Three-dimensional imaging of orientational order by fluorescence confocal polarizing microscopy

Applications for photonic crystals and metamaterials put stringent requirements on the characteristics of advanced optical materials, demanding tunability, high Q factors, applicability in visible range, and large-scale self-assembly. Exploiting the interplay between structural and optical properties, colloidal lattices embedded in liquid crystals (LCs) are promising candidates for such materials. Recently, stable two-dimensional colloidal configurations were demonstrated in nematic LCs. However, the question as to whether stable 3D colloidal structures can exist in an LC had remained unanswered. We show, by means of computer modeling, that colloidal particles can self-assemble into stable, 3D, periodic structures in blue phase LCs. The assembly is based on blue phases providing a 3D template of trapping sites for colloidal particles. The particle configuration is determined by the orientational order of the LC molecules: Specifically, face-centered cubic colloidal crystals form in type-I blue phases, whereas body-centered crystals form in type-II blue phases. For typical particle diameters (approximately 100 nm) the effective binding energy can reach up to a few 100 kBT, implying robustness against mechanical stress and temperature fluctuations. Moreover, the colloidal particles substantially increase the thermal stability range of the blue phases, for a factor of two and more. The LC-supported colloidal structure is one or two orders of magnitude stronger bound than, e.g., water-based colloidal crystals.

https://www.pnas.org/content/pnas/108/13/5188.full.pdf

Three-dimensional colloidal crystals in liquid crystalline blue phases

Topological defects in liquid crystals (LCs) have been widely used to organize colloidal dispersions and template polymerization, leading to a range of assemblies, elastomers and gels. However, little is understood about molecular-level assembly processes within defects. Here, we report that nanoscopic environments defined by LC topological defects can selectively trigger processes of molecular self-assembly. By using fluorescence microscopy, cryogenic transmission electron microscopy and super-resolution optical microscopy, we observed signatures of molecular self-assembly of amphiphilic molecules in topological defects, including cooperativity, reversibility and controlled growth. We also show that nanoscopic o-rings synthesized from Saturn-ring disclinations and other molecular assemblies templated by defects can be preserved by using photocrosslinkable amphiphiles. Our results reveal that, in analogy to other classes of macromolecular templates such as polymer-surfactant complexes, topological defects in LCs are a versatile class of three-dimensional, dynamic and reconfigurable templates that can direct processes of molecular self-assembly.

Topological defects in liquid crystals as templates for molecular self-assembly

Point defects in nematics, also called hedgehogs, are topological entities that have no equivalent in ordered atomic solids, despite the homonymy. They have been the subject of intense experimental and, above all, theoretical (analytical and computational) investigations in the last thirty years. They are present in bulk specimens and at the specimen boundaries. This review article stresses the importance of the core structure of the defect, the possibility of it splitting into a disclination loop, and boundary conditions, as well as taking stock of the recent advances on point defects in nematic colloidal suspensions. An important topic is the formation of strings between opposite hedgehogs (radial and hyperbolic), and their role in the dynamic properties of nematics.

/pdf/topological-point-defects-in-nematic-liquid-crystals-1jag0r3rpb.pdf

Topological point defects in nematic liquid crystals

Supramolecular and macromolecular functional helical superstructures are ubiquitous in nature and display an impressive catalog of intriguing and elegant properties and performances. In materials science, self-organized soft helical superstructures, i.e., cholesteric liquid crystals (CLCs), serve as model systems toward the understanding of morphology- and orientation-dependent properties of supramolecular dynamic helical architectures and their potential for technological applications. Moreover, most of the fascinating device applications of CLCs are primarily determined by different orientations of the helical axis. Here, the control of the helical axis orientation of CLCs and its dynamic switching in two and three dimensions using different external stimuli are summarized. Electric-field-, magnetic-field-, and light-irradiation-driven orientation control and reorientation of the helical axis of CLCs are described and highlighted. Different techniques and strategies developed to achieve a uniform lying helix structure are explored. Helical axis control in recently developed heliconical cholesteric systems is examined. The control of the helical axis orientation in spherical geometries such as microdroplets and microshells fabricated from these enticing photonic fluids is also explored. Future challenges and opportunities in this exciting area involving anisotropic chiral liquids are then discussed.

Stimuli-Driven Control of the Helical Axis of Self-Organized Soft Helical Superstructures.

We study phase separation from a nematic liquid crystal with spatially nonuniform director gradients. Particles of a phase-separated component, which is either an isotropic fluid (silicone oil) or a nonmesogenic photopolymer, accumulate in the regions with the strongest director distortions, thus reducing the overall energy of the system.

/pdf/effect-of-director-distortions-on-morphologies-of-phase-vjmegbq61c.pdf

Effect of Director Distortions on Morphologies of Phase Separation in Liquid Crystals

We report the observation of optical vortices in a laser beam propagating through the stripe pattern of a cholesteric liquid crystal. The liquid crystal is confined in a cell with homogeneous boundary conditions and forms a diffraction phase grating. Optical vortices are produced by edge dislocations of the cholesteric grating. The vortices show up as spots of zero light intensity in the diffraction maxima. There is one spot in each +1 and -1 diffraction maximum and two spots in diffraction maxima +2 and -2.

Optical vortices generated by dislocations in a cholesteric liquid crystal.

We study director structures of electrically-controlled cholesteric diffraction gratings (CDG's). The cholesteric liquid crystal (LC) is confined between two transparent non-patterned electrodes with either planar or homeotropic anchoring. In cells with planar alignment, the applied electric field causes reorientation of the initial (zero field) planar state and creates one-dimensionally modulated structures in the plane of the cell. The modulations occur via two distinct scenarios: (I) nucleation and expansion of ‘stripes’ and (II) undulation of quasinematic layers in the plane normal to the cell's plates. In the cells with homeotropic surface alignment, modulated structures exist without applied voltage. Magnetic field is used to provide uniform orientation of these structures. Confocal microscope studies and computer simulations are carried out to reveal the fine structure of the CDG's.

Director structures of cholesteric diffraction gratings

We study photopolymerization in a low-molecular weight liquid crystal with non-uniform director gradients Phase separation results in spatially non-uniform distribution of polymer density controlled by the distorted nematic matrix Director-gradient-controlled polymerization provides a new and useful technique to assemble micron-scale polymer architectures

D. Voloschenko

Papers

Effect of Director Distortions on Morphologies of Phase Separation in Liquid Crystals

Optical vortices generated by dislocations in a cholesteric liquid crystal.

Director structures of cholesteric diffraction gratings

Effect of Director Distortions on Polymer-Liquid Crystal Phase Separation