Liquid crystal technology has a long well-established history of applications in visible light, primarily in the display industry. In contrast, applications at lower frequencies (microwave through infrared) are less common. In this paper, we examine non-display applications and review the use of liquid crystal materials for tunable signal processing devices operating at microwave and millimeter wave frequencies. These devices include tunable phase shifters, filters (bandpass and bandstop), antennas and antenna arrays, resonators, and frequency selective surfaces. We also give a short review of the typical characteristics of liquid crystals in these frequency ranges.

Lenses with tunable focal length play important roles in nature by helping species avoid predators and capture prey. Many practical devices mimic lens concept for imaging, sensing, and detection. This review covers fundamental optics of lenses and its extension to lenses made of liquid crystals (LCs). Three main types of LC lenses are described, namely, lenses with curved surfaces, flat gradient-index lenses and composite lenses. The review discusses advantages of LC lenses over their isotropic counterparts, challenges in their fabrication and control, as well as a variety of potential applications. We also discuss the current challenges associated with nematic LC lenses and their solutions. LC lenses are already having significant impacts on optics and optometry, and these impacts will grow with discovering new LC materials and new lens designs.

With growth of interest in functional materials possessing highly organized and well-defined nanostructures, lyotropic mesophases have received much attention. Amphiphiles and chromogens can spontaneously construct ordered lyotropic liquid crystal (LLC) and lyotropic chromonic liquid crystal phases in both organic and inorganic solvents. The combined properties of self-organization with facile orientation and intriguing optical adjustments grant lyotropic mesophases with many potential applications, including use in coatable polarizers, micropatterned films, soft actuators, biomimetic chirophotonic crystals, and energy harvesting devices. In this review, we discuss the general concept of self-assembled lyotropic mesophases and their corresponding structural evolutions in aqueous and non-aqueous systems. Recent research progress in the investigations of polymer-stabilized LLC is also discussed.

Electric field-induced patterns in liquid crystals have been observed and studied for about 50 years. During this time, a great variety of structures, detected under different conditions, have been described; theoretical descriptions were also developed parallel with the experiments and a huge number of papers have been published. The non-vanishing interest in the topic is due to several factors. First, most experimentalists working with new (or even well-known) liquid crystals apply sooner or later an electric field for different purposes and, as a response, often (maybe undesirably or unexpectedly) have to face with emergence of patterns. Second, understanding the complexity of the formation mechanism of regular patterns in a viscous, anisotropic fluid is an extremely challenging theoretical task. Third, specialists in display fabrication or in other applications are also interested in the results; either to make use of them or in order to avoid field-induced patterns. In this review, we attempt to provide a systematic overview of the large amount of published results, focusing on recent achievements, about the three main types of electric field-induced patterns: transient patterns during the Freedericksz transition, flexoelectric domains and electroconvection. As a result of different instability mechanisms, a variety of pattern morphologies may arise. We address the physical background of the mechanisms, specify the conditions under which they may become effective, discuss the characteristics of the patterns, and summarize the possibilities of morphological transitions induced by frequency, voltage or temperature variations. Special emphasis is given to certain topics, which recently have gained enhanced interest from experimental as well as theoretical point of view, like driving with ultra-low frequencies or non-sinusoidal (superposed) waveforms, and the dynamics of defects and embedded colloidal particles. Assisting newcomers to the field, we also mention some, yet unresolved, problems, which may need further experimental and/or theoretical studies.

AbstractIn real samples, focal conic domains (FCDs) and double helical domains (DHDs) in smectic A phases are frequently distorted. Distortions show up as imperfections of conjugated disclinations. These are induced imperfections in difference with proper imperfections, which in terms of the extended Volterra process are inevitable structural features of any curved disclination, including conjugated disclinations of an ideal FCD and DHD. The paper reviews experimental observations of induced imperfections of disclinations in FCDs and DHDs in non-chiral and chiral smectic A and A* phases. Imperfections of FCDs and DHDs are documented as giant kinks on their disclinations or due to the deviation of the shapes of disclinations from those geometrically predicted for conjugated conics. The main message of this paper is that the induced imperfections of FCDs and DHDs are due to the interaction of disclinations with dislocations. The interaction of disclinations with dislocations is suggested to be the driving mechanism for the temperature evolution of FCDs in nematogenic SmA as well as DHDs* in chiral SmA* phases under the temperature variation and applied electric field.

Liquid crystals (LCs) are widely used in nonlinear optics because of their sensitive responses to optical stimulation. Combined with other optoelectronic materials such as azo dyes, quantum dots (QDs), etc., LCs show very large optical nonlinearity which makes them suitable for applications in optical information processing, including holographic display and holographic storage. In this review, we present an overview of recent efforts on holographic display and storage based on liquid crystalline materials. Emphasis is placed on the dynamic holographic display which uses fast-response LCs doped with azo dye and QD. In this application, we discuss how to improve the response speed, resolution, and diffraction efficiency of liquid crystalline materials to realize high frame rate, large-size and high-brightness holographic displays. Some LC materials for holographic storage are also introduced, including polymer-dispersed liquid crystals, blue-phase liquid crystals, and azobenzene polymer liquid crystals, which have permanent response to light stimulation. The mechanisms of optical nonlinearities are discussed in detail, as a foundation for the physical process of LC-based holographic display and storage.

