Nematic Domains Research Articles

Spool-Nematic Ordering of dsDNA and dsRNA under Confinement.

The ability of double-stranded DNA or RNA to locally melt and form kinks leads to strong nonlinear elasticity effects that qualitatively affect their packing in confined spaces. Using analytical theory and numerical simulation we show that kink formation entails a mixed spool-nematic ordering of double-stranded DNA or RNA in spherical capsids, consisting of an outer spool domain and an inner, twisted nematic domain. These findings explain the experimentally observed nematic domains in viral capsids and imply that nonlinear elasticity must be considered to predict the configurations and dynamics of double-stranded genomes in viruses, bacterial nucleoids or gene-delivery vehicles. The nonlinear elastic theory suggests that spool-nematic ordering is a general feature of strongly confined kinkable polymers.

Optical transmission modulation and spectral properties of self-organized nematic domains

The modulation and spectral characteristics of self-organized ensembles of nematic domains with the disclination lines non-aligned and aligned in a magnetic field have been studied using an electric field. The obtained experimental voltage dependences of the light transmission agree well with the dependences calculated using the model considerations. These dependences have been compared with the electro-optical characteristics of a homogeneous planar nematic layer. The amplitude component of the optical transmission determined by light scattering and the oscillating component caused by the phase modulation have been examined. The effect of the spectral dispersion of transmission on the electro-optical characteristics of domain ensembles has been evaluated.

3D Printing of Flow-Inspired Anisotropic Patterns with Liquid Crystalline Polymers.

Anisotropic materials formed by living organisms possess remarkable mechanical properties due to their intricate microstructure and directional freedom. In contrast, human-made materials face challenges in achieving similar levels of directionality due to material and manufacturability constraints. To overcome these limitations, an approach using 3D printing of self-assembling thermotropic liquid crystal polymers (LCPs) is presented. Their high stiffness and strength is granted by nematic domains aligning during the extrusion process. Here, a remarkably wide range of Young's modulus from 3 to 40 GPa is obtained by utilizing directionality of the nematic flow the printing process. By determining a relationship between stiffness, nozzle diameter, and line width, a design space where shaping and mechanical performance can becombined is identified. The ability to print LCPs with on-the-fly width changes to accommodate arbitrary spatially varying directions is demonstrated. This unlocks the possibility to manufacture exquisite patterns inspired by fluid dynamics with steep curvature variations. Utilizing the synergy between this path-planning method and LCPs, functional objects with stiffness and curvature gradients can be 3D-printed,offering potential applications in lightweight sustainable structures embedding crack-mitigation strategies. This method also opens avenues for studying and replicatingintricate patterns observed in nature, such as wood or turbulent flow using 3D printing.

Modulation of Light Transmission in Self-Organized Ensembles of Nematic Domains

Using an electric field, the transmission of light passing through self-organized ensembles of nematic domains is studied. Modulation characteristics of the ensembles containing domains with non-oriented and magnetic-field-oriented disclination lines are compared. A significant decrease in light scattering is shown when disclination lines are oriented. The calculated and well agreeing with them experimental dependences of light transmission on electric voltage applied to liquid crystal cells are obtained and presented. Oscillations on light transmission curves are studied. Superpositions of ordinary and extraordinary waves propagating through the domain ensembles and homogeneous planar liquid crystal layers are considered. The spectral characteristics of the ensembles are presented.

Cascade Formation of Topological Defects and Satellite Droplets in Liquid Crystals at Dynamic Capillary Instability

The formation of topological defects at the nematic–isotropic liquid interface and near satellite droplets has been detected at the breakup and fragmentation of the bridge of the isotropic phase between nematic domains. This process has been implemented in thin optical cells filled with a liquid crystal. The critical width of the bridge at which a universal time dependence of its width is determined by the capillary velocity (ratio of the surface tension to the viscosity) has been determined.

Thermoreversible Gel-Dispersed Liquid Crystals.

A simple model is introduced to describe phase behaviours of binary mixtures of a thermoreversible gel and a low-molecular-weight liquid crystal (LC). We predict novel phase diagrams on the temperature-concentration plane, including sol-gel transition, nematic-isotropic phase transition, and phase separation. At high temperatures, the phase separation between the isotropic sol and gel phases appears. As the temperature decreases, we have the phase separation between nematic sol and isotropic gel phases, in which the nematic domains are dispersed in the isotropic gel phase. We suggest that thermoreversible gelation of reactive molecules mixed with LCs will become one of the new classes of polymer-dispersed liquid crystals.

