Chiral Nematic Liquid Crystal Microlenses

Nematic liquid crystals (NLCs) of achiral molecules and racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids and immersed in water. On addition of chiral dopants to the liquid crystal, the films exhibit optical textures with concentric ring patterns and radial variation of the birefringence color. Both are related to a biconvex shape of the chiral liquid crystal film; the rings are due to interference. The curvature radii of the biconvex lens array are in the range of a few millimeters. This curvature leads to a radial variation of the optical axis along the plane of the film. Such a Pancharatnam-type phase lens dominates the imaging and explains the measured focal length of about one millimeter. To our knowledge, these are the first spontaneously formed Pancharatnam devices. The unwinding of the helical structure at the grid walls drives the lens shape. The relation between the lens curvature and material properties such as helical pitch, the twist elastic constant, and the interfacial tensions, is derived. This simple, novel method for spontaneously forming microlens arrays can also be used for various sensors

A Liquid Crystal Biosensor for Specific Detection of Antigens

Following the principle of the enzyme-linked immunosorbent assay (ELISA) pathogen detection method, we demonstrate specific sensing of goat Immunoglobulin G (IgG) by a nematic liquid crystal material. Sensing occurs via the visually-striking realignment of a pre-fabricated liquid crystal film, suspended in grids and coated with biotinylated lipids followed by biotinylated anti-goat IgG. Realignment occurs when the targeted goat IgG is added to the cell, but not when rat or rabbit serum IgG is added to the same surface. In principle, this method can be generalized to provide an inexpensive, fast and sensitive prefabricated sensor for any pathogen

Thermotropic Liquid Crystal-Assisted Chemical and Biological Sensors

In this review article, we analyze recent progress in the application of liquid crystal-assisted advanced functional materials for sensing biological and chemical analytes. Multiple research groups demonstrate substantial interest in liquid crystal (LC) sensing platforms, generating an increasing number of scientific articles. We review trends in implementing LC sensing techniques and identify common problems related to the stability and reliability of the sensing materials as well as to experimental set-ups. Finally, we suggest possible means of bridging scientific findings to viable and attractive LC sensor platforms

Liquid Metals and Liquid Crystals Subject to Flow: From Fundamental Fluid Physics to Functional Fibers

Technology over the past few decades has pushed strongly towards wearable technology, one such form being textiles which incorporate a functional component. There are several ways to produce polymer fibers on both laboratory and industrial scales, but the implementation of these techniques to spin fibers incorporating a functional heterocore has proven challenging for certain combinations of materials. In general, fiber spinning from polymer solutions, regardless of the method, is a multifaceted process with concerns in chemistry, materials science, and physics, both from fundamental and applied standpoints, requiring balancing of flow parameters (interfacial tension, viscosity, and inertial forces) against solvent extraction. This becomes considerably more complicated when multiple interfaces are present. This thesis explores the concerns involved in the spinning of fibers incorporating functional materials from several standpoints. Firstly, due to the importance of interfacial forces in jet stability, I present a microfluidic interfacial tensiometry technique for measuring the interfacial tension between two immiscible fluids, assembled using glass capillary microfluidics techniques. The advantage of this technique is that it can measure the interfacial tension without reliance on sometimes imprecise external parameters and data, obtaining interfacial tension measurements solely from experimental observations of the deformation of a droplet into a channel and the pressure needed to induce the same. Using the knowledge gained from both microfluidic device assembly and the interfacial tension, I then present the wet spinning of polymer fibers using a glass capillary spinneret. This technique uses a polymer dope flowed along with a coagulation bath tooled to extract solvent, leaving behind a continuous polymer fiber. We were able to spin both pure polymer fibers and elastomer microscale fibers containing a continuous heterocore of a liquid crystal, with the optical properties of the liquid crystal maintained within the fiber. While we were not able to spin fibers of a harder polymer containing a continuous core, either liquid crystalline or of a liquid metal, I present analysis of why the spinning was unsuccessful and analysis that will lead us towards the eventual spinning of such fibers.La recherche en technologie se tourne actuellement vers la technologie portable, qui inclut les fibres des textiles incorporant des matériaux qui répondent aux stimuli. Que l’on produise ces fibres synthétiques en laboratoire ou de façon industrielle, le processus de fabrication engage à la fois la chimie, la physique, et les sciences des matériaux, et se complexifie quand on veut réaliser des fibres coaxiales avec des noyaux fonctionnels. Cette thèse se penche de plusieurs façons sur les aspects de la production des fibres dotées d’un noyau de matériaux fonctionnels. Dans un premier temps, en raison de l'importance de la tension superficielle pour la stabilisation des jets liquides, on présente une technique pour mesurer la tension superficielle entre deux fluides immiscibles. Cette technique utilise des capillaires en verre dans une géométrie microfluidique. L'avantage de cette technique est de permettre de mesurer la tension superficielle uniquement sur la base des observations de la déformation des gouttelettes et de la pression nécessaire pour réaliser cette déformation dans le capillaire, sans avoir à compter sur les valeurs externes, comme la densité et la viscosité. Les valeurs obtenues sont similaires à celles disponibles dans la littérature. En exploitant les informations obtenues lors des premières expériences, on présente le filage au mouillé, qui produit des fibres en polymère avec une filière construite des capillaires en verre. On a réussi à produire des fibres de polymère pur ainsi que des fibres incorporant un noyau de cristal liquide nématique ou cholésterique, et les fibres obtenues avec les cristaux liquides conservent les propriétés optiques caractéristiques. Il n’a en revanche pas été possible d’obtenir des fibres avec des noyaux continus de métal liquide ; on présente une analyse des causes de cet échec et on suggère une autre méthode pour parvenir à réaliser éventuellement des microfibres avec un noyau continu de métal liquide

