Lyotropic Nematic Liquid Crystals Research Articles

Time Dependence of Gel Formation in Lyotropic Nematic Liquid Crystals: From Hours to Weeks.

The combination of lyotropic liquid crystals (LLCs) and low-molecular-weight gelators (LMWGs) for the formation of lyotropic liquid crystal gels (LLC gels) leads to a versatile and complex material combining properties of both parent systems. We gelled the calamitic nematic NC phases of a binary and ternary system using the LMWG 3,5-bis-(5-hexylcarbamoyl-pentoxy)-benzoic acid hexyl ester (BHPB-6). This binary system consists of the surfactant N,N-dimethyl-N-ethyl-1-hexadecylammonium bromide (CDEAB) and water, whereas the ternary system consists of the surfactant N,N,N-trimethyl-N-tetradecylammonium bromide (C14TAB), the cosurfactant n-decanol, and water. Though containing similar surfactants, the gelled NC phases of the binary and ternary systems show differences in their visual and gel properties. The gelled NC phase of the binary system remains clear for several days after preparation, whereas the gelled NC phase of the ternary system turns turbid within 24 h. We investigated the time evolution of the gel strength with oscillation rheology measurements (a) within the first 24 h and (b) up to two weeks after gel formation. The shape of the fibers was investigated over different time scales with freeze fracture electron microscopy (FFEM). We demonstrate that despite their similarities, the two LLC gels also have distinct differences.

Flow-induced periodic chiral structures in an achiral nematic liquid crystal

Supramolecular chirality typically originates from either chiral molecular building blocks or external chiral stimuli. Generating chirality in achiral systems in the absence of a chiral input, however, is non-trivial and necessitates spontaneous mirror symmetry breaking. Achiral nematic lyotropic chromonic liquid crystals have been reported to break mirror symmetry under strong surface or geometric constraints. Here we describe a previously unrecognised mechanism for creating chiral structures by subjecting the material to a pressure-driven flow in a microfluidic cell. The chirality arises from a periodic double-twist configuration of the liquid crystal and manifests as a striking stripe pattern. We show that the mirror symmetry breaking is triggered at regions of flow-induced biaxial-splay configurations of the director field, which are unstable to small perturbations and evolve into lower energy structures. The simplicity of this unique pathway to mirror symmetry breaking can shed light on the requirements for forming macroscopic chiral structures.

Confinement twists achiral liquid crystals and causes chiral liquid crystals to twist in the opposite handedness: cases in and around sessile droplets.

We study the chiral symmetry breaking and metastability of confined nematic lyotropic chromonic liquid crystals (LCLCs) with and without chiral dopants. The isotropic-nematic coexistence phase of the LCLC renders two confining geometries: sessile isotropic (I) droplets surrounded by the nematic (N) phase and sessile nematic droplets immersed in the isotropic background. In the achiral system with no dopants, LCLC's elastic anisotropy and topological defects induce a spontaneous twist deformation to lower the energetic penalty of splay deformation, resulting in spiral optical textures under crossed polarizers both in the I-in-N and N-in-I systems. While the achiral system exhibits both handednesses with an equal probability, a small amount of the chiral dopant breaks the balance. Notably, in contrast to the homochiral configuration of a chirally doped LCLC in the bulk, the spiral texture of the disfavored handedness appears with a finite probability both in the I-in-N and N-in-I systems. We propose director field models explaining how chiral symmetry breaking arises by the energetics and the opposite-twist configurations exist as meta-stable structures in the energy landscape. These findings help us create and control chiral structures using confined LCs with large elastic anisotropy.