This article reviews the main results from the investigations performed on solid chiral microparticles based on polymerized cholesteric droplets. The procedures of particles generation, the structural characterization, optomechanics and microphotonics investigations are shown. The aim of this work is to give a picture of the innovation introduced by exploring the combination of chirality, self-organization and solid structure.

This review presents the main results that were achieved over the past decade in the new field of liquid-crystal micro-photonics. After a general introduction to some aspects of state-of-the-art micro-photonics technologies, nematic colloids are discussed in terms of their self-assembly and photonic properties. Liquid-crystal lasers, based on spatially periodic, liquid-crystal phases, are reviewed, and microlasers based on liquid-crystal microdroplets are presented and discussed. We show that optical microfibres can be self-grown in water/liquid-crystal dispersions and present their waveguiding and lasing properties. The review concludes with a discussion of the resonant transfer of light across different liquid-crystal micro-objects and presents the ultra-fast optical Kerr and STED effects in bulk nematic liquid crystals.

This article re-examines a classic question in liquid-crystal physics: What are the elastic modes of a nematic liquid crystal? The analysis uses a recent mathematical construction, which breaks the director gradient tensor into four distinct types of mathematical objects, representing splay, twist, bend, and a fourth deformation mode. With this construction, the Oseen-Frank free energy can be written as the sum of squares of the four modes, and saddle-splay can be regarded as bulk rather than surface elasticity. This interpretation leads to an alternative way to think about several previous results in liquid-crystal physics, including: (1) free energy balance between cholesteric and blue phases, (2) director deformations in hybrid-aligned nematic cells, (3) spontaneous twist of achiral liquid crystals confined in a torus or cylinder, and (4) curvature of smectic layers.

(2020). Memoirs of my time at Kent State University during formation of the Liquid Crystal Institute. Liquid Crystals Reviews: Vol. 8, No. 1, pp. 1-4.

Cellulose, the most abundant natural polymer on earth, is used in numerous applications in our day-to-day life. However, the discovery that cellulose-based systems could lead to the formation of liquid crystalline phases only dates to the 1970s. Compared with all known applications of cellulose, the liquid crystalline behavior has been less considered. Associated with this are the low solubility of cellulose and the existence of a chiral nematic precursor solution and its processing under the action of a shear field, which is used to produce fibers and films. In this review, we first conduct a short review of the main features of cellulosic liquid crystalline phases including the main textures observed by polarizing optical microscopy and the cholesteric phase characteristics of thermotropic and lyotropic systems observed for cellulose and cellulose derivatives. Then, we focus on the rheological properties of liquid crystalline solutions and special attention is given to the formation of striations developed during shear and the formation of the band texture, which appears during the relaxation process. Among the different techniques used, special emphasis is given to the results obtained by coupling rheology with optical microscopy (Rheo-optics) and nuclear magnetic resonance (Rheo-NMR) techniques. Some examples described in the literature, related to the use of cellulose and cellulose derivatives liquid crystals to the production of structural color scaffolds, stimuli-responsive films and fibers, are addressed. In these systems, the initial cholesteric phase determines the unique properties exhibited by the films and the fibers produced from cellulosic liquid crystalline systems.

(2016). Biaxial nematic liquid crystals – theory, simulation and experiment. Liquid Crystals Reviews: Vol. 4, No. 1, pp. 80-81.

A major challenge in the delivery of cargo (genes and/or drugs) to cells using nanostructured vehicles is the ability to safely penetrate plasma membranes by escaping the endosome before degradation, later releasing the payload into the cytoplasm or organelle of interest. Lipids are a class of bio-compatible molecules that self-assemble into a variety of liquid crystalline constructs. Most of these materials can be used to encapsulate drugs, proteins, and nucleic acids to deliver them safely into various cell types. Lipid phases offer a plethora of structures capable of forming complexes with biomolecules, most notably nucleic acids. The physichochemical characteristics of the lipid molecular building blocks, one might say the lipid primary structure, dictates how they collectively interact to assemble into various secondary structures. These include bilayers, lamellar stacks of bilayers, two-dimensional (2D) hexagonal arrays of lipid tubes, and even 3D cubic constructs. The liquid crystalline materials can be present in the form of aqueous suspensions, bulk materials or confined to a film configuration depending on the intended application (e.g. bolus vs surface-based delivery). This work compiles recent findings of different lipid-based liquid crystalline constructs both in films and particles for gene and drug delivery applications. We explore how lipid primary and secondary structures endow liquid crystalline materials with the ability to carry biomolecular cargo and interact with cells.

(2019). Lehmann effect in nematic and cholesteric liquid crystals: a review. Liquid Crystals Reviews: Vol. 7, No. 2, pp. 142-166.

