Thermotropic Nematic Liquid Crystals Research Articles

Charge Modulation at the Liquid Crystal Droplet-Aqueous Interface Enables Ultrasensitive, Nonspecific Protein Detection.

Thermotropic nematic liquid crystals (LC) have been utilized to sense/detect various analytes such as polymers, surfactants, lipids, etc. However, their use for protein detection depends on pre-adsorbed molecules, co-nematogens, or biomolecular agents for specificity.This approach impedes the platform's sensitivity with a detection limit for the folded proteins generally reported in the micromolar concentration range. Here, this work provides fundamental insights into the type of molecular interactions and their modulation that can drive ultrasensitive protein detection at an LC microdroplet/aqueous interface formed without adding an auxiliary co-nematogen. Using ultraviolet (UV) light treated 4-cyano-4'-pentylbiphenyl (5CB) LC and a flow-focused microfluidic device, we prepared different populations of monodisperse and highly negatively charged microdroplets in water. Adding an aqueous solution of various model proteins(α-synuclein, α-chymotrypsin, myoglobin, or bovine serum albumin, BSA) with different secondary structures and surface charges triggers a rapid radial- to bipolar-defect transition in these microdroplets. Isothermal titration calorimetry measurement and molecular dynamic simulation studies attribute this to the dominant electrostatic force-mediated adsorption of proteins at the LC/aqueous interface.Further, bioconjugation-based variation of protein surface charge allows tuning their detection limit.These findings can provide crucial physical cues for designing responsive LC systems and establishing a foundation for developing versatile, molecularly tailored, and highly specific biomolecular detection platforms.

Nano and micro-structural complexity of nematic liquid crystal configurations

Of our interest are frustration-driven pattern generating mechanisms in systems which in bulk equilibrium display spatially homogeneous long-range orientational order in absence of perturbations. As testbed material, we select thermotropic nematic liquid crystals. In bulk, they exhibit weakly discontinuous order–disorder phase transformation on varying temperature where the ordered nematic phase features spatially uniform axial order along an arbitrary symmetry breaking direction. However, due to continuous symmetry breaking (CSB) the established order is extremely susceptible to various perturbations which are in real systems in general always present. We theoretically illustrate how diverse complex patterns could be excited. Particularly intriguing configurations could appear if topological defects are present that could be generated via CSB. Our analysis is based on a relatively simple Lebwohl-Lasher-type model in which we could get analytical insight into phenomena of our interest. Using it we illustrate history dependent early stage isotropic-nematic phase evolution and final patterns in presence of “impurities” (e.g., nanoparticles). We show how characteristic effective interaction characteristics predict qualitatively different emerging patterns. Our analysis is based on CSB which is ubiquitous in nature. Consequently, demonstrated mechanisms are expected to manifest also in other condensed matter systems whose ordered phase is formed via CSB. We illustrate how kinetics and impurities could impact key structural properties of the systems of our interest.

Effect of doping of organo-soluble carbon dots on ionic relaxation and conductivity of planar anchored cyanobiphenyl based nematic liquid crystal

We demonstrate here the striking effect of organo-soluble carbon dots (CDs, diameter ∼7- 8 nm) on the tunability of ionic relaxation and conductivity of the host, planar anchored thermotropic nematic liquid crystal (5CB, 4-Cyano-4′-pentylbiphenyl). The optical texture, frequency, temperature, and concentration dependent dielectric studies of pristine 5CB and CDs-5CB composites have been performed using the polarising optical microscope and high-resolution dielectric spectroscopy, respectively. The doping of 0.5 wt% CDs into planar anchored 5CB is resulted into the vertical alignment confirmed through the cross-polarised optical microscope. The doping of CDs in 5CB liquid crystal is resulted into an unprecedented faster ionic relaxation with an abatement of the ionic amplitude at room temperature. The shift in the relaxation frequency for 0.5 wt% CDs-5CB composite (induced vertical alignment) as compared to pristine 5CB (planar aligned) at 28 °C (nematic) is found to be ∼13 folds which is further increased to ∼18 folds at 38 °C (isotropic). A similar trend in the conductivity curve is also observed which further confirms the significant enhancement in conductivity value with increase in temperature and concentration of dopant CDs. The observation of enhanced ionic conductivity and faster ionic relaxation in CDs-5CB composites as compared to pristine 5CB are attributed to the complete change of planar to vertical alignment by doping of CDs on planar anchored liquid crystal sample cells. It appears that the plausible strong interaction between hydrocarbon chains of CDs and 5CB molecules through anchored substrate surfaces with increasing concentration of CDs and decrease in rotational viscosity of host 5CB with increasing temperature certainly plays a pivotal role in the tuning of ionic relaxation and conductivity in the liquid crystal sample cells. We certainly believe that our results would catalyze the cognizance of the ionic relaxation and conductivity in CDs-5CB composites and other related systems. Moreover, such composites with tunable ionic relaxation/conductivity would be certainly helpful in the fabrication of various tuneable devices such as wave front corrector, low frequency oscillation generators, dynamic light scattering, and so on.

