Nematic Solutions Research Articles

Biomimetic Spun Silk Ionotronic Fibers for Intelligent Discrimination of Motions and Tactile Stimuli.

Innovation in the ionotronics field has significantly accelerated the development of ultraflexible devices and machines. However, it is still challenging to develop efficient ionotronic-based fibers with necessary stretchability, resilience, and conductivity due to inherent conflict in producing spinning dopes with both high polymer and ion concentrations and low viscosities. Inspired by the liquid crystalline spinning of animal silk, this study circumvents the inherent tradeoff in other spinning methods by dry spinning a nematic silk microfibril dope solution. The liquid crystalline texture allows the spinning dope to flow through the spinneret and form free-standing fibers under minimal external forces. The resultant silk-sourced ionotronic fibers (SSIFs) are highly stretchable, tough, resilient, and fatigue-resistant. These mechanical advantages ensure a rapid and recoverable electromechanical response of SSIFs to kinematic deformations. Further, the incorporation of SSIFs into core-shell triboelectric nanogenerator fibers provides outstanding stable and sensitive triboelectric response to precisely and sensitively perceive small pressures. Moreover, by implementing a combination of machine learning and Internet of Things techniques, the SSIFs can sort objects made of different materials. With these structural, processing, performance, and functional merits, the SSIFs prepared herein are expected to be applied in human-machine interfaces.

Rheo-NMR velocimetry characterisation of PBLG/m-cresol

ABSTRACTNMR spectroscopy was used to characterise the velocity profile in nematic solutions of poly(γ-benzyl-L-glutamate) (PBLG) in m-cresol during the application of a simple shear-flow at constant shear-rate values. In this study, the rheo-NMR technique was used that allows an insight over the fluid dynamics during the application of a shear flow in the presence of an external magnetic field, in a perpendicular direction to the shear gradient. Using this Magnetic Resonance Imaging (MRI) based technique, the velocity profile developed inside the system is accessed.

Lyotropic Nematic Phases of Isotropic Nanoparticles via Semiflexible Polymer Ligands.

Lyotropic liquid crystalline(LC) nanomaterials are normally achieved through particle shape anisotropy. Herein, it is shown that lyotropic nematic rather than cubic phases are produced from sphericalnanoparticles (NPs) with semi-flexible polymer ligands. ZrO2 nanocrystals (4nm dia.) are coated with a dense shell of poly(hexyl isocyanate) (PHIC), a helical rod-like polymer that forms lyotropic LC phases in a range of organic solvents. Solvent casted NPs with PHIC ligands above the persistence length form linear assemblies, separated by a characteristic distance related to the chain length while NPs with shorter, rigid rod PHIC ligands pack hexagonally. Concentrated NP-PHIC dispersions present nematic textures similar to the free PHIC nematic solutions but at lower critical concentrations, widening the isotropic-nematic biphasic region. 2 H NMR spectra of the NPs dispersed in a deuterated solvent display quadrupolar splittings that increase with NP concentration, showing that the PHIC ligands are magnetically aligned. The high degree of orientation order is evidence that splaying of the ligand shell transforms the spherical NPs to rod-like shapes that assemble to produce nematic lyotropic LC phases and linear NP arrays. This approach to creating anisotropic assemblies can be extended to other types of spherical NPs and semiflexible polymers.

Dendritic patterns from shear-enhanced anisotropy in nematic liquid crystals.

Controlling the growth morphology of fluid instabilities is challenging because of their self-amplified and nonlinear growth. The viscous fingering instability, which arises when a less viscous fluid displaces a more viscous one, transitions from exhibiting dense-branching growth characterized by repeated tip splitting of the growing fingers to dendritic growth characterized by stable tips in the presence of anisotropy. We controllably induce such a morphology transition by shear-enhancing the anisotropy of nematic liquid crystal solutions. For fast enough flow induced by the finger growth, the intrinsic tumbling behavior of lyotropic chromonic liquid crystals can be suppressed, which results in a flow alignment of the material. This microscopic change in the director field occurs as the viscous torque from the shear flow becomes dominant over the elastic torque from the nematic potential and macroscopically enhances the liquid crystal anisotropy to induce the transition to dendritic growth.

Elastic anisotropy in the reduced Landau-de Gennes model.

We study the effects of elastic anisotropy on Landau–de Gennes critical points, for nematic liquid crystals, on a square domain. The elastic anisotropy is captured by a parameter, , and the critical points are described by 3 d.f. We analytically construct a symmetric critical point for all admissible values of , which is necessarily globally stable for small domains, i.e. when the square edge length, , is small enough. We perform asymptotic analyses and numerical studies to discover at least five classes of these symmetric critical points—the , , and solutions, of which the , and solutions can be stable. Furthermore, we demonstrate that the novel solution is energetically preferable for large and large , and prove associated stability results that corroborate the stabilizing effects of for reduced Landau–de Gennes critical points. We complement our analysis with numerically computed bifurcation diagrams for different values of , which illustrate the interplay of elastic anisotropy and geometry for nematic solution landscapes, at low temperatures.

Multipolar nematic state of nonmagnetic FeSe based on DFT+U

Clarifying the origin of nematic state in FeSe is one of urgent problems in the field of iron-based superconductivity. Motivated by the discovery of a nematic solution in the density-functional theory implemented by on-site Coulomb interaction ($\mathrm{DFT}+U$) [npj Quantum Mater. 5, 50 (2020)], we reexamine the $U$ dependence of electronic states in the nonmagnetic normal state of FeSe and perform full multipolar analyses for the nematic state. We find that with increasing $U$ the normal state experiences a topological change in the Fermi surfaces before the emergence of a nematic ground state. The resulting nematic ground state is a multipolar state having both antiferrohexadecapoles in the $E$ representation and ferromultipoles in the ${B}_{2}$ representation on each Fe site. Cooperative coupling between the $E$ and the ${B}_{2}$ multipoles in the local coordinate with the ${D}_{2d}$ point group will play an important role in the formation of the ${d}_{xz},{d}_{yz}$ orbital-splitting nematic state not only in FeSe, but also in other iron pnictides.