Research works on new discoid molecules, comprehending an aromatic/heteroaromatic rigid core with flexible peripheral chains, have been gathered with the multiplying attentiveness due to their prime importance as prototype systems for the charge and energy transport investigation and owing to the prospect of their organo-electronic applications. This critical review article delineates the recent headway in the fundamental design ideas and the available synthetic approaches to obtain the most often encountered triazine-based discotic liquid crystals. The major focal point of the review is to explore the exhilarating research on 1,3,5-triazine-based mesogens that has been reported in the last 5 years. Moreover, the current review not only highlights the variety of structural modifications that are undertaken by the various researchers across the globe in the field of 1,3,5-triazine-based discoid molecules, but also facilitates the fascinating and beneficial altering of properties, both for the rudimentary intentions of demonstrating structure–property relationships and for materials focused in the direction of commercially triumphant liquid crystal display and other applications.

ABSTRACTThe long-range orientational order of liquid crystals (LCs) makes them excellent responsive materials for amplification and transduction of various biochemical events occurring at their interfaces. LC interfaces have been vastly explored for sensing of various macromolecules such as endotoxin, proteins, and disease markers. LC-based sensors possess several advantages over conventional methods in terms of not requiring intensive labour, label-free detection, and low-cost fabrication. However, the first challenge is to ensure the specificity of LC response towards a particular analyte. The second challenge is to design sensing platforms for the detection of small molecule analytes with equal selectivity and sensitivity. In this regard, aptamers have emerged as promising recognition probes due to their high binding affinity towards small molecules, high stability over antibodies, and fast response. The integration of aptamer-based sensing with LC interfaces has enabled the detection of deoxyribonucleic acid (DNA) targets, biologically important toxins, environmental pollutants, heavy metal ions, etc., with applicability in clinical samples. In this review, we recapitulate the recent advancements in the design of aptamer-based LC biosensors for the detection of DNA targets (by DNA hybridization), various biomolecules, and metal ions.

We present a comprehensive review of nonlinear optics and photonics of two ordered phases of liquid crystals, namely, Cholesteric and Blue-phase liquid crystals that exhibit 1- and 3-D photonic crystalline properties. We delve into the ultrafast individual molecular electronic nonlinearity as well as crystalline non-electronic nonlinearities arising from laser induced electrostriction, thermal-, density- and order parameter changes, director axis reorientation and lattice distortion. Emphasis is placed on exploring the unique advantages associated with their liquid crystalline and photonic crystal properties. Exemplary feasibilities demonstration of all-optical passive eye/sensor protection, femtoseconds and picoseconds pulse modulations, lasing actions, aberration-free optical phase conjugation and all-optical image processing and holographic recording are presented along with a critical review and comparison with their nematic counterparts and other electro-optical or nonlinear optical materials.

Liquid crystals (LCs) are an intermediate state of matter that exists between conventional solids and liquids. They are vital to the existence of life as several critical components in living organisms such as cell wall and biochemical fluids are liquid crystalline in nature. Drug delivery based on LCs is a vast field of research. In recent years there has been a huge leap in interest into using LCs, particularly lyotropic liquid crystals (LLCs), as nanoparticles (cubosomes and hexosomes) for drug delivery applications. Such nanoparticle-based drug delivery promise efficient, controlled and target selective release of drugs. This paper reviews the concepts and techniques involved in LLC-based drug delivery. The influence of physical properties of LCs on the drug carrier design and efficiency, key aspects of the methods used to identify, characterize and analyse lyotropic nanoparticles and the feasibility of production of nanoparticles for their widespread usage are discussed. The study suggests that LC-based nanoparticles have the potential to revolutionize drug delivery industry, however a reliable method for production of nanoparticles on a large scale needs to be explored further.

Refraction of light at a boundary between an isotropic dielectric and an optically anisotropic material – liquid crystalline or metamaterial – is elaborated, especially the dependence on the angle between the anisotropic material birefringence optical axis and the material surface. Different regimes of negative and positive refraction are shown, caused by the liquid-crystalline optical response or potential negative-positive anisotropic metamaterial, identifying distinct regimes of refraction and reflection. The theoretical analysis is verified with finite-difference time-domain simulations and presented in the context of selected related literature. More broadly, this work is a contribution towards developing and understanding anisotropic liquid crystal-type soft metamaterials to achieve novel photonic phenomena based on optical anisotropy.

Recent experimental developments show that without reticulation links or entanglements, the isotropic phase of liquid crystals can work mechanically as an elastomer and optically as a harmonic oscillator at the sub-millimetre scale. The strength of the elastic response depends on the molecular architecture and is enhanced when the liquid-crystal molecules are attached as side-chain moieties to a chain backbone. This elastomeric behaviour of the isotropic phase is evidenced via stress and optical responses to a low-frequency mechanical excitation. Visible far away from any phase transition, this spectacular effect contains important information: the liquid state is a low threshold elastic medium; molecules are dynamically correlated and this elastic correlation is observed up to the sub-millimetre scale. This discovery sheds a new light on the origin of well-known effects as flow birefringence, shear banding in complex fluids or flow instabilities in simple liquids. The present paper emphasizes the key role of long range elastic interactions for a comprehensive approach to the flow birefringence, from its origins and to new possible applications.