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.

Synthesis and mesomorphic properties of swallow-tailed liquid crystal monomers and polymers

In this article, we designed and synthesized two novel swallow-tailed liquid crystal monomers (M1 and M2) with different chain lengths and rigidities centered on 3,4,5-trihydroxybenzoic acid and two series of side-chain liquid crystals polymers PN1 and PN2. All liquid crystal monomers and polymers were conformed to the molecular designs by FT-IR and NMR. The liquid crystal properties were tested by DSC and POM, and the results showed that M1 and M2 were thermotropic enantiotropic nematic liquid crystals and the side-chain liquid crystal polymers PN1 and PN2 series were typical amorphous liquid crystal polymers. With the increase of monomer content, the glass transition temperatures and clearing points of PN1 and PN2 series enhanced. With the increase of the number of mesogens in the liquid crystal monomer, the rigidity was enhanced, and the liquid crystal performance of the polymer was better.

Phase behavior of nematic-nanoparticle mixtures

We study the effects of nanoparticles (NPs) on thermotropic nematic liquid crystals (LCs) in relatively dilute NP–LC mixtures. We are interested in the fundamental generic mechanisms that quantitatively and qualitatively affect the phase behavior of LCs. A simple molecular field analysis shows that a phase transition will likely occur upon entry into the ordered phase. Moreover, the interaction between nematogenic NPs and LCs could force a sergeant–soldier-like behavior, in which only the phase behavior of one component is affected despite the symmetric appearance of the coupling term. When NPs are anisotropic, their influence on LC phase behavior can be qualitatively different depending on the anchoring, even in the absence of the disorder. We illustrate numerically that a random-field-type disorder might impose either short-range, quasi-long-range, or even long-range order, which might survive.

Efficient methodology with potential uses of Fresnel diffractometry for real-time study of uniaxial nematic liquid crystal phase transitions

Due to the different features of their various phases and expanding physical understanding, liquid crystals (LCs) play a fundamental and crucial role in contemporary technology. Recently, they have also been utilised in adaptive optics, active switching, and next-generation displays for augmented and virtual reality. In order to determine relevant quantities for thermotropic uniaxial nematic liquid crystals (NLCs), our aim is to assess the applicability of a ground-breaking method. The method being discussed is based on Fresnel diffraction (FD) from phase objects, which has been employed over the past 20 years in several accurate and precise metrological applications. Using a phase step and quantitatively registering the visibility of the diffraction patterns, diffractometry can transform any change in the order of LCs brought on by a change in temperature into a change in the optical phase. Owing to its low sensitivity to environmental vibrations, inherent compactness, and ease of set up, diffractometry can be used much more effectively than interferometry. Additionally, as a special major feature, if the various phases of a LC have the required transparency, it is possible to record the number of phases in the bulk of the LC, the surface ordering, and the approximate temperature of the phase transitions in a single-shot imaging by applying a suitable temperature gradient. The numerical computations and practical data comparisons from our theoretical considerations demonstrate a very high level of agreement with the output from other currently used methodologies. As we shall see, by addressing some of the faults and inadequacies of existing techniques, this strategy has the potential to both complement and strengthen them.

Stable Assemblies of Topological Defects in Nematic Orientational Order.

We considered general mechanisms enabling the stabilization of localized assemblies of topological defects (TDs). There is growing evidence that physical fields represent fundamental natural entities, and therefore these features are of interest to all branches of physics. In general, cores of TDs are energetically costly, and consequently, assemblies of TDs are unfavorable. Owing to the richness of universalities in the physics of TDs, it is of interest to identify systems where they are easily experimentally accessible, enabling detailed and well-controlled analysis of their universal behavior, and cross-fertilizing knowledge in different areas of physics. In this respect, thermotropic nematic liquid crystals (NLCs) represent an ideal experiment testbed for such studies. In addition, TDs in NLCs could be exploited in several applications. We present examples that emphasize the importance of curvature imposed on the phase component of the relevant order parameter field. In NLCs, it is represented by the nematic tensor order parameter. Using a simple Landau-type approach, we show how the coupling between chirality and saddle splay elasticity, which can be expressed as a Gaussian curvature contribution, can stabilize Meron TDs. The latter have numerous analogs in other branches of physics. TDs in 2D curved manifolds reveal that the Gaussian curvature dominantly impacts the assembling and stabilization of TDs. Furthermore, a strong enough curvature that serves as an attractor for TDs is a respective field that could be imposed in a fast enough phase transition. Assemblies of created TDs created in such a disordered environment could be stabilized by appropriate impurities.