Shear-induced flow and dynamics of the nematic phase of Sunset Yellow lyotropic chromonic liquid crystal

ABSTRACT Fundamental understanding of rheological properties of lyotropic chromonic liquid crystals is inadequate compared to their thermotropic counterparts in spite of the growing interest in these materials. Here, we investigate the rheological properties of a lyotropic chromonic nematic liquid crystal (LCLC), prepared by dissolving 32 wt% of Sunset Yellow (SSY) in water. The flow curves (stress-strain rate, σ − γ ˙ ) at different temperatures exhibit three flow regimes R1, R2 and R3, respectively. At room temperature, in R1 ( γ ˙ < γ ˙ cl ) and R3 ( γ ˙ > γ ˙ cu ) the sample exhibits Newtonian flow behaviour ( σ ∝ γ ˙ ), whereas, in R2 ( γ ˙ cl < γ ˙ < γ ˙ cu ), it shows a power-law dependence ( σ ∼ γ ˙ β ). With increasing shear rate the change of stress from R1 to R2 is continuous, whereas the change is discontinuous from R2 to R3. We observe stress fluctuations under steady shear in regime R2 similar to those reported in worm-like micellar systems and in thermotropic twist-bend nematic liquid crystals. The non-monotonic flow behaviour and the stress dynamics are corroborated using in situ rheo-microscopy.

Gelled lyotropic nematic liquid crystals

ABSTRACT Lyotropic liquid crystal gels (LLC gels) are versatile materials which combine the anisotropy of a liquid crystal with the mechanical stability of a gel. We succeeded in gelling the rare calamitic nematic NC phase of two different micellar LLC systems with the low molecular weight gelator (LMWG) 3,5-bis-(5-hexylcarbamoyl-pentoxy)-benzoic acid hexyl ester (BHPB-6). The systems of choice are: (1) a binary LLC system with the surfactant N,N-dimethyl-N-ethyl-1-hexadecylammonium bromide (CDEAB) and water; (2) a ternary LLC system with the surfactant N,N,N-trimethyl-N-tetradecylammonium bromide (C14TAB), n-decanol, and water. The obtained gels show optical birefringence and no macroscopic flow. Freeze fracture electron microscopy (FFEM) images show ribbon-like gel fibres that form the gel network and can stack into thicker bundles, while small-angle X-ray scattering (SAXS) confirms the coexistence of the nematic NC phase and the gel network. The growth of the gel network was monitored with time-resolved SAXS measurements. Additionally, significant gel ageing effects were observed in rheology measurements, showing that the gel strengthens with time. With this study, we considerably broaden the pool of available surfactant-based LLC gels and pave a way to macroscopically aligned LLC gels with the goal to apply them as stimuli-responsive actuators and sensors.

Cover Feature: Chirality Transfer from an Innately Chiral Nanocrystal Core to a Nematic Liquid Crystal 2: Lyotropic Chromonic Liquid Crystals (ChemPhysChem 3/2023)

The Cover Feature illustrates the formation of homochiral nematic lyotropic chromonic liquid crystal tactoids formed by disodium cromoglycate in water by the addition of cromoglycate surface-modified cellulose nanocrystals (CNCs) as chiral solutes. Using CNCs with an inherently chiral core reveals that biorenewable chiral solutes can serve as suitably powerful and increasingly multipurpose chiral additives. More information can be found in the Research Article by Diana P. N. Gonçalves, Timothy Ogolla, and Torsten Hegmann.

Chirality Transfer from an Innately Chiral Nanocrystal Core to a Nematic Liquid Crystal 2: Lyotropic Chromonic Liquid Crystals.

The importance of and the difference between molecular versus structural core chirality of substances that form nanomaterials, and their ability to transmit and amplify their chirality to and within a surrounding condensed medium is yet to be exactly understood. Here we demonstrate that neat as well as disodium cromoglycate (DSCG) surface-modified cellulose nanocrystals (CNCs) with both molecular and morphological core chirality can induce homochirality in racemic nematic lyotropic chromonic liquid crystal (rac-N-LCLC) tactoids. In comparison to the parent chiral organic building blocks, D-glucose, endowed only with molecular chirality, both CNCs showed a superior chirality transfer ability. Here, particularly the structurally compatible DSCG-modified CNCs prove to be highly effective since the surface DSCG moieties can insert into the DSCG stacks that constitute the racemic tactoids. Overall, this presents a highly efficient pathway for chiral induction in an aqueous medium and thus for understanding the origins of biological homochirality in a suitable experimental system.

