Session F25: Liquid Crystals II

Theoretical Model of Vertically Stacked Domains in Nematic Liquid Crystals

In the typical nematic liquid crystal (LC) geometry, the director profile under the effect of an applied field has a quasi-parabolic configuration. Previously, other possible profiles of the LC director were explored under the effects of mutually perpendicular electric and magnetic fields. We discovered the metastable states with an oscillatory director profile in the vertical direction are possible under the effect of an applied electric field only. We explore these states theoretically both through energy minimization and a differential equation approach. Under weak applied electric fields, we find small oscillations, which evolve into standard soliton-like domain walls as the applied field is increased. We develop a simple analytic model to predict the thickness of the domain wall that compares well with our numerical calculations. Furthermore, we investigate effects of domain walls on the optical characteristics of LC-based devices. For instance, we find the shift in the transmittance through the LC cell, from low to high voltages as domain walls are introduced, which can be used to create an optical shutter.   

Numerical modeling of colloid aggregation in isotropic to nematic phase transition

Colloidal aggregates in a nematic liquid crystal form unique structures owing to the long-range orientational ordering of the nematic molecules. Motivated by recent experimental results in the self-assembly of aggregates of colloidal shells of nanoparticles, we study the mechanism behind the formation of these aggregates in the isotropic to nematic phase transition. The far-field interaction between colloidal particles and the nematic liquid crystal allows us to use Brownian dynamics simulations to model the formation of a variety of colloid structures, which are dependent on temperature quench depth and concentration. Additionally, we use Landau-de Gennes free energy minimization to model the aggregation kinetics in the isotropic to nematic phase transition, and to understand how topological defects mediate in the assembly of colloidal aggregates. The mentioned numerical methods allow us to look at the kinetics and aging of colloidal aggregate structures in a liquid crystal.   

A generic approach to directed self-assembly of polymerizable blue-phase liquid crystals

Blue-phases (BPs) occur in a narrow temperature window between the chiral nematic and the isotropic liquid crystalline phases. They exhibit unit cell sizes of order ~100 nm, making them soft photonic crystals. BPs are promising candidates for displays, sensors, and lasing. Polycrystallinity and thermal instability have still greatly limited their applicability. Chemically patterned surfaces with alternating planar and homeotropic anchoring regions can be used for the epitaxial growth of monocrystalline BPs. We show that such stripe-like patterns direct the self-assembly of BPs with different chemical compositions into single crystals. Experiments together with the support of free energy calculations propose that the strategy of nano-patterned substrates is generally applicable and may serve as a general platform for nucleation and growth of the single-crystalline area with controlled crystal orientation. Each single crystal reflects the light differently which results in the formation of a colorful spectrum. We also demonstrate that the thermal stability of such prepared single crystals can be improved by photopolymerization. Combining the directed self-assembly and the photopolymerization lead to the preparation of thermally stable single crystals of a desire lattice orientation.   

How confinement-induced chiral director configurations respond to magnetic fields

Confinement can make achiral liquid crystals (LCs) twist to exhibit chiral configurations. Lyotropic chromonic liquid crystals (LCLCs) of unusual elastic properties, such as large saddle-splay and small twist modulus compared to splay and bend modulus, provide archetypal examples of this confinement-induced chiral symmetry breaking. This experimental work investigates how applied magnetic fields transform LCLC's double-twist director configuration in a cylinder. Because of LCLC's negative magnetic anisotropy, upon increasing the field strength, the twist profile along the capillary radius changes from a convex to concave curve, and the twist angle at the cylindrical wall converges to 90 deg. Elastic free-energy calculation explains well our experimental data in terms of the delicate interplay between elasticity, geometry, and external field.   

