Nayani Karthik, Fu Jinxin, Chang Rui, Park Jung Ok, Srinivasarao Mohan
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332.
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332.
Proc Natl Acad Sci U S A. 2017 Apr 11;114(15):3826-3831. doi: 10.1073/pnas.1614620114. Epub 2017 Mar 23.
Tactoids are nuclei of an orientationally ordered nematic phase that emerge upon cooling the isotropic phase. In addition to providing a natural setting for exploring chromonics under confinement, we show that tactoids can also serve as optical probes to delineate the role of temperature and concentration in the aggregation behavior of chromonics. For high concentrations, we observe the commonly reported elongated bipolar tactoids. As the concentration is lowered, breaking of achiral symmetry in the director configuration is observed with a predominance of twisted bipolar tactoids. On further reduction of concentration, a remarkable transformation of the director configuration occurs, wherein it conforms to a unique splay-minimizing configuration. Based on a simple model, we arrive at an interesting result that lower concentrations have longer aggregates at the same reduced temperature. Hence, the splay deformation that scales linearly with the aggregate length becomes prohibitive for lower concentrations and is relieved via twist and bend deformations in this unique configuration. Raman scattering measurements of the order parameters independently verify the trend in aggregate lengths and provide a physical picture of the nematic-biphasic transition.
类晶区是在冷却各向同性相时出现的取向有序向列相的核。除了为在受限条件下探索发色团提供自然环境外，我们还表明类晶区也可作为光学探针来描绘温度和浓度在发色团聚集行为中的作用。对于高浓度，我们观察到通常报道的细长双极类晶区。随着浓度降低，观察到指向矢构型中非手性对称性的破坏，其中扭曲双极类晶区占主导。随着浓度进一步降低，指向矢构型发生显著转变，其中它符合一种独特的使张开最小化的构型。基于一个简单模型，我们得出一个有趣的结果，即在相同的约化温度下，较低浓度具有更长的聚集体。因此，与聚集体长度成线性比例的张开变形对于较低浓度变得过高，并通过这种独特构型中的扭曲和弯曲变形得以缓解。序参量的拉曼散射测量独立验证了聚集体长度的趋势，并提供了向列双相转变的物理图像。