Lyotropic isotropic to columnar phase transition in RNA solutions

ABSTRACT RNA oligomers exhibit lyotropic chiral nematic and columnar liquid crystal phases. The lyotropic chiral nematic and columnar phases with special emphasize on the isotropic to columnar phase transition in RNA solutions are described based on Landau theory. A free energy is constructed based on the order parameters to describe the chiral nematic and columnar liquid crystal phases and isotropic to columnar phase transition. The lyotropic isotropic to columnar phase transition is found to be always first order. The birefringence has been calculated in the isotropic phase of isotropic to columnar phase transition. Theoretical observation of the existence of the lyotropic isotropic to columnar phase transition in RNA solutions is confirmed with experimental results.

Electro-optical properties of nematic lyotropic chromonic liquid crystals

The electro-optical properties of liquid crystals play an important role in liquid crystal displays. Lyotropic liquid chromonic crystals are attracting the interest of researchers because of its extending application in areas of chemistry, medicine, and biology. An experimental apparatus is constructed based on nematic lyotropic chromonic liquid crystal sunset yellow filled in the twisted nematic liquid crystal cell to obtain two primary parameters of the electro-optical properties, the threshold voltage and relaxation time. The threshold voltage and relaxation time of nematic lyotropic chromonic liquid crystals decrease with the increase of temperature or the decrease of concentration. The concentration of neutral ionic dopants like sodium chloride and magnesium sulfate is positively correlated with the threshold voltage and relaxation time of nematic lyotropic chromonic liquid crystals, while the concentration of alkaline ionic dopants like sodium hydroxide exhibits a negative correlation. The changes of the electro-optical properties with respect to the temperature, solute concentration, and dopant concentration are analyzed based on the alteration of the molecular self-assembly behaviors of lyotropic chromonic liquid crystals. In addition, the viscoelastic theory of nematic thermotropic liquid crystals is proved to be also applicable on nematic lyotropic chromonic liquid crystals for the first time.

Anchoring-induced nonmonotonic velocity versus temperature dependence of motile bacteria in a lyotropic nematic liquid crystal.

The elastic and viscous properties of lyotropic chromonic liquid crystals have a very sharp, often exponential temperature dependence. Self-propelled bacteria swimming in this viscoelastic medium induce director deformations which can strongly influence their velocity, and we study the temperature behavior of their motility in the whole range of the nematic phase. We observe experimentally that, with increasing temperature, while the viscosity drops exponentially and the frequency of the flagellum rotation grows linearly, the swimmers' speed first conventionally increases but then, above some crossover temperature, slows down and at the same time bacteria-induced director distortions become visible. It is shown that the physics behind this temperature-driven effect is in a sharp rise in the ability of the bacterium's flagellum to induce director deformations. As temperature increases, the splay and bend elastic constants sharply decrease and the anchoring extrapolation length of the flagellum surface gets shorter and shorter. At the crossover temperature the resulting effective anchoring effect dominates the fast dropping viscosity and the distortion strengthens. As a result, a fraction of the torque the flagellum applies for the propulsion is spent for the elastic degrees of freedom, which results in a bacterium slowdown. To find the director distortions, the flagellum is presented as a collection of anchoring-induced elastic monopoles, and the bacterium velocity is found from the balance of the energy spent for the propulsion and the viscous drag and nematodynamic dissipation.

Rotational viscosity of nematic lyotropic chromonic liquid crystals

Lyotropic chromonic liquid crystal has great application potential in the areas of the biology, medicine, and optics due to its biocompatibility, nontoxicity, biodegradability and birefringence. The rotating magnetic field method was used to measure the rotational viscosity of nematic Sunset Yellow, disodium cromoglycate and Sunset Yellow doping of magnesium sulfate, sodium chloride and sodium hydroxide at various temperatures and concentrations. The rotational viscosity of nematic lyotropic chromonic liquid crystals is positively correlated with the square of the average length of columnar aggregates, increasing with the increase of concentration, but decreasing exponentially as temperature increases. This paper established a universal empirical expression of the rotational viscosity of nematic lyotropic chromonic liquid crystals and presented that the largest relative deviation between theoretical calculation and experimental results is only 22.60%. The rotational viscosity of nematic lyotropic chromonic liquid crystals becomes intensive with the increasing of magnesium sulfate and sodium chloride concentrations, while it reduces with the increase of sodium hydroxide concentration. The “one-step method” is proposed here that one sample of nematic LCLCs at various temperatures can replace normal materials to evaluate the influences of the shape anisotropy of molecules or colloidal particles on the research target, such as physical property parameters, depletion interaction potentials, and so on. This method is based on the self-assembly ability difference of lyotropic chromonic liquid crystal molecules at various temperatures and has been proved to be reliable by comparison with the results obtained from many samples of nematic 5CB, CBP5, 5PCB, et al.