Magnetic induced terahertz modulation characteristics based on ferromagnetic nematic liquid crystals

In recent years, solid state terahertz (THz) modulators have obtained rapid progress with the widespread use of two-dimensional (2D) materials in the field of THz; however, challenges remain in preparing flexible THz modulators. In this study, flexible ferromagnetic nematic materials were prepared by doping thermotropic nematic liquid crystals 5CB into magnetic fluids, and the influence of water was reduced by a self-made cyclic olefin copolymer (COC) microfluidic chip. THz modulation characteristics of magnetic fluid and ferromagnetic nematic liquid crystal (FNLC) under the induction of external magnetic field were compared using a THz time domain spectroscopy system. Under the action of a 91 mT magnetic field, the magnetic fluid has a maximum modulation depth (MD) of 54%. Under the same magnetic field, the maximum MD of the FNLC materials increase to 78% because of the rearrangement of Fe3O4 nanoparticles induced by the topological defect of the liquid crystal. We demonstrate that the magneto-optical effect is significantly enhanced in the ferromagnetic nematic liquid crystal hybrid system. This strategy of doping thermotropic nematic liquid crystals to enhance the magneto-optical effect has great potential for THz filtering, modulation, and sensing applications.

Synthesis and phase behavior of Y-shaped Schiff base liquid crystal oligomers

ABSTRACT In this paper, we designed and synthesised three Schiff base mesogenic units X-YP and six Y-shaped liquid crystal oligomers X-AYP and X-BYP (×= -CH3O, -CH3, -Cl). Molecular structures of the prepared compounds were confirmed via FT-IR, 1H NMR and elemental analysis. The mesomorphic properties were investigated by differential scanning calorimetry (DSC) and polarised light microscopy (POM). The photochemical properties of Y-shaped liquid crystal oligomers were preliminarily studied by UV-Vis spectrum. The results indicated that X-YP containing OCH3 and Cl substituents exhibited mesophase, while X-YP with CH3 substituent was non-mesogenic. The synthesized Y-shaped liquid crystals X-AYP and X-BYP both had liquid crystal properties and exhibited nematic phase during heating and cooling, which belonged to thermotropic enantiotropic nematic liquid crystals. The polarity of the terminal groups and the number of mesogenic units had a significant effect on the mesophase behaviour of Y-shaped liquid crystals. The absorption intensities of X-AYP and X-BYP in the UV spectra were affected by the terminal substituents.

Liquid crystal elastomers containing azobenzene homologues as crosslinkers—synthesis and characterization

ABSTRACT In this paper, we designed and synthesised three H-shaped liquid crystal monomers ABOP-n (n = 4,6,8) and a straight-chain liquid crystal monomer M. Then, we used three H-shaped liquid crystal monomers as crosslinking agents, which were graft-copolymerised with M and polymethylhydrosiloxane (PMHS), respectively, to synthesise three series of liquid crystal elastomers P 1 –P 3 . The products were structurally well characterised by elemental analysis, 1HNMR, and FT-IR. The phase behaviours were investigated using the polarising microscope (POM) and differential scanning calorimetry (DSC). The results showed that ABOP-8, ABOP-6, ABOP-4 and M were thermotropic enantiotropic nematic liquid crystals, exhibiting schlieren texture during heating, and spherulite texture and schlieren texture during cooling. The photochemical properties of H-shaped liquid crystal monomers were preliminarily studied by UV-Vis spectra, and it was found that they have good photosensitivity. Both the P 1 –P 3 series were typical non-crystalline polymers with anisotropic coloured textures. With the increase of crosslinking monomer content, the glass transition temperature of the P 1 –P 3 series both increased, while the isotropic temperature showed a trend of first decreasing and then increasing.

