Francesca Serra

Topological defects of nematic liquid crystals confined in porous networks

We study the formation of topological defects in nematic liquid crystals confined in open bicontinuous networks produced in glass by femtosecond laser micromachining. We obtain a careful classification of the number and localisation of the defects as a function of the topological properties of the network. Our findings lead to a general formula that predicts the total topological charge in open complex networks, thus complementing the classic Stein–Gauss theorem. Our result provides a justification for the observed multistability of nematics confined in porous networks.

The emergence of memory in liquid crystals

The nematic phase of liquid crystals (LC), used in most LC display applications, is a fluid state formed by orientationally ordered molecules. The direction of their alignment, and hence the overall optical response of the material, is easily modified by the application of an electric field and elastically relaxes back to a well-defined off-state when the field is removed. It has been recently shown that hybrid materials formed by nematic LCs incorporated in complex micro-structured porous matrices are often capable of indefinitely retaining the alignment direction imposed by an electric field. Such multistability is ultimately due to the interactions of the porous material with the lines of topological defects that develop within the confined nematic. Controlling the defect lines and their interactions is crucial to the design of materials whose optical properties are electrically driven but spontaneously preserved.

Defect science and engineering of liquid crystals under geometrical frustration

Spontaneous symmetry breaking while preserving flow ability is a remarkable feature of nematic liquid crystals. When a nematic liquid crystal coexists with a solid, the surface field of the solid tends to anchor the director direction on the surface: anchoring effects. If geometrical frustration between nematic ordering and anchoring is strong enough, stable topological defects are formed. Defects in an ordered state are usually regarded as undesirable features. However, recent studies reveal that defects stabilized by a topological constraint from the solid surfaces are actually quite useful and open up novel possibilities for defect engineering of liquid crystals: self-organization of soft matter by defects, memory effects of topological origin, and control of flow of nematic liquid crystals and colloid motion by defects. For example, defect reconfiguration accompanying the change in the topology costs a very high energy far beyond the thermal energy, which overwhelms a typical energy scale in soft matter. This provides extreme stability for structures assembled by defects and information memorized in defect topology. Furthermore, effects of topological defects can easily be removed perfectly by a nematic-to-isotropic transition, which provides switchable functions. Defects also affect the motion of colloids immersed in a liquid crystal and flow behaviour of a liquid crystal. Here we review recent developments in science and engineering of topological defects in nematic liquid crystals, mainly based on our numerical simulation studies.

Curvature and defects in nematic liquid crystals

ABSTRACT The use of nematic liquid crystals for directed assembly of particles and for the creation of multistable systems depends on the ability to control the topological defects and the distortions of the director field. These are not only driven by topological constraints and by anchoring energies but also by the curvature field created by the bounding surfaces. This review explores the interaction between defects, topology, inclusions and curvature in nematics. I will introduce the relationship between curvature and the Frank elastic energy in nematics, and then I will give an overview of specific examples that show how this coupling can create unexpected behaviours, such as lock-and-key interactions, anchoring transitions on curved surfaces and memory effects. GRAPHICAL ABSTRACT

DIRECT MAPPING OF LOCAL DIRECTOR FIELD OF NEMATIC LIQUID CRYSTALS AT THE NANOSCALE

Significance It has been challenging to directly probe the molecular-scale organization of nonglassy nematic liquid crystal (LC) molecules without altering the LC directors. Here, we design and synthesize a new type of stable nematic liquid crystal monomer (LCM) system with strong dipole–dipole interactions. The new LCMs can achieve faithful anchoring and alignment control at various boundaries, analogous to that of small molecule LCs. Upon photo-cross-linking, the orientational order of mesogens is effectively and faithfully locked, allowing for direct visualization of the LC director field and defect structures by scanning electron microscopy (SEM) with 100-nm resolution. Further, we use SEM imaging to calculate the extrapolation length of the LCM for planar and homeotropic anchoring. Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of wide interest in both the display industry and soft matter as a route to more sophisticated optical objects, to direct phase separation, and to facilitate colloidal assemblies. However, it remains challenging to directly probe the molecular-scale organization of nonglassy nematic LC molecules without altering the LC directors. We design and synthesize a new type of nematic liquid crystal monomer (LCM) system with strong dipole–dipole interactions, resulting in a stable nematic phase and strong homeotropic anchoring on silica surfaces. Upon photopolymerization, the director field can be faithfully “locked,” allowing for direct visualization of the LC director field and defect structures by scanning electron microscopy (SEM) in real space with 100-nm resolution. Using this technique, we study the nematic textures in more complex LC/colloidal systems and calculate the extrapolation length of the LCM.

Phases and structures of sunset yellow and disodium cromoglycate mixtures in water

We study phases and structures of mixtures of two representative chromonic liquid crystal materials, sunset yellow FCF (SSY) and disodium cromoglycate (DSCG), in water. A variety of combinations of isotropic, nematic (N), and columnar (also called M) phases are observed depending on their concentrations, and a phase diagram is made. We find a tendency for DSCG-rich regions to show higher-order phases while SSY-rich regions show lower-order ones. We observe uniform mesophases only when one of the materials is sparse in the N phases. Their miscibility in M phases is so low that essentially complete phase separation occurs. X-ray scattering and spectroscopy studies confirm that SSY and DSCG molecules do not mix when they form chromonic aggregates and neither do their aggregates when they form M phases.

Smectic Gardening on Curved Landscapes

Focal conic domains (FCDs) form in smectic-A liquid crystal films with hybrid anchoring conditions with eccentricity and size distribution that depend strongly on interface curvature. Assemblies of FCDs can be exploited in settings ranging from optics to material assembly. Here, using micropost arrays with different shapes and arrangement, we assemble arrays of smectic flower patterns, revealing their internal structure as well as defect size, location, and distribution as a function of interface curvature, by imposing positive, negative, or zero Gaussian curvature at the free surface. We characterize these structures, relating free surface topography, substrate anchoring strength, and FCD distribution. Whereas the largest FCDs are located in the thickest regions of the films, the distribution of sizes is not trivially related to height, due to Apollonian tiling. Finally, we mold FCDs around microposts of complex shape and find that FCD arrangements are perturbed near the posts, but are qualitatively similar far from the posts where the details of the confining walls and associated curvature fields decay. This ability to mold FCD defects into a variety of hierarchical assemblies by manipulating the interface curvature paves the way to create new optical devices, such as compound eyes, via a directed assembly scheme.

Effect of configuration of the microchannels fabricated by femtosecond laser micromachining on topological defects in confined liquid crystals

The Femtosecond laser micromachining is a versatile tool for fabrication of microfluidic channel network; we exploit the fast prototyping capability of this technology to produce various channel configurations and study the alignment and topological defects in microchannels filled with Liquid crystals. The configurations consist of multiple intersections of microchannels to form networks both in 2D and 3D. The effect of each configuration on the defect formations is also studied.

Bistability of nematic liquid crystals confined in 3D scaffold produced by two-photon polymerization

We show that nematic liquid crystals confined inside cubic scaffolds made by two-photon polymerization exhibit bistability and large memory effects in response to electric fields, due to topological defects interacting with the solid structure.