Thermotropic ionic liquid crystals as lithium battery electrolytes

Novel organic electrolyte platforms are being investigated for use in next-generation lithium-ion batteries. A major target is the reduction of use of volatile organic solvents, towards less flammable electrolytes and safer electrochemical energy storage systems without sacrificing energy density. Here we report on a family of ionic liquid and thermotropic ionic liquid crystalline electrolytes with the general chemical structure alkyl-ALi, where "A" is a monovalent anion with differing chemistry. It is found that the thermal, structural, and ion transport properties of these solvent-free electrolytes is a strong function of both the alkyl chain length and the chemical nature of the bound anion. For certain compositions, the materials are found to have liquid crystalline behavior over a wide temperature range. These surfactant-like salts may be diluted with common electrolyte solvents to create super concentrated electrolytes with enhanced ionic conductivity compared with the solvent-free electrolyte and enhanced electrochemical stability compared with conventional electrolytes based on the same solvents.   

Electro-optic HAN device utilizing carbon nanotube arrays and 2D hexagonal boron nitride nanosheet as alignment substrates

Hybrid-aligned nematic (HAN) liquid crystal (LC) devices have both fundamental and technological importance for their applications in LC adaptive lenses, low voltage LC displays, smart windows, and many more.  We report the fabrication and characterization of a novel nanostructure-based HAN device employing vertically aligned carbon nanotube (VA-CNT) arrays as the homeotropic alignment agent on one side and 2D hexagonal boron nitride (h-BN) as the planar alignment agent on the other side of the LC cell.  The LC achieves the HAN configuration in the cell, i.e., homeotropic alignment at the VA-CNT side due to the π-π stacking interaction between the LC and CNTs, and planar alignment at the h-BN side due to the π-π stacking interaction between the LC and h-BN.   When an applied electric field is ramped up across this VA-CNT/h-BN HAN cell, the LC (positive anisotropic) obtains a homeotropic state, requiring no threshold voltage to start the reorientation process—this effect is similar to that of a traditional polyimide (PI)-based HAN device.  This VA-CNT/h-BN HAN cell successfully demonstrates the optical, electro-optical operations and the electric field-induced dynamic response.  This study reveals that two inorganic nanostructured surfaces, i.e., VA-CNT arrays and 2D h-BN, can efficiently replace the organic PI alignment agents when needed and retain the HAN device's necessary electro-optical performances.  These results substantially expand the fundamental understanding and the scope of utilizing various nanostructured surfaces for LC alignment mechanisms.   

Relation of the correlation length to the size of the crystallites at the buried interface? A study of the in-plane structure of the TiO2-EPM6R8 liquid crystal system

We observed the changes that occur in the TiO2 – EPM6R8 system along the in-plane structure using grazing incidence x-ray scattering for concentrations of n = 7, 15 and 30 wt % of TiO2. The EPM6R8 has been electropolymerized with the TiO2 present, resulting in a structure where the hanging M6R8 monomers, and the monomers mixed with the TiO2 are mostly oriented along the in-plane direction. The region of q = 0.15 – 0.5 Ǻ^-1 covers the (002) and (003) peaks of the EPM6R8 polymer, and the (001) and (002) peaks of the M6R8 monomer, plus the monomer peak mixed with the TiO2. We observe that as a function of the wt % of TiO2, the polymer peaks in the in-plane direction disorder. This is marked by a crossover of peaks from a Gaussian (crystalline) shape to a Lorentzian (disordered) shape when crossing from 7 to 15 wt%. The peaks disappear completely in the in-plane direction for the 30 wt% sample. The peak mixed with the TiO2 sample, as well as the monomer (001) and (002) peaks remain crystalline. The peaks mixed with the TiO2 sample show a crystalline structure that varies between 20 to 30 nm, whereas the (001) and (002) monomer show a crystalline structure that varies between 61 to 69 nm. These in-plane values are smaller than the numbers calculated averaging over the three dimensions [1]. The sizes suggest that the in-plane structures may consist of nanowires [2,3].

[1] P. Romero-Hasler, et al., Liquid Crystals, 2020, 47, 423-432.

[2] C. J. Takacs, et al., ACS Nano, 2014, 8141-8152.