Synthesis and phase behaviour of a class of comb-like cholesteric azobenzene liquid crystalline polymers

ABSTRACT In this paper, we designed and synthesised two azo liquid crystal monomers (M1 and M2) and a chiral liquid crystal monomer N, followed by side-chain graft copolymerisation with polymethylhydrosiloxane (PMHS) to synthesize comb-like liquid crystalline polymers PN1 and PN2. The structures of all liquid crystal monomers and polymers conform to the molecular designs by FT-IR and 1H NMR. The liquid crystal properties were tested by DSC and POM, and the results showed that M1 and M2 were thermotropic enantiotropic nematic liquid crystals and N was a thermotropic enantiotropic cholesteric liquid crystal. Both the PN1 and PN2 series were amorphous polymers. The PN1 and PN2 series exhibited a typical Grand-jean texture. The glass transition temperature and isotropic temperature of the PN1 and PN2 series decreased first and then increased with the increase of azo monomer M content. And the glass transition temperature and isotropic temperature of the PN1 series were higher than the PN2 series. Both the PN1 and PN2 series had good thermal stability. The mesophase range of the PN1 series was 173.5°C~181.5°C, and the PN2 series was 162.5°C~172.9°C.

Enhancing physical characteristics of thermotropic nematic liquid crystals by dispersing in various nanoparticles and their potential applications

Enhancing physical characteristics of thermotropic nematic liquid crystals by dispersing in various nanoparticles and their potential applications

Label-Free Detection of Ochratoxin A Using Aptamer as Recognition Probe at Liquid Crystal-Aqueous Interface

Aqueous interfaces of stimuli-responsive, thermotropic nematic liquid crystals (LC) have been utilized in the design of biosensing platforms for a range of analytes. Owing to the orientational changes in LC, these interfaces can easily report aptamer-target binding events at the LC-aqueous interface. We demonstrate a label-free, simple and robust technique for the detection of Ochratoxin A (OTA) using aptamer as the recognition probe. The self-assembly of CTAB (cetyltrimethylammonium bromide; cationic surfactant) at aqueous-LC interface gives a homeotropic orientation of LC. In presence of negatively charged OTA specific aptamer, aptamer forms a complex with CTAB. Formation of aptamer-CTAB complex results in ordering transition of LCs to planar/tilted. In presence of OTA, OTA forms a strong and stable G-quadruplex structure of aptamer that results in the redistribution of CTAB at LC-aqueous interface and leads to homeotropic orientation of LC. The designed LC aptasensor exhibits a detection limit of 0.1 nM. We observed that the sensitivity of LC aptasensor was affected by the pH and ionic strength. In addition, we demonstrated the applicability of the designed LC aptasensor for the detection of OTA in tap water and apple juice. This approach offers advantages over the conventional detection methods in terms of fabrication, ease of operation, and analysis.

Double-twisted nematic director configurations in cylindrical capillaries with a photocontrollable angle of twist.

The orientational configurations of thermotropic nematic liquid crystal in cylindrical capillaries with nondegenerated planar surface anchoring are investigated. The boundary conditions were determined by a photoaligning coating on the inner wall of the capillary treated with a linearly polarized UV light while rotating the capillary around its long axis, thus providing the easy alignment axis perpendicular to the polarization direction of illuminating light. By changing the angle between the incident light polarization and the capillary axis, this procedure allows us to realize axially symmetric twisted structure with any angle of twist, ranging from a trivial axial alignment with zero twist (when the UV polarization is perpendicular to the capillary axis) to the 180^{∘}-twisted configuration (when the UV polarization is along the capillary). These two extreme configurations together with an intermediate configuration induced by UV light polarized at an angle of 45^{∘} to the capillary axis were produced and their director profiles were studied using polarizing microscopy techniques. UV light intensity and polarization distribution over inner capillary surface covered with photoaligning layer were analyzed using optical ray tracing calculations. An analytical model of the observed configurations of the liquid crystal director field is proposed, which takes into account the finite anchoring on the capillary surface. By comparing the theoretically calculated and experimentally estimated director profiles, the liquid crystal anchoring energy on the photoaligned capillary surface was estimated.