Supramolecular Aggregates: Hardness Plus Softness.

The properties of supramolecular aggregates cross several disciplines, embracing the sciences of nature and joining theory, experiment, and application. There are few articles centering on the problems of interdisciplinarity, and this paper gives an alternative approach, starting with scientific divulgation, bringing concepts from their origin, to facilitate the access of young scientists to the scientific content. Didactic examples are taken from the experience of the author in changing directions of research due to several circumstances of life (including maternity), starting from the view of a rigorous student of physics and evolving to several subjects in chemistry. The scientific part starts with concepts related to nuclear interactions, using the technique of neutron scattering in reactors, and evolves to research in molecular physics. Finally, it arrives at the academic context, with research in condensed matter physics, with X-ray and other techniques, starting with detergents forming nematic lyotropic liquid crystals and the phase transition sequence of isotropic to nematics to hexagonal. The scientific subjects evolved to biological and bio-inspired liquid crystals, including DNA and also specific lipids and phospholipids in biomimetic membranes. Special attention is given to the question of distribution of matter in these complex systems and the non-trivial connections between biochemistry, structures, auto-aggregation, and biology.

Interplay between Confinement, Twist Elasticity, and Intrinsic Chirality in Micellar Lyotropic Nematic Liquid Crystals.

Recent studies have shown that lyotropic nematic liquid crystals (LLCs) are exceptional in their viscoelastic behavior. In particular, LLCs display a remarkable softness to twist deformations, which may lead to chiral director configurations under achiral confinement despite the absence of intrinsic chirality. The twisted escaped radial (TER) and the twisted polar (TP) are the two representative reflection symmetry breaking director configurations in the case of cylindrical confinement with homeotropic anchoring. We demonstrate how such reflection symmetry breaking of micellar LLCs under cylindrical confinement is affected by intrinsic chirality, introduced by the addition of a chiral dopant. Similarities and differences between the effects of intrinsic chirality on the defect-free TER configuration, and on the TP configuration incorporating two half-unit twist disclination lines, are discussed. In the TP case, topological constraints facilitate stable heterochiral systems even in the presence of a small amount of chiral dopant, with unusual regions of rapidly reversing handedness between homochiral domains. At moderate dopant concentrations, the TP structure becomes homochiral. At high dopant concentrations, for which the induced cholesteric pitch is much smaller than the diameter of the capillary, the cholesteric fingerprint structure develops.

Lyotropic nematic liquid crystals: interplay between a small twist elastic constant and chirality effects under confined geometries

ABSTRACT There are two classes of liquid crystals (LCs): Thermotropic LCs where the organic molecule represents the anisotropically shaped mesogen and lyotropic LCs, where the mesogens are non-spherical supramolecular assemblies dispersed in a solvent. Various kinds of lyotropic nematic liquid crystals (LLCs) exist, differing in shape and composition of the aggregates: Chromonic, polymeric or micellar LLCs.However, micellar LLCs are the most ubiquitous kind of LLCs, occurring in soaps and biological systems. Although these nematic lyotropics are so different, they exhibit – contrary to thermotropic nematics – spontaneous reflection symmetry breaking under confinement. We discuss spontaneous mirror symmetry breaking under capillary confinement for a standard micellar ND phase and give a detailed director field structure of the twisted polar configuration. Furthermore, we measure for the first time a complete set of viscoelastic properties of a micellar nematic LLC, finding a small twist elastic constant K 22, which is at least one order of magnitude smaller than K 11 and K 33. As demonstrated by the following work, a small K 22 and therefore spontaneous mirror symmetry breaking under confinement is rather the rule than the exception in lyotropic nematics and constitutes a unique difference between lyotropic and typical thermotropic nematics, for which the well-known one-constant approximation holds. Abbreviations: LC: Liquid Crystal; LLC: Lyotropic liquid crystal; TER: twisted escaped radial director configuration; TP: twisted polar director configuration; : director; K 11: splay elastic constant of the nematic phase; K 22: twist elastic constant of the nematic phase; K 33: bend elastic constant of the nematic phase; s: strength of a defect/disclination; ER: escaped radial director configuration; CDEAB: N,N-ethylhexadecylammonium bromide; DOH: decan-1-ol; ND: nematic phase of disk-shaped building blocks; NC: nematic phase of rod-shaped building blocks; H-field: magnetic field; PR: planar radial director configuration; PP: planar polar director configuration; RR: right handed – right handed; LL: left handed – left handed; RL: right handed – left handed; LR: left handed – right handed.

