# Chiral Chromonics Confined in Spherical Geometries
## 4 Confinement of Nematic Chromonics and Native Chiral Chromonics in Microspheres  
Compared to the thermotropic liquid crystals, the chromonic liquid crystal droplets are less studied because it is difficult to confine them in a matrix and prevent both coalescence and water evaporation. In 2013, [PERSON] and his co-workers carried out detailed studies on the chromonic liquid crystal Sunset Yellow confined in the microspheres with planar anchoring conditions at the interface [40]. The microdroplets containing SSY in water in a percentage of 31% in weight were stabilized in a hexadecane matrix containing a non-ionic surfactant: sorbitran monooleate (Span 80). As mentioned above, the nematic director field in a typical bipolar configuration observed in a thermotropic nematic LC presents two boojums located at opposite poles of the drop. These are represented in Figure 10 by the black dots, while the yellow rods represent the director configuration. Instead, the authors observed a distorted chiral bipolar configuration in the chromonic nematic microspheres in which the director rotated in a helicoidal manner with the axis of the helix perpendicular to the boojums (Figure 10b). As underlined, in chromonics, the twist elastic constant is an order of magnitude smaller than the bend and splay constants. This explain why, when they are confined in curved geometries, a large reflection symmetry breaking is observed, i.e., the strong splay and bend deformations of the director are relaxed trough the twist deformation. Their findings are extremely interesting since [PERSON] et al. demonstrated that the confinement in a curved geometry highlights a spontaneous formation in the nematic phase of the chiral patterns from the achiral building blocks. There are few examples in the literature of chiral chromonics that were confined in a spherical geometry. The most remarkable one is the work published by [PERSON] and his co-workers in 2016 [41]. The authors presented an extensive investigation on cellulose  
Figure 10: Schematic diagrams of (**a**) the bipolar configuration and (**b**) the twisted bipolar configuration. Reprinted with permission from ref. [40]. Copyright 2014, the National Academy of Sciences. Figure 9: Diagram of the observed textures at various droplet sizes as a function of the relative helicity parameter N\({}_{0}\). Reprinted with permission from ref. [37]. Copyright 2005, the Royal Society of Chemistry. nanocrystals (CNCs) confined in micrometric spheres. CNCs are stiff rod-like particles consisting of cellulose chain segments that present an almost perfect crystalline structure and have an average length of 183 nm and a diameter of 23 nm. CNCs are intrinsically chiral and form at the proper concentrations in a chiral liquid crystalline phase [42]. The confinement was obtained using an aqueous 7 wt% CNC suspension following a precise protocol, together with the exploitation of the microfluidic techniques. The immiscible matrix was a mixture of a fluorinated oil (F-oil) containing 1.0 wt% of a surfactant triblock copolymer perfluoropolyether and poly(ethylene oxide-co-propylene oxide). The authors demonstrated that, in the presence of a strong planar anchoring, the textures obtained showed spherical layers arranged in a concentric manner with a defect at the center of the sphere and a radial disclination line. The textures were size dependent, in particular for the droplets with a radius in the range between 40 \(\upmu\)m and 115 \(\upmu\)m, a Frank-Pryce-like texture was obtained. Interestingly, the core of the drop was in the isotropic phase, while the rest of the drop was in the chiral phase, showing a phase separation between the core and the rest of the droplet. The optical microscope image in polarized light (Figure 11) of a droplet exhibiting Frank-Pryce texture showed the characteristic structures of the cholesteric phase, a Maltese cross and a shell with alternating light and dark concentric rings corresponding to the surfaces at a constant refractive index.