[3] M. Roders, et al., J. Phys. Chem. C, 2019, 27305-27316

  

Substitution of sub-molecular structure can alter surface anchoring of liquid crystals: Impact of cyclohexyl vs phenyl groups on surface anchoring of nematic LC

In the design for responsive and functional materials, the ability to explain their macroscopic behavior from the nanoscale structure of their components is a still challenging but promising approach. Here we demonstrate experimentally that the macroscopic orientational ordering of a liquid crystal (LC) in its interaction with perfluoroalkane oils can be altered by the substitution of sub-molecular structures from phenyl to alkyl rings. Using molecular dynamics simulations and enhanced sampling methods, we can reconstruct the LC interface with perfluoroalkane oils. Based on compartmentalization of the mesogens and potential of mean force between each compartment and fluorocarbon molecules, we provide an explicit explanation of the formation of molecular ordering where affinity between perfluorocarbons and cyclohexyl group is favored over phenyl groups.   

Antiferroelectric Smectic Ordering as a Prelude to the Ferroelectric Nematic: Introducing the Smectic Z Phase

In 2020, the Boulder group demonstrated direct evidence of ferroelectricity in the nematic phase of the molecule RM734 [1,2]. DIO [3] is one of the few other molecules demonstrating nematic ferroelectricity, and the Boulder group has recently shown that RM734 and DIO have the same ferroelectric nematic (NF) phase [4]. As previously reported [3], there is also an intermediate phase of unknown structure in DIO, between the nematic and NF phases. Here we report that this phase is a new class of smectic liquid crystal, one having the molecular long axes on average parallel to the layers. Synchrotron-based non resonant SAXS and electro-optical microscopy experiments show that the intermediate phase is density modulated with a 90Å periodicity. The layers are parallel to director n which is the limit opposite to that of the smectic A, so we call this phase the smectic Z. The space-filling periodic structure of the SmZ phase results in textures familiar in smectics, such as the bookshelf layer organization, including the formation of chevron textures having zig-zag walls. The SmZ is antiferroelectric, with polarization along n and oppositely directed in neighboring layers.   

Electrically Tunable Reflection Color of Chiral Ferroelectric Nematic Liquid Crystals

The recently discovered ferroelectric nematic (N_F) liquid crystals (LCs) with over 0.04 C/m² ferroelectric polarization and 10⁴ relative dielectric constants, coupled with sub-millisecond switching, offer potential applications in high-power super capacitors and low voltage driven fast electro-optical devices. This talk will present electrical, optical, and electro-optical studies of a ferroelectric nematic LC material doped with commercially available chiral dopants. While the N_F phase of the undoped LC is only monotropic, the chiral N_F phase is enantiotropic, indicating a chirality induced stabilization of the polar nematic order. Compared to undoped N_F material, a remarkable improvement of the electro-optical switching time is demonstrated in the chiral doped materials. The color of the chiral mixtures that exhibit a selective reflection of visible light in the chiral N_F phase, can be reversibly tuned by 0.02-0.1V/µm in-plane electric fields, which are much smaller than typically required in full-color cholesteric LC displays and do not require complicated driving scheme. The fast switchable reflection color at low fields has potential applications for LC displays without backlight, smart windows, shutters and e-papers.

  

Polarization monodomains and electric field response in wedge samples of a ferroelectric nematic liquid crystal.

A ferroelectric nematic (N_F) liquid crystal is formed by highly polar molecules which produce macroscopic electric polarization. The long-range polar ordering leads to a broad range of phenomena that are absent in typical nematics such as the formation of polarization domains, polar surface anchoring, and polar response to the electric field, etc. In this work, we report on the ferroelectric properties of the material abbreviated DIO [1] under different confinement conditions in the presence and absence of an electric field. In cells with the parallel assembly of rubbing directions, the polar mesophase of DIO forms N_F monodomain upon cooling with a rate below 5 ^oC/min if the thickness is less than 5 μm. The monodomain polarization is antiparallel to the rubbing direction and is realigned in the plane of the cell by applying a dc electric field through a system of patterned electrodes. The electric field realignment demonstrates that the in-plane surface anchoring energy is polar with a local minimum for polarization parallel to the rubbing direction and a global minimum for antiparallel alignment. The bulk and surface properties of DIO show that the material is a ferroelectric nematic. The work is supported by NSF grant ECCS-2122399.