Production of giant unilamellar vesicles and encapsulation of lyotropic nematic liquid crystals

We describe a modified microfluidic method for making Giant Unilamellar Vesicles (GUVs) via water/octanol-lipid/water double emulsion droplets. At a high enough lipid concentration we show that the de-wetting of the octanol from these droplets occurs spontaneously (off-chip) without the need to use shear to aid the de-wetting process. The resultant mixture of octanol droplets and GUVs can be separated by making use of the buoyancy of the octanol. A simpler microfluidic device and pump system can be employed and, because of the higher flow-rates and much higher rate of formation of the double emulsion droplets (∼1500 s-1 compared to up to ∼75 s-1), it is easier to make larger numbers of GUVs and larger volumes of solution. Because of the potential for using GUVs that incorporate lyotropic nematic liquid crystals in biosensors we have used this method to make GUVs that incorporate the nematic phases of sunset yellow and disodium chromoglycate. However, the phase behaviour of these lyotropic liquid crystals is quite sensitive to concentration and we found that there is an unexpected spread in the concentration of the contents of the GUVs obtained.

STUDY ON ROTATIONAL VISCOSITY OF NEMATIC LYOTROPIC LIQUID CRYSTAL

Lyotropic liquid crystal exhibits excellent biocompatibility, non-toxicity, biodegradability, optical anisotropy, and electromagnetic anisotropy. It has been widely used in the areas of biology, medical engineering and liquid crystal displaying, such as cell interaction, nerve stimulation transmission, fat absorption and intelligent drug transport. In this paper, the rotational viscosity of the lyotropic liquid crystal Sunset Yellow in the nematic phase at different temperature and solution concentration is measured by the rotating magnetic field method. Combined with the self-assembly process of lyotropic liquid crystal molecules, the variation of rotational viscosity of lyotropic liquid crystal in the nematic phase with temperature and solution concentration is analyzed theoretically. The results show that the rotational viscosity of the lyotropic liquid crystal is positively correlated with the square of the average length of the self-assembled columns, increasing with the increase of concentration, but decreasing exponentially with the increase of temperature. The empirical expression of rotational viscosity related to temperature and concentration of nematic lyotropic liquid crystal is constructed. The calculated results of the empirical expression are in good agreement with the experimental values, with the maximum error of 18.56${\%}$. A new indirect of obtaining the shear energy of lyotropic liquid crystal by using rotating rheometer is proposed. The shear energy of lyotropic liquid crystal increases with the increase of solution concentration, but it is hardly related with the change of temperature in the experimental range. The shear energy obtained in this paper is in good agreement with the results of Joshi et al. who used the X-ray detection method, with the maximum error of 3${\%}$. The influence of length-diameter ratio of the columns on the rotational viscosity is investigated by using the variation of self-assembly capacity of liquid crystal molecules with temperature. The one-step method measurement is proposed, which greatly reduces the expenses and complexity of related experimental researches.

