减小，这种增强变得更加显著。对低浓度下具有弱向列序的硬棒取向局部分布的分析表明，硬棒优先沿着最近纳米柱两个曲面之间变窄区域的纳米柱排列；在通道和柱间空间中心，硬棒的有序性较差。可以得出结论，在低浓度下，密集排列的纳米柱所产生的限制首先在变窄区域诱导局部向列序，随着浓度进一步增加，向列序扩展到整个区域。在密集排列的纳米柱中低浓度硬棒形成有序相，可为制备用作各向异性分散介质的向列相液晶提供一条低浓度制备的新途径。
