Nematic Phase Research Articles

Polymerisation of twist-bend nematic textures for electro-optical applications.

Polymer-stabilised liquid crystals (PSLCs) have recently been used to maintain the focal conic domains (FCDs) typical of the smectic A phase in the nematic phase for smart window applications. The newly discovered twist-bend nematic phase of bent-shaped dimers also exhibits FCDs due to its pseudo-layered structure. The variety of topological defects in the NTB phase is arguably even greater than in the smectic A phase, but the NTB phase is often metastable and usually crystallises at room temperature, which hinders its use in electro-optical applications. Here we show how different textures (FCDs, rope-like texture, double helices) of the NTB phase can be polymerised and then maintained in the nematic phase, at room temperature. This allows us to combine in PSLCs the optical properties of these defects, the thermal stability of the nematic phase and its reversible response to an electric field. We also show that the polymerised FCDs of the NTB phase could be used in smart glass applications and that the polymerised rope-like texture could be of interest for optical modulators and beam steering. In addition, the polymerisation of double helices could help to better understand their formation and structure in the NTB phase. More fundamentally, our work shows that despite the lack of density modulation, the textures of the NTB phase, thanks to its periodic character, can be exploited in the same way as those of the smectic A phase.

Motile bacteria crossing liquid-liquid interfaces of an aqueous isotropic-nematic coexistence phase.

In nature, bacteria often swim in complex fluids, but our understanding of the interactions between bacteria and complex surroundings is still evolving. In this work, rod-like Bacillus subtilis swims in a quasi-2D environment with aqueous liquid-liquid interfaces, i.e., the isotropic-nematic coexistence phase of an aqueous chromonic liquid crystal. Focusing on the bacteria motion near and at the liquid-liquid interfaces, we collect and quantify bacterial trajectories ranging across the isotropic to the nematic phase. Despite its small magnitude, the interfacial tension of the order of 10 μN m-1 at the isotropic-nematic interface justifies our observations that bacteria swimming more perpendicular to the interface have a higher probability of crossing the interface. Our force-balance model, considering the interfacial tension, further predicts how the length and speed of the bacteria affect their crossing behaviors. Investigating how a phase change affects bacterial motion, we also find, as soon as the bacteria cross the interface and enter the nematic phase, they wiggle less, but faster, and that this occurs as the flagellar bundles aggregate within the nematic phase. Given the ubiquity of multi-phases in biological environments, our findings will help to understand active transport across various phases.

Chiral cyanobiphenyl dimers – significance of the linking group for mesomorphic properties and helical induction

Chiral cyanobiphenyl dimers display rarely observed room-temperature twist-bend nematic (NTB) phase when the linking group is an ester, while the amide analogues are strong inducers of helical organization in the chiral nematic phase.

How reciprocity impacts ordering and phase separation in active nematics?

Active nematics undergo spontaneous symmetry breaking and show phase separation instability. Within the prevailing notion that macroscopic properties depend only on symmetries and conservation laws, different microscopic models are used out of convenience. Here, we test this notion carefully by analyzing three different microscopic models of apolar active nematics. They share the same symmetry but differ in implementing reciprocal or non-reciprocal interactions, including a Vicsek-like implementation. We show how such subtle differences in microscopic realization determine if the ordering transition is continuous or first order. Despite the difference in the type of phase transition, all three models exhibit fluctuation-dominated phase separation and quasi-long-range order in the nematic phase.

Activity-induced phase transition and coarsening dynamics in dry apolar active nematics.

Using the Lebwohl-Lasher interaction for reciprocal local alignment, we present a comprehensive phase diagram for a dry, apolar, active nematic system using its stochastic off-lattice dynamics. The nematic-isotropic transition in this system is first-order and occurs alongside a fluctuation-dominated phase separation. Our phase diagram identifies three distinct regions based on activity and orientational noise relative to alignment strength: a homogeneous isotropic phase, a nematic phase with giant density fluctuations, and a coexistence region. Using mean-field analysis and hydrodynamic theory, we demonstrate that reciprocal interactions lead to a density fluctuation-induced first-order transition and derive a phase boundary consistent with numerical results. Quenching from the isotropic to nematic phase reveals coarsening dynamics where nematic ordering precedes particle clustering. Both the nematic and density fields exhibit similar scaling behaviors, exhibiting dynamic exponents zS ≈ 2.5 and zρ ≈ 2.34, consistently falling within the range of 2 and 3.

Morphology and line tension of twist disclinations in a nematic liquid crystal.

