Liquid crystals in micron-scale droplets, shells and fibers

Abstract

The extraordinary responsiveness and large diversity of self-assembled structures of liquid crystals are well documented and they have been extensively used in devices like displays. For long, this application route strongly influenced academic research, which frequently focused on the performance of liquid crystals in display-like geometries, typically between flat, rigid substrates of glass or similar solids. Today a new trend is clearly visible, where liquid crystals confined within curved, often soft and flexible, interfaces are in focus. Innovation in microfluidic technology has opened for high-throughput production of liquid crystal droplets or shells with exquisite monodispersity, and modern characterization methods allow detailed analysis of complex director arrangements. The introduction of electrospinning in liquid crystal research has enabled encapsulation in optically transparent polymeric cylinders with very small radius, allowing studies of confinement effects that were not easily accessible before. It also opened the prospect of functionalizing textile fibers with liquid crystals in the core, triggering activities that target wearable devices with true textile form factor for seamless integration in clothing. Together, these developments have brought issues center stage that might previously have been considered esoteric, like the interaction of topological defects on spherical surfaces, saddle-splay curvature-induced spontaneous chiral symmetry breaking, or the non-trivial shape changes of curved liquid crystal elastomers with non-uniform director fields that undergo a phase transition to an isotropic state. The new research thrusts are motivated equally by the intriguing soft matter physics showcased by liquid crystals in these unconventional geometries, and by the many novel application opportunities that arise when we can reproducibly manufacture these systems on a commercial scale. This review attempts to summarize the current understanding of liquid crystals in spherical and cylindrical geometry, the state of the art of producing such samples, as well as the perspectives for innovative applications that have been put forward.

Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets

“…The mismatch between these two tilt values indicates that other phenomena also play an important role in defining the initial alignment β i of the cholesteric domains under magnetic field. We believe that the anchoring will get stronger upon further drying as the vertical confinement increases, [62,75] contingent that it occurs before the sample becomes kinetically arrested. [30,36,72] Moreover, the alignment of cholesteric fields under both external fields and conflicting anchoring conditions usually involves a Fréedericksz transition.…”

“…[73,74] Such transition is characterized by a magnetic field threshold required to tilt the liquid crystalline alignment, while the resulting alignment profile is usually given by a compromise between both conditions and the minimization of the director distortion. We believe that the anchoring will get stronger upon further drying as the vertical confinement increases, [62,75] contingent that it occurs before the sample becomes kinetically arrested. This can cause a complex self-assembly behavior upon drying that could impact the tilt of the final cholesteric structure.…”

Laser‐Induced Nanodroplet Injection and Reconfigurable Double Emulsions with Designed Inner Structures

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strategy can in principle be applied to soft matter, [1][2][3][4] including liquid crystals (LCs) [5,6] to fabricate multilevel hierarchical structures with broad applicability in drug delivery, [7] food processing, [8] cosmetics, [9] and microchemistry. [10] However, microfabrication in liquid and soft systems is severely constrained by their peculiar properties such as fluidity, zero or weak resistance to shear, surface tension, and diffusion.

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Elucidating the fine details of cholesteric liquid crystal shell reflection patterns

“…While liquid crystals were studied in spherical shape very soon after their discovery [2], with a number of seminal works published also in the 1970s-1990s [3][4][5][6][7][8][9][10], not least in connection to polymer-dispersed liquid crystals [11], the success of liquid crystals in the flat panel display industry has for a quite long time set a paradigm in which liquid crystals are studied primarily in a flat sample geometry. This is today changing and curved geometries like droplets, cylinders or shells are now drawing more attention [12,13] (and references therein), due to the many interesting effects arising from confinement within one (droplet) or two (shell) self-closing surfaces. Among the issues that have attracted the largest attention is the topological defects that must appear at the surface or in the bulk of spherical samples of nematic and smectic liquid crystals [12][13][14][15][16][17][18][19][20][21][22][23].…”

“…This is today changing and curved geometries like droplets, cylinders or shells are now drawing more attention [12,13] (and references therein), due to the many interesting effects arising from confinement within one (droplet) or two (shell) self-closing surfaces. Among the issues that have attracted the largest attention is the topological defects that must appear at the surface or in the bulk of spherical samples of nematic and smectic liquid crystals [12][13][14][15][16][17][18][19][20][21][22][23]. While liquid crystal droplets must have at least one topological defect within the droplet, on the surface and/or in the bulk, shells (containing an immiscible isotropic droplet as core) must contain topological defects in case of planar alignment on at least one of their surfaces (inner and outer), while homeotropic shells (i.e.…”