Structures and topological defects in pressure-driven lyotropic chromonic liquid crystals

Lyotropic chromonic liquid crystals are water-based materials composed of self-assembled cylindrical aggregates. Their behavior under flow is poorly understood, and quantitatively resolving the optical retardance of the flowing liquid crystal has so far been limited by the imaging speed of current polarization-resolved imaging techniques. Here, we employ a single-shot quantitative polarization imaging method, termed polarized shearing interference microscopy, to quantify the spatial distribution and the dynamics of the structures emerging in nematic disodium cromoglycate solutions in a microfluidic channel. We show that pure-twist disclination loops nucleate in the bulk flow over a range of shear rates. These loops are elongated in the flow direction and exhibit a constant aspect ratio that is governed by the nonnegligible splay-bend anisotropy at the loop boundary. The size of the loops is set by the balance between nucleation forces and annihilation forces acting on the disclination. The fluctuations of the pure-twist disclination loops reflect the tumbling character of nematic disodium cromoglycate. Our study, including experiment, simulation, and scaling analysis, provides a comprehensive understanding of the structure and dynamics of pressure-driven lyotropic chromonic liquid crystals and might open new routes for using these materials to control assembly and flow of biological systems or particles in microfluidic devices.

Controlling the shapes and internal complexity of the polymeric particles using liquid crystal-templates confined into microwells

Formation and positioning of the topological defects within liquid crystalline (LC) medium confined into microwells were shown to critically effect the shapes and symmetry of the particles obtained after polymerization. Water-soluble, sacrificial poly(vinyl alcohol) microwells with different sizes and shapes were filled with a nematic solution of reactive (RM257) and non-reactive mesogens (5CB) and photopolymerized under ultraviolet light. The characterization of the LC director profile and defect locations were done using a polarized optical microscope. We found that the positions of the defects could be controlled by the elasticity and surface anchoring by tuning the size and the shape of the microwells. Polymerization followed by the extraction of the unreacted part and drying, the particles maintained their final shapes where we observed a critical dependence on the location and charge of the defects. Beyond the three-dimensional shapes, SEM imaging of the particle surfaces revealed that the LC director profile around the defects of different charges to be reflected in the surface morphology. Overall, the outcomes of this study provide a method for the synthesis of three-dimensional polymeric microparticles in different size and shape with predetermined internal structures. The findings herein may be used in different applications such as drug delivery, sensors, soft-robotics.

Fingerprinting the nonlinear rheology of a liquid crystalline polyelectrolyte

We report on the rheology of isotropic and nematic aqueous solutions of a sulfonated all-aromatic polyamide, poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT), that forms high-aspect-ratio rod-like assemblies. For quiescently isotropic solutions, the concentration dependence of the zero-shear viscosity, longest relaxation time, and terminal modulus shows deviations in comparison to the Doi-Edwards theory for hard rods. For quiescently nematic solutions, we characterize the flow behavior through steady-state and transient nonlinear rheological measurements in conjunction with small-angle neutron scattering under shear. The steady-state flow curve is characterized by two anomalous shear thickening responses, one at moderate shear rates and the other immediately prior to flow alignment at high shear rates. We assign the origin of these shear thickening response to director “kayaking” and “out-of-plane steady” states, using predictions from prior high-resolution numerical simulations of sheared nematic rods. Utilizing transient shear flow reversals and step-down experiments, we characterize the oscillatory response of the nematic director through these flow regimes. When the first normal stress difference is plotted versus the shear stress during a transient step-down, the so-called dynamic stress path, the counterclockwise versus clockwise rotation has previously been shown to reveal the relative dominance of viscous versus elastic contributions to the stress tensor, respectively. Our measurements strongly suggest that the anomalous shear thickening behavior in nematic PBDT solutions arises from viscous stresses developed as the ensemble of rods undergoes periodic oscillatory motion under shear, rather than elastic stresses due to broadening of the orientational distribution function.

A first-principle perspective on electronic nematicity in FeSe

Electronic nematicity is an important order in most iron-based superconductors, and FeSe represents a special example, in which nematicity disentangles from spin ordering. A first-principle description of this order remains elusive. Here, we show that by carefully searching the paramagnetic energy landscape within the density functional theory, a nematic solution stands out at either the +U or hybrid functional level with the lowest energy. The band structure and Fermi surface can be well compared with the recent experimental results. Symmetry analysis assigns the dominant order parameter to the Eu irreducible representations of the D4h point group. Distinct from the B1g Ising nematicity as widely discussed in the context of vestigial stripe antiferromagnetic order, the two-component Eu vector order features mixing of the Fe d-orbitals and inversion symmetry breaking, which lead to striking experimental consequences, e.g., missing of an electron pocket.

Phase-dependent shear-induced order of nanorods in isotropic and nematic wormlike micelle solutions.

Small angle X-ray scattering with in situ shear was employed to study the assembly and ordering of dispersions of gold nanorods within wormlike micelle solutions formed by the surfactant cetylpyridinium chloride (CPyCl) and counter-ion sodium salicylate (NaSal). Above a threshold CPyCl concentration but below the isotropic-to-nematic transition of the micelles, the nanorods self-assembled under quiescent conditions into isotropically oriented domains with hexagonal order. Under steady