Characterization of an anionic membrane mimetic with natural phospholipid content and magnetic orienting capabilities

The cellular membrane is a highly complex and dynamical structure, with variable and inhomogeneous composition, which makes detailed structural studies a very difficult task. 2H-NMR has become one of the most useful tools for many of such studies, however this requires the use of simpler structures that behave as close as possible to the cellular membrane, i.e. membrane mimetics. In this article we present a new anionic nematic lyotropic liquid crystal, with an important content of a natural mixture of phospholipids isolated from soybean, which can be employed as membrane mimetic. The capability of the mesophase to spontaneously orient when exposed to an external magnetic field, enables it for studies about distribution, dynamics and mobility of mesophase components, as well as dissolved substrates, employing 2H-NMR. A 2H-NMR characterization of the mimetic is performed by using different deuterated probes. To validate its permeability properties as membrane, the permeating capability of Benzocaine and the inability of Levodopa to do so were tested. For an atomic detailed characterization of both, the new mimetic and the process of crossing the interface, we also present a molecular dynamics simulation calibrated to reproduce experimental 2H-NMR results.

Extremely small twist elastic constants in lyotropic nematic liquid crystals

Recent measurements of the elastic constants in lyotropic chromonic liquid crystals (LCLCs) have revealed an anomalously small twist elastic constant compared to the splay and bend constants. Interestingly, measurements of the elastic constants in the micellar lyotropic liquid crystals (LLCs) that are formed by surfactants, by far the most ubiquitous and studied class of LLCs, are extremely rare and report only the ratios of elastic constants and do not include the twist elastic constant. By means of light scattering, this study presents absolute values of the elastic constants and their corresponding viscosities for the nematic phase of a standard LLC composed of disk-shaped micelles. Very different elastic moduli are found. While the splay elastic constant is in the typical range of 1.5 pN as is true in general for thermotropic nematics, the twist elastic constant is found to be one order of magnitude smaller (0.30 pN) and almost two orders of magnitude smaller than the bend elastic constant (21 pN). These results demonstrate that a small twist elastic constant is not restricted to the special case of LCLCs, but is true for LLCs in general. The reason for this extremely small twist elastic constant very likely originates with the flexibility of the assemblies that are the building blocks of both micellar and chromonic lyotropic liquid crystals.

Correlation of interfacial dilational rheology and processing of 2D Materials liquid crystals: A case-study of graphene oxide liquid crystal phases

Due to the high aspect-ratio of 2D graphene oxide nanosheets in water, the lyotropic nematic liquid crystal phases of graphene oxide dispersions can be spontaneously formed. The unique visco-elastic characteristics of such liquid crystals can make them a novel category of soft materials. The fundamental insights ensued in this work can be used as a basis for the development of new guidelines for the processing of soft 2D materials by means of vastly available traditional fabrication methods. The concentration range of isotropic, biphasic and nematic phases was determined by employing polarized optical microscopy. Using 2D sheets with a high aspect ratio (over 35000) resulted in the formation of the biphasic region at a concentration as low as 0.05 g/l and the fully nematic region at concentrations higher than 0.25 g/l. Shear rotational rheology and interfacial dilational rheology were employed, as the tools of choice, to correlate the nematic phase formation with the processability and the change in modulus. Our results underpin the argument that the combination of the low concentration of 2D sheets in the supporting media and high elastic modulus can facilitate the use of graphene oxide based formulations for an array of processing and fabrication techniques including but not limited to wet-spinning, electro-spraying, inkjet printing, and 3D printing.

Self-assembly properties of carboxylated tunicate cellulose nanocrystals prepared by ammonium persulfate oxidation and subsequent ultrasonication

Tunicate cellulose, extracted from the marine animal, has drawn increasing attention as the high crystallinity and aspect ratio. However, it is hard to prepare tunicate cellulose nanocrystals (tCNCs) with narrow size distribution in the traditional way, especially for the carboxylated samples, which also affects their lyotropic liquid crystal behavior to a certain extent. Herein, carboxylated tCNCs with uniform nanoscale dimensions and high surface charges density were prepared through ammonium persulfate (APS) oxidation and ultrasonic post-processing. Of particular interest, the formation of carboxylated tCNCs lyotropic chiral nematic liquid crystals was observed for the first time, which displayed obvious birefringence and fingerprint texture. Meanwhile, it was found that the critical concentration of phase separation for tCNCs suspension was around 3.5 wt% from the phase diagram. This study provides an efficient way to fabricate carboxylated tCNCs, and the self-assembly properties may lead to great potential applications in constructing advanced functional materials.