Elastic sheath–liquid crystal core fibres achieved by microfluidic wet spinning

While coaxial polymer sheath–liquid crystal core fibres attract interest for fundamental research as well as applied reasons, the main method for achieving them so far, electrospinning, is complex and has significant limitations. It has proven particularly challenging to spin fibres with an elastic sheath. As an alternative approach, we present a microfluidic wet spinning process that allows us to produce liquid crystal core–polyisoprene rubber sheath fibres on a laboratory scale. The fibres can be stretched by up to 300% with intact core–sheath geometry. We spin fibres with nematic as well as with cholesteric liquid crystal in the core, the latter turning the composite fibre into an elastic cylindrical photonic crystal. Iridescent colours are easily observable by the naked eye. As this coaxial wet spinning should be amenable to upscaling, this could allow large-scale production of innovative functional fibres, attractive through the various responsive characteristics of different liquid crystal phases being incorporated into an elastic textile fiber form factor

Microfluidic Wet Spinning of Core-Sheath Elastomer-Liquid Crystal Fibers

Liquid crystals encapsulated in fibers have a wide variety of applications in sensing. In order to produce these, several methods have been explored. Electrospinning is among the better-known techniques with considerable successes. Only a limited range of polymers, though, has been used for electrospinning with liquid crystal cores, and the process of electrospinning has many obstacles to its utility at an industrial scale. On the other hand, wet-spinning techniques are better suited for industrial applications and are widely used in textile manufacturing, but are not commonly used for coaxial fiber production, especially with the large experimental scales that are difficult to replicate in a standard liquid crystal research laboratory. We therefore propose a method for wet-spinning coaxial core-sheath liquid crystal-filled elastomer fibers using a microfluidic set-up. Based on the flow-focusing method used for the production of liquid crystal shells and emulsions, this technique generates coaxial filaments by pumping a core-sheath flow of a liquid crystal surrounded by a rubbery polymer solution into a co-flowing coagulation bath. The coagulation bath is tuned to quickly extract the elastic polymer solution solvent, leaving behind a dry, continuous fiber. We have employed this method to produce fibers of polybutadiene and polyisoprene containing a core of a liquid crystal, such as 4-cyano-4'-pentylbiphenyl (5CB). Investigations into the choice of polymer solution, i.e. both the polymer and solvents used, will be presented in addition to discussion on parameters affecting the contiguity of the core