We deploy optical microscopy with and without an applied magnetic field to characterize the three-dimensional morphology and measure the line tension of twist disclinations in twisted nematic liquid crystal (LC) sample cells. Twist disclinations are generated by quenching the LC, 5CB (4-cyano-4'-pentylbiphenyl), into the nematic phase; 5CB is confined between substrates with in-plane anchoring directions perpendicular to one another. The disclinations form loops separating domains of opposite twist handedness. Many segments of these loops are pinned to the substrates, and the ends of some pinned segments connect to free disclination segments that penetrate into the bulk nematic. We use confocal microscopy to measure the profiles of these free disclinations and test theoretical predictions about their shape, yielding a lower bound of ∼32 nm for the disclination core radius. We then use an applied magnetic field to deform the free disclinations into circular arcs whose curvature increases with magnetic field strength and depends on the field-induced energy difference between opposite twist domains. The line tension of the disclinations is derived from an energy-balance equation that relates disclination curvature to magnetic field. The measured line tension increases logarithmically with sample cell thickness; it ranges from 75 to 200 pN in samples with thickness spanning from 6 to 27 μm. In total, the investigation introduces new non-invasive methodologies for studies of defects in LCs, and it provides new information about the line tension and character of isolated twist disclinations, thereby testing theory and laying experimental foundation for the study of ensembles of disclinations.

Polar nematic phases with enantiotropic ferro- and antiferroelectric behaviour

The first synthesized liquid crystal compounds with enantiotropic ferro- and antiferroelectric nematic phases.

Observation of ferroelectric behaviour in non-symmetrical cholesterol-based bent-shaped dimers.

Non-symmetrical cholesterol-based dimers have emerged as crucial materials in the field of liquid crystal research, owing to their remarkable ability to stabilize various exotic mesophases, including the blue phases (BPIII, BPII, BPI), cholesteric nematic (N*) phase, smectic blue phase (SmBP), twist grain boundary (TGB) phase, smectic A/smectic A* (SmA/SmA*) phase, and smectic C/smectic C* (SmC/SmC*) phase. These mesophases have garnered considerable attention due to their diverse applications in spatial light modulation, chiro-optical devices, optical switching, thermochromic materials, and more. In this study, we present the synthesis and comprehensive characterization of a series of non-symmetrical cholesterol-based bent-shaped dimers (1/12, 1/14, 1/16) in which the cholesterol unit is intricately linked to an aromatic mesogenic core through a flexible spacer. These novel materials exhibit the intriguing ability to stabilize a variety of mesophases, including the N*, TGBA, SmA, and SmC* phases. The chiro-optical properties of the helical SmC* phase have been meticulously investigated through temperature-dependent chiro-optical measurements, shedding light on their potential for advanced optoelectronic applications. Additionally, we have conducted a thorough examination of the physical characteristics of these cholesterol-based dimers, including static permittivity measurements, dielectric spectroscopy, and electro-optical performance analysis. Remarkably, two homologues (1/14, 1/16) exhibit negative dielectric anisotropy, a crucial parameter for liquid crystal devices. Furthermore, our investigation reveals that these materials exhibit ferroelectric behaviour in the SmC* phase, with compounds 1/14 and 1/16 demonstrating substantial spontaneous polarization (PS) values of approximately 132 nC cm-2 and 149 nC cm-2, respectively. These findings underscore the potential of non-symmetrical cholesterol-based dimers as versatile components for the development of innovative electro-optical devices.

Defect-influenced particle advection in highly confined liquid crystal flows.

We study the morphology of the Saturn ring defect and director structure around a colloidal particle with normal anchoring conditions and within the flow of the nematic host phase through a rectangular duct of comparable size to the particle. The changes in the defect structures and director profile influence the advection behaviour of the particle, which we compare to that in a simple Newtonian host phase. These effects lead to a non-monotonous dependence of the differential velocity of particle and fluid, also known as retardation ratio, on the Ericksen number.

Competing synclinic and anticlinic interactions in smectic phases of bent-core mesogens

Presented molecules combine two features, the bent-core rigidity with the dimeric character, and reveal variety of specific phases. Longer homologues exhibit tilted smectic phases, short homologues manifest a propensity for helical 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.

Modeling nematic phase main-chain liquid crystal elastomer synthesis, mechanics, and thermal actuation via coarse-grained molecular dynamics.

This paper presents a coarse-grained molecular dynamics simulation study of the synthesis, mechanics, and thermal actuation of nematic phase main-chain liquid crystal elastomers (LCEs), a type of soft, temperature-responsive, polymeric actuating material. The simulations herein model the crosslinking, mechanical stretching, and additional crosslinking synthesis process, following which, the simulated LCE exhibits a direction-dependent thermal actuation and mechanical response. The thermal actuation response shows good qualitative agreement with experimental results, including the variation of a global order parameter that describes the orientation of the mesogen domains comprising the LCE. The mechanical response due to applied deformation shows less agreement, but manifests the key features observed in experiments on LCEs, namely soft strain and hyperelasticity that is present when loaded perpendicularly and in-line, respectively, to the mesogen alignment direction. We also present a topological analysis of the simulated LCEs, which, in conjunction with the simulated thermomechanical responses, allows us to infer the relative contribution of entanglements and chemical crosslinks on those responses. We suggest that the model proposed herein will help enable improved LCE formulations via mechanistic insights that can be gained via the use of such a relatively computationally inexpensive coarse-grained molecular dynamics model, which may be of further value to application areas including soft robotics, bio-mimicking devices, artificial muscles, and adaptive materials.

Phases and correlations in active nematic granular systems.