1.Nishikawa, et al. Adv. Mater.(2017)29:1702354   

Multiple ferroelectric nematic phases of a highly polar liquid crystal compound.

Ferroelectric nematic liquid crystals represent not only interesting fundamental science, but they also hold promise for storage capacitors with high power density or new information display technology having sub-millisecond switching. In this work we describe the synthesis and measurements of the physical properties of a new highly polar ferroelectric nematic compound, 4-nitrophenyl 4-[(2,4-dimethoxylbenzoyl)oxy]-2-fluorobenzoate (RT11001). We employ a wide range of physical characterization methods including differential scanning calorimetry (DSC), mass density measurement, optical birefringence, polarizing optical microscopy (POM), dielectric spectroscopy, electric current analysis, electro-optical switching, small-angle and wide-angle x-ray scattering measurements to show that RT11001 has three distinct ferroelectric states, N_F1, N­_F2 and N_F3. The  phase is characterized by a polar nematic fluid medium and non-polar smectic clusters. The  is also a polar nematic continuum but with polar smectic clusters. The phase differs from the  phase by the volume of the polar clusters that increases in as evidenced by a broad DSC peak observed on cooling.   

Surface patterning of ferroelectric nematic

Nishikawa et al discovered a novel LC material abbreviated DIO with highly polar nematic order[H. Nishikawa, 2017]. We report on patterned surface alignment of DIO using plasmonic metamasks [Y. Guo, Adv. Mater, 28, 2016]. The patterns include topological defects of strength  ½,  1. In the polar nematic phase, the patterns are accompanied by domains of opposite direction of polarization, as established by applying the in-plane electric field. The observation proves that the polar phase is a ferroelectric nematic N_F. The isolated  ½ disclinations in the nematic phase transform into composite defects in NF, with domain walls emanating from the disclination cores. Patterns of +1 circular defects show domain structures that depend strongly on whether the defect is of a bend or splay type, which illustrates that the domain structure is affected by the space charge associated with the spatial divergence of polarization.      

Wide temperature range tunable selective reflection of light by chiral ferroelectric nematic phase

A chiral mixture composed of ferroelectric nematic mesogen and chiral additives shows a right-angle chiral nematic (N*) followed by a chiral ferroelectric nematic (N_F*) at lower temperatures  [1–3]. The N_F* phase is a recently realized new chiral nematic with a polar helical order.  We explore both the N* and N_F* phases in three chiral mixtures containing ferroelectric nematic mesogen  RM734 doped with different chiral additives. In all three chiral mixtures, the helical pitch measured in a Cano wedge cell shows a strong temperature dependence. In one chiral mixture, the pitch decreases monotonously in both the N* and N_F* phases with a temperature decrease. However, in the other two mixtures, the pitch shows a non-monotonous trend with a decrease of temperature; pitch decreases in the N* phase while increases in N_F* phase. The selective Bragg reflection of light from the N_F* cell with hybrid surface alignment shows a continuous redshift by 200 nm driven by a temperature decrease.   

Mesomorphic Ceramics via Blade Coating of Lyotropic Nanorod Suspensions

Mesomorphic ceramics are inorganic polycrystalline solids with static liquid crystalline superstructures. Here, we fabricate mesomorphic ceramic thin films over centimeter length scales by blade coating nematic lyotropic suspensions of ZnO nanorods, followed by calcination. The coating velocity was optimized in the Laundau-Levich regime to obtain defect-free ceramics green bodies.  The resulting mesomorphic ceramic thin films are optically transparent with a smooth surface finish and exhibit in-plane birefringence of 0.075 ± 0.002. Form birefringence is shown to dominate over the intrinsic birefringence of ZnO. Uniaxial orientation of ZnO crystallites along the coating direction was confirmed by texture analysis and SEM imaging. The reported method to fabricate birefringent, mesomorphic ceramics is cost-effective, scalable, and expected to be broadly applicable to nematic lyotropic suspensions of various nanorods. Moreover, it could benefit diverse applications including thin-film electronics, optoelectronics, catalysis, and optics for high-power lasers.