Hybrid microstructure of smectite clay gels revealed using neutron and synchrotron X-ray scattering

Aqueous suspensions of swelling clays display a nematic sol-gel transition at very low solid concentrations. The underlying microstructure of the gel has remained a point of contention since the time of Irving Langmuir and has been a major obstacle to fully realizing the potential of clays for practical applications. Here, we comprehensively probe the microstructure of a smectite clay suspension using ultra-small angle neutron/X-ray scattering and find that the nematic gel is structurally ordered and contains entities that are at least an order of magnitude larger than the individual particles. Complementary cryo-electron microscopy shows the presence of domains having particle-particle ordering responsible for nematic texture and regions of particle-particle aggregation responsible for gel-like behavior. We find that the smectic clay gels have a hybrid microstructure with co-existing repulsive nematic domains and attractive disordered domains.

Low-Voltage Haze Tuning with Cellulose-Network Liquid Crystal Gels.

Being key components of the building envelope, glazing products with tunable optical properties are in great demand because of their potential for boosting energy efficiency and privacy features while enabling the main function of allowing natural light indoors. However, windows and skylights with electric switching of haze and transparency are rare and often require high voltages or electric currents, as well as not fully meet the stringent technical requirements for glazing applications. Here, by introducing a predesigned gel material we describe an approach dubbed "Haze-Switch" that involves low-voltage tuning of the haze coefficient in a broad range of 2-90% while maintaining high visible-range optical transmittance. The approach is based on a nanocellulose fiber gel network infiltrated by a nematic liquid crystal, which can be switched between polydomain and monodomain spatial patterns of optical axis via a dielectric coupling between the nematic domains and the applied external electric field. By utilizing a nanocellulose network of nanofibers ∼10 nm in diameter we achieve <10 V dielectric switching and <2% haze in the clear state, as needed for applications in window products. We characterize physical properties relevant to window and smart glass technologies, like the color rendering index, haze coefficient, and switching times, demonstrating that our material and envisaged products can meet the stringent requirements of the glass industry, including applications such as privacy windows, skylights, sunroofs, and daylighting.

Mechanism of Pressure-Sensitive Adhesion in Nematic Elastomers.

Nematic liquid crystal elastomers (LCEs) have anomalously high vibration damping, and it has been assumed that this is the cause of their anomalously high-pressure-sensitive adhesion (PSA). Here, we investigate the mechanism behind this enhanced PSA by first preparing thin adhesive tapes with LCE of varying cross-linking densities, characterizing their material and surface properties, and then studying the adhesion characteristics with a standard set of 90° peel, lap shear, and probe tack tests. The study confirms that the enhanced PSA is only present in (and due to) the nematic phase of the elastomer, and the strength of bonding takes over 24 h to fully reach its maximum value. Such a long saturation time is caused by the slow relaxation of local stress and director orientation in the nematic domains after pressing against the surface. We confirm this mechanism by showing that freshly pressed and annealed tape reaches the same maximum bonding strength on cooling, when the returning nematic order is forming in its optimal configuration in the pressed film.

Cascade Formation of Topological Defects and Satellite Droplets in Liquid Crystals at Dynamic Capillary Instability

The formation of topological defects at the nematic–isotropic liquid interface and near satellite droplets has been detected at the breakup and fragmentation of the bridge of the isotropic phase between nematic domains. This process has been implemented in thin optical cells filled with a liquid crystal. The critical width of the bridge at which a universal time dependence of its width is determined by the capillary velocity (ratio of the surface tension to the viscosity) has been determined.

Structural, Magnetic, Electric and Optical Coherences in Self-Organized Ensembles of Nematic Domains

Structural, Magnetic, Electric and Optical Coherences in Self-Organized Ensembles of Nematic Domains

Origin of the Monolayer and Bilayer Arrangement in Smectic A and C Mesophases and Nematic Cybotactic Domains: The Case Study of Benzyloxy-Terminated Isoxazole Liquid Crystals

Origin of the Monolayer and Bilayer Arrangement in Smectic A and C Mesophases and Nematic Cybotactic Domains: The Case Study of Benzyloxy-Terminated Isoxazole Liquid Crystals

Dual-emitting cellulose nanocrystal hybrid materials with circularly polarized luminescence for anti-counterfeiting labels

Cellulose nanocrystal (CNC) hybrid materials with numerous optical states have great potential as anti-counterfeiting labels and information encryption materials. However, it is challenging to construct multicolor emitting materials with tunable behaviors, which can dramatically enhance anti-counterfeiting abilities. Here, free-standing composite films with vivid multi-structural colors and dual-emitting fluorescence are successfully fabricated through a host-guest coassembly strategy. The lanthanide complex and an aggregation-induced emission molecule (tetraphenylethylene derivative, TPEC) are selected as luminescent guests, which are integrated into the chiral nematic structure of CNCs. The obtained photonic films display broadband reflection across the visible spectrum, which may be attributed to the chiral nematic domains with variations in the helical pitches and helical axis orientations. Under 254 nm excitation, the film exhibits bright red emission, while blue–green emission switching occurs under 365 nm excitation. The broad reflection band of the film covers both the green and red fluorescent emission centers, and right circularly polarized luminescence emission with different dissymmetry factors is produced due to the selective reflection of the left chiral nematic structure. A large glum value up to −0.21 at 600 nm was realized. Additionally, CNC-based materials with tailored shapes are further used in anti-counterfeit tags and decorative applications.

