Microfluidic encapsulation of cholesteric liquid crystals for photonic applications

Abstract

Cholesteric liquid crystals (CLCs) are nematic liquid crystals whose molecular orientation is periodically rotated by a chiral dopant. As the helical nanostructure has the spatial modulation of refractive index, CLCs have photonic stop band along the helical axis. The wavelength for the stop band is easily controllable by external stimuli, such as magnetic and electric fields, light, and temperature as the CLC molecules are highly mobile, enabling the use of the CLCs in various optical applications. However, the CLCs are fluidic, which restricts the ease of processing and structural stability. To overcome the limitations while maintaining the stop band tunability, the fluidic CLCs have been encapsulated by a solid membrane utilizing emulsion templates. Here, I designed a stable 3-dimensional CLC microcapsules which were generated thorough microfluidic approaches. In chapter 1, I discuss optical property of CLCs and the influence of external stimuli and molecular alignment on the property. Afterward, I describe various methods for shell formation on the surface of CLC drops in bulk emulsification processes. Finally, I introduce a microfluidic technology to generate emulsion drops in a highly controlled manner for encapsulation of cholesteric liquid crystals. In chapter 2, I report a microfluidic approach to encapsulate CLCs with robust hydrogel membrane. With capillary microfluidic devices, monodisperse oil-in-water-in-oil (O/W/O) double-emulsion drops were generated to have innermost oil of CLCs and aqueous shell of photo-polymerizable hydrogel precursors. Upon UV irradiation, the gel precursors were cross-linked in the water shell, thereby enclosing the CLC core with a hydrogel membrane. The microcapsules were stable even in the air. Also, structural color of CLC microcapsules can response to temperature change. In chapter 3, I report reconfigurable microcapsules containing CLCs with planar alignment. With a glass-capillary microfluidic device with precisely controllable wettability, I prepared oil-in-waterin-oil-in-water (O/W/O/W) triple-emulsion drops with an ultrathin inner water shell through single-step emulsification. The triple emulsion consisted of an innermost CLC core, an aqueous alignment shell, a photocurable oil shell, and a continuous water phase. The helical axes of CLC in the core have radial orientation along the interface with the inner water shell, which was further encapsulated by an elastic polymer membrane through photopolymerization of the outer shell. Owing to the radial orientation of the helical axes, the resultant microcapsules exhibit omnidirectional structural colors and photonic cross-communication between the microcapsules. Moreover, the photonic microcapsules can be elastically deformed while the planar alignment is maintained, rendering both the optical properties and the capsule shape highly reconfigurable. In chapter 4, I design a CLC resonator in a capsule format to simultaneously achieve high air-stability, wavelength- and intensity tunability, and lasing-direction controllability. The capsule resonators have a triple-layered structure which comprises a CLC core, an ultrathin alignment shell, and a thick elastic solid shell. The capsules were microfluidically created to have uniform size and composition by using oil-in-water-in-oil-in-water (O/W/O/W) triple-emulsion drops as a template. The silicone elastomer shell formed by photocross-linking provides the shape reconfigurability and the high mechanical stability of the capsule structure. Therefore, the CLC capsules enable a stable omnidirectional lasing in an air environment. At the same time, the fluidic CLC core provides wavelength tunability along with an external stimulus of temperature. As the elastic shell allows reversible deformation of the capsules from spherical to nonspherical shapes while maintaining a planar alignment, the lasing direction can be adjusted from the omnidirectional to bi- or multidirectional. Consequently, one can control the intensity of laser on the target location by adjusting the degree and shape of the deformation. In chapter 5, I report core-shell microcapsules which have dual structural color. The core-shell microcapsules comprised a left handed CLC core, thin aqueous layer, and right-handed CLC shell. To minimize optical cross-talk between CLCs, the core CLC was rendered right-handed (R-CLC), whereas the shell CLC was left-handed (S-CLC). The aqueous layer separated the distinct CLCs in the core and shell, and helped the LC molecules to align parallel to the interfaces. The outer CLC shell was further stabilized by polymerizing a reactive mesogen to provide stable core-shell capsules. The capsules display dual structural colours that are switchable, depending on the selection of light-handedness.