The yielding of defect-entangled dispersions in a nematic solvent

Oscillatory rheology, at both small and large (LAOS) amplitudes, was performed to measure the dynamic response of a soft solid, formed on dispersing colloids into a thermotropic nematic liquid crystal at volume fractions of ϕ>18%. Due to weak homeotropic anchoring of nematogens at colloid surfaces, a Saturn-ring defect line, known as a “disclination,” encircles each particle and entangles with neighboring Saturn-ring disclinations [Wood et al., Science 334, 79–83 (2011)]. We present the first experimental investigation of the yielding behavior of the resulting gel to reveal the underpinning physics. Results reveal that the frequency response of the composite is independent of volume fraction ϕ, an indication that the dispersed phase simply increases the density of disclinations spanning the composite without further effect. Beyond the linear viscoelastic regime (LVR), LAOS experiments indicate the composite is an elastoplastic fluid exhibiting both strain-hardening and shear-thinning behaviors with Chebyshev coefficients e3>0 and ν3<0, respectively. We deduce that the disclination number density nd is constant until the strain amplitude is sufficient to break disclinations leading to shear-thinning behavior beyond the LVR. A simple theory is introduced revealing that the Ericksen number Er determines the onset of flow, when Er>1, generating a strain-hardening response since Frank elasticity resists reorientation of molecular alignment within confined nematic domains. Above a critical frequency ωc, the loss modulus G″ increases slowly due to enhanced viscosity within confined nematic domains, G″∝ω1/2 [Larson, R. G., The Structure and Rheology of Complex Fluids (Oxford University, 1999), p. 463]. Observation of this behavior in a small-molecule nematic solvent provides insights into the physics of flow behavior in other, more complex, defect-mediated liquid crystalline structures exhibiting similar properties [Colby, Europhys. Lett. 54, 269–274 (2001); Sahoo and Dhara, Liq. Cryst. 44, 1582–1591 (2017); and Romo-Uribe, Polym. Adv. Technol. 32, 651–662 (2021)].

Synthesis and mesomorphic properties of new azo H-shaped liquid crystal compounds

ABSTRACT Using tartaric acid as the core, six azo H-shaped liquid crystal compounds M1-M6 with different terminal substituents were synthesised. All products were structurally well characterised by elemental analysis, 1HNMR, and FT-IR. The phase behaviours were investigated by means of polarised optical microscopy and differential scanning calorimetry. M1-M6 were thermotropic enantiotropic nematic liquid crystals. They all showed schlieren texture in the process of heating up, and exhibited spherulite texture in the process of cooling down. M1 and M4 whose terminal substitution was nitro had the widest liquid crystal range in the same series of liquid crystal compounds, which were 135.1°C and 96.1°C, respectively, under heating cycle and 167.4°C and 112.7°C, respectively, under cooling cycle. The results show that the terminal substituents and flexible chain have important influence on the mesogenic phase and range. With the increase of flexibility, the mesogenic range of liquid crystal compounds becomes smaller when the end groups are the same. And when the flexible chain is the same, the greater the induced and polarised force of the terminal substituents, the better the mesogenic properties of the liquid crystal compounds.

Flexoelectric effects on escaped twisted configurations of nematic liquid crystal 5CB with chiral dopants in a cylindrical cavity

ABSTRACT In a cylindrical cavity with a degenerate planar anchoring, the lyotropic chromonic nematic liquid crystals without intrinsic chirality tend to exhibit an escaped twisted configuration for the large saddle-splay-to-twist elastic modulus ratio, which is not found in most thermotropic liquid crystals. In this work, we consider thermotropic nematic liquid crystal 5CB with a chiral dopant confined in a cylindrical cavity with a degenerate planar anchoring. Based on the elastic theory of liquid crystal and the variation method, Euler equation and boundary conditions are obtained. The numerical results show that the director of 5CB forms an escaped twisted configuration, and the twist angle is influenced by the chiral parameter and the saddle-splay elastic constant. Under an axial electric field, the dielectric effect and the flexoelectric effect affect the LC director distribution. The single flexoelectric coefficient e 1 has a large radial orientation effect on the director. The results provide a clearer understanding of the director configuration in the cylindrical cavity under the electric field and a theoretical basis for the measurement of the flexoelectric coefficients.

Broadband Spectral Domain Interferometry for Optical Characterization of Nematic Liquid Crystals

In this paper, broadband Spectral Domain Interferometry provides single-shot optical characterization of dispersive thermotropic nematic liquid crystals. The proposed experimental setup enables measuring the chromatic dispersion, the extended Cauchy equation parameters knowing the optical index for one wavelength, and the thermo-optical coefficients to ascribe the dependence of the optical index with the inner temperature. The analysis is applied to the commonly known E 7 mixture and to M L C 2132 , whose chromatic properties are not referenced although the mixture is commercial, demonstrating the latter’s interest for electro-optical or thermo-optical applications of thick nematic liquid crystal cells.

First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics

We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.