We investigate the statistical behavior of a system comprising fore-aft symmetric rods confined between two vertically vibrating plates using numerical simulations closely resembling the experimental setup studied by V. Narayan et al., Science, 2007, 317, 105-108. Our focus lies in studying the phase transition of the system from an isotropic phase to a nematic phase as we increase either the rod length or the rod concentration. Our simulations confirm the presence of long-range ordering in the ordered phase. Furthermore, we identify a phase characterized by a periodic ordering profile that disrupts translation symmetry. We also provide a detailed analysis of the translational and rotational diffusive dynamics of the rods. Interestingly, the translational diffusivity of the rods is found to increase with the rod concentration.

A one-pot telescopic synthesis of benzo[b]carbazoles and exploration of their liquid crystalline properties.

We describe a diversity-oriented one-pot telescopic synthesis of various benzo[b]carbazoles with the naphthannulation of indoles as the key step, enabled by an intramolecular furan-olefin Diels-Alder reaction. This strategy is general and efficient across a wide range of substrates. We applied this method to synthesize and characterize the first benzo[b]carbazole-based liquid crystalline materials, where the unique molecular design led to the formation of a rare nematic phase at room temperature.

Room-temperature ferroelectric nematic liquid crystal showing a large and diverging density.

The ferroelectric nematic phase (NF) is a recently discovered phase of matter in which the orientational order of the conventional nematic liquid crystal state is augmented with polar order. Atomistic simulations suggest that the polar NF phase would be denser than conventional nematics owing to contributions from polar order. Using an oscillating U-tube densitometer, we obtain detailed temperature-dependent density values for a selection of conventional liquid crystals with excellent agreement with earlier reports. Having demonstrated the validity of our method, we then record density as a function of temperature for M5, a novel room-temperature ferroelectric nematic material. We present the first experimental density data for a NF material as well as density data for a nematic that has not previously been reported. We find that the room-temperature NF material shows a large (>1.3 g cm-3) density at all temperatures studied, notably including phases without polar order. An increase in density at phase transitions is observed. The magnitude of the increase for the intermediate-to-ferroelectric nematic (NX-NF) transition is an order of magnitude smaller than the isotropic-nematic (I-N) transition. We then probe potential consequences that may result from an elevated density through measurement of the refractive indices (no and ne). The navg of M5 is compared with 5CB and polar smectic liquid crystals. We observe how the highly polar nature of the system counteracts the effects of an increase in density. With knowledge of experimental density, we are able to derive an approximation that yields the polar order parameter, 〈P1〉, from polarisation measurements. Present results may be typical of ferroelectric nematic materials, potentially guiding material development, and is especially relevant for informing ongoing studies into this emerging class of materials.

Uniqueness of global weak solutions to the frame hydrodynamics for biaxial nematic phases in $\mathbb{R}^2$

Uniqueness of global weak solutions to the frame hydrodynamics for biaxial nematic phases in $\mathbb{R}^2$

Visualizing localized nematic states in twisted double bilayer graphene.

Direct visualization of the states originating from electron-electron interactions is of great importance for engineering the surface and interfacial properties of graphene-based quantum materials. For instance, the rotational symmetry breaking or nematic phase inferred from spectroscopic imaging has confirmed the existence of correlated states in a wide range of moiré materials. Here, we study the atomic-scale spatial distributions and symmetry of wave functions in gate-tunable twisted double bilayer graphene by employing scanning tunneling microscopy/spectroscopy and continuum model calculations. A series of spectroscopic imaging analyses are used to identify dominant symmetry breaking of the emergent states. Interestingly, in non-integer hole fillings, a completely new localized electronic state with rotational symmetry breaking is observed on the left side of the valence flat band. The degree of anisotropy is found to increase from the conduction flat band through the valence flat band to the new state. Our results provide an essential microscopic insight into the flat band and its adjacent state for a full understanding of their electric field response in twisted graphene systems.

Evidence for an odd-parity nematic phase above the charge-density-wave transition in a kagome metal

Evidence for an odd-parity nematic phase above the charge-density-wave transition in a kagome metal

Ferroelectric nematogens containing a methylthio group

We report for the first-time a series of methylthio terminated materials which exhibit the ferroelectric nematic phase.

Relaxation processes in nematic liquid crystals in the nonstationary regime of a rotation of magnetic field

The prospects of using the nonstationary regime of motion of the director for studying the relaxation properties of liquid crystals in the low-temperature interval of the nematic phase are shown. In the nonstationary regime, the phase shift between the direction and the wave vector is 45 and does not depend on the angular velocity of rotation of the magnetic field. A eutectic mixture of nematic liquid crystals with a wider temperature range of the nematic phase compared to individual components has been studied. A decrease in the anisotropy of the absorption coefficient in the nonstationary regime is associated with a partial violation of the homogeneous orientation of the sample. The nature of the dependence of the low-frequency component of the phase characteristic on the thermodynamic parameters of the state is established. Within the framework of the hydrodynamics of the nematic phase, relations were obtained that allow calculating the dissipative coefficients responsible for the performance of liquid crystal technical devices.