# Chiral Chromonics Confined in Spherical Geometries
## 1 Introduction  
This review is focused on a peculiar state of matter: chromonic liquid crystals. These materials are columnar liquid crystals that self-assemble in water. One typical example is DNA. There are several reasons to study them. First of all, the molecules are biocompatible and their mesophases could be applied in sensor and food safety fields. However, the self-aggregation mechanism is not fully understood and the behavior of these compounds when confined among two glasses is unpredictable. Furthermore, apart from native cholesteric chromonics, the chirality can be induced in the nematic compounds doping them with a suitable chiral amino acid. However, the connection between the chirality at the molecular scale and the macroscopic chiral structure of the liquid crystalline phases is not yet understood [1]. In recent years, researchers demonstrated that chromonics assume a spontaneous torque when they are confined to cylindrical or spherical geometries. Most of the studies focused on tactoids or cylindrical capillaries [2; 3; 4; 5]. Tactoids are spindle-like droplets in which an ordered nematic phase appears when cooling the isotropic phase. The nematic phase coexists with the isotropic phase in a broad range of concentrations and temperatures. The director configuration and the shape of the tactoids are determined by the balance between the elastic volume and the surface energies [2]. Since chromonics have a small twist elastic constant with respect to the splay and bend constants, the nematic director field is twisted. Since the system is not natively chiral, the number of tactoids with the left and right twist is approximately the same. In 2015, [PERSON] and [PERSON] [5] demonstrated that a small amount of R or S chiral molecules can amplify the chirality, turning chiral tactoids into either right- or left-twisted structures. The chirality amplification is demonstrated by an increased optical activity of these structures. In the last twenty years, there has been an increasing interest in thermotropic chiral liquid crystals confined in curved geometries. Several investigations have been carriedout allowing researchers to theoretically predict and experimentally observe a plethora of topological configurations [6; 7; 8; 9]. In particular, the optical properties of chiral nematic microspheres have been exploited in several application fields. For example, chiral microdroplets can be used as tunable microlasers. Microdroplets containing a dye-doped cholesteric liquid crystal exhibit omni-directional lasing from the center of the drop when they are optically pumped, with the lasing wavelength that depends on the pitch of the cholesteric. [10; 11]. Additionally, it is possible to create band edge lasers using dye-doped chiral nematic liquid crystals emulsified within a polymer matrix and deposited onto a rigid and flexible substrate [12] or encapsulated in free-standing films [13]. CLC droplets can be used not only for laser application but also for sensing. [PERSON] et al. demonstrated their use as biosensors [14]. [PERSON] et al. investigated the possibility of using an emulsion of dye-doped chiral liquid crystals microdroplets dispersed in a glycerol matrix as a sensor for the presence of nitric acid molecules [15]. Finally, in 2016, [PERSON] et al. proposed an anti-counterfeiting system based on the optical properties of a cholesteric liquid crystal emulsion [16]. The different chiral microspheres and microshells in random positions when illuminated by visible light, owing to the cross talk and selective reflection phenomena, exhibited a peculiar optical pattern that was irreproducible and could not be tampered with. In this perspective, the possibility of confining or encapsulating the chromonics materials in the microspheres represents an opportunity for the creation of biocompatible devices to be exploited, expanding the aforementioned applications into the biological and pharmaceutical fields. This type of confinement has been achieved in chromonics using tactoids encapsulated in a polymeric matrix.