Inhomogeneous Superconductivity Onset in FeSe Studied by Transport Properties

Heterogeneous superconductivity onset is a common phenomenon in high-Tc superconductors of both the cuprate and iron-based families. It is manifested by a fairly wide transition from the metallic to zero-resistance states. Usually, in these strongly anisotropic materials, superconductivity (SC) first appears as isolated domains. This leads to anisotropic excess conductivity above Tc, and the transport measurements provide valuable information about the SC domain structure deep within the sample. In bulk samples, this anisotropic SC onset gives an approximate average shape of SC grains, while in thin samples, it also indicates the average size of SC grains. In this work, both interlayer and intralayer resistivity were measured as a function of temperature in FeSe samples of various thicknesses. To measure the interlayer resistivity, FeSe mesa structures oriented across the layers were fabricated using FIB. As the sample thickness decreases, a significant increase in superconducting transition temperature Tc is observed: Tc raises from 8 K in bulk material to 12 K in microbridges of thickness ∼40 nm. We applied analytical and numerical calculations to analyze these and earlier data and find the aspect ratio and size of the SC domains in FeSe consistent with our resistivity and diamagnetic response measurements. We propose a simple and fairly accurate method for estimating the aspect ratio of SC domains from Tc anisotropy in samples of various small thicknesses. The relationship between nematic and superconducting domains in FeSe is discussed. We also generalize the analytical formulas for conductivity in heterogeneous anisotropic superconductors to the case of elongated SC domains of two perpendicular orientations with equal volume fractions, corresponding to the nematic domain structure in various Fe-based superconductors.

Coassembly of Cellulose Nanocrystals and Neutral Polymers in Iridescent Chiral Nematic Films.

Photonic materials based on composite films of cellulose nanocrystals (CNCs) and polymers are promising as they can be renewable and show tunable optical and mechanical properties. However, the influence of polymers on CNC self-assembly is not always well understood, and conflicting results are present in the literature. In this study, we incorporate three neutral, water-soluble polymers-poly(ethylene glycol) (PEG), poly(vinyl pyrrolidone) (PVP), and poly(acrylic acid) (PAA)-with different molecular weights into CNC suspensions at various concentrations prior to obtaining iridescent composite thin films by solvent evaporation. Through spectroscopic, potentiometric, and rheological analyses, we find that PVP physically adsorbs to the surface of CNCs resulting in a bathochromic shift in film color with both increasing concentration and polymer molecular weight. In contrast, PEG induces depletion interactions that result in a decrease in the size of chiral nematic CNC domains, with a negligible change in film color. Finally, PAA hydrogen bonds to the hydroxyl groups of CNCs, resulting in a bathochromic color shift along with interesting rheological and liquid-state properties. This work demonstrates a deeper understanding of CNC-polymer interactions during coassembly and formation of iridescent chiral nematic films, allowing for greater control over optical properties of future CNC-based materials.

Polarization-insensitive liquid crystal Fresnel lens based on self-assembly polymer gravels and chiral dopant.

A polarization-insensitive liquid crystal (LC) Fresnel lens is developed with binary LC configurations of 90°-twisted nematic (TN) and vertically-aligned (VA) domains in the adjacent zones. A LC mixture comprised of nematic host, photopolymer and chiral material is initially filled into the VA cell with orthogonal rubbing treatment. After the ultraviolet irradiation on the filled LC cell through a photomask with Fresnel zone plate pattern, the interactions among orthogonal rubbing treatment, self-assembly polymer gravels, and chiral material induce the 90°-TN structure in the odd zones, whereas the initial VA structures are maintained in the even zones. The fabricated LC Fresnel lens with binary configuration emerges a maximum diffraction efficiency of around 35% at a voltage of 2.3 V, close to the theoretical diffraction limit of around 41%. The diffractive focus of the LC Fresnel lens is polarization-insensitive at the voltage above 2 V. When the voltage reaches 10 V, the diffractive focus vanishes. The numerical calculation confirms that the polarization-insensitive property appears in the primary focus of the LC Fresnel lens. This work reports a simple method to develop a highly efficient, polarization-insensitive, and electrically tunable LC Fresnel lens which is favorable for imaging system.

Quantifying nematic order in the evaporation-driven self-assembly of halloysite nanotubes: nematic islands and the critical aspect ratio.

Halloysite nanotubes (HNTs) are naturally occurring clay minerals found in the Earth's crust that typically exist in the form of high aspect-ratio nanometer-long rods. Here, we investigate the evaporation-driven self-assembly process of HNTs and show that a highly polydisperse collection of HNTs self-sort into a spatially inhomogeneous structure, displaying a systematic variation in the resulting nematic order. Through detailed quantification using the nematic order parameter S and nematic correlation functions, we show the existence of well-defined isotropic-nematic transitions in the emerging structures. We also show that the onset of these transitions gives rise to the formation of nematic islands, a phase resembling ordered nematic domains surrounded by an isotropic phase, which grow in size with S. Detailed image analysis indicates a strong correlation between local S and the local aspect ratio, L/D, with nematic order possible only for rods with L/D ≥ 6.5 ± 1. Finally, we conclude that the observed phenomena directly result from aspect ratio-based sorting in our system. Altogether, our results provide a unique method of tuning the local microscopic structure in self-assembled HNTs using L/D as an external parameter.

Angle-resolved resistivity method for precise measurements of electronic nematicity

Electronic nematicity is an intriguing phenomenon found in unconventional superconductors and its mechanism invokes intense studies. The precise measurement of the director and amplitude of electronic nematicity by electric transport demands a full mapping of the longitudinal and transverse resistivity, $\ensuremath{\rho}$ and ${\ensuremath{\rho}}_{T}$, along different directions. Fabricating Hall bars along every possible in-plane direction would be so labor intensive and vulnerable to sample to sample variations that it becomes impractical. Instead, here we make the local current density $\mathbit{J}$ continuously rotate in plane by independently controlling its in-plane components along two orthogonal axes and measure $\ensuremath{\rho}$ and ${\ensuremath{\rho}}_{T}$ as a function of the azimuth angle $\ensuremath{\phi}$ between $\mathbit{J}$ and the crystallographic $[100]$ direction. Clear substantial angular oscillations in $\ensuremath{\rho}(\ensuremath{\phi})$ and ${\ensuremath{\rho}}_{T}(\ensuremath{\phi})$ were observed from the optimally doped ${\mathrm{La}}_{1.84}{\mathrm{Sr}}_{0.16}\mathrm{Cu}{\mathrm{O}}_{4}$ film, in stark contrast to those of the control gold film. This angle-resolved resistivity method demonstrates unprecedented precision in determining the director and amplitude of electronic nematicity and is applicable to anisotropic transport due to mechanisms other than electronic nematicity as well. This method paves the way for the studies of the nematic domains and fluctuations of the nematic order in films and bulk crystals.

Spontaneous electric-polarization topology in confined ferroelectric nematics

Topological textures have fascinated people in different areas of physics and technologies. However, the observations are limited in magnetic and solid-state ferroelectric systems. Ferroelectric nematic is the first liquid-state ferroelectric that would carry many possibilities of spatially-distributed polarization fields. Contrary to traditional magnetic or crystalline systems, anisotropic liquid crystal interactions can compete with the polarization counterparts, thereby setting a challenge in understating their interplays and the resultant topologies. Here, we discover chiral polarization meron-like structures, which appear during the emergence and growth of quasi-2D ferroelectric nematic domains. The chirality can emerge spontaneously in polar textures and can be additionally biased by introducing chiral dopants. Such micrometre-scale polarization textures are the modified electric variants of the magnetic merons. Both experimental and an extended mean-field modelling reveal that the polarization strength plays a dedicated role in determining polarization topology, providing a guide for exploring diverse polar textures in strongly-polarized liquid crystals.

Rotation of the dislocation grid in multilayer FeSe films and visualization of electronic nematic domains via orbital-selective tunneling

Understanding the interplay of structural and electronic symmetry breaking in Fe-based high temperature superconductors remains of high interest. In this work we grow strain-patterned multilayer FeSe thin films in a range of thicknesses using molecular beam epitaxy. We study the formation of electronic nematic domains and spatially-varying strain using scanning tunneling microscopy and spectroscopy. We directly visualize the formation of edge dislocations that give rise to a two-dimensional edge dislocation network in the films. Interestingly, we observe a 45 degree in-plane rotation of the dislocation network as a function of film thickness, yielding antisymmetric strain along different directions. This results in different coupling ratios between electronic nematic domains and antisymmetric strain. Lastly, we are able to distinguish between different orthogonal nematic domains by revealing a small energy-dependent difference in differential conductance maps between the two regions. This could be explained by orbital-selective tip-sample tunneling. Our observations bring new insights into the dislocation network formation in epitaxial thin films and provide another nanoscale tool to explore electronic nematicity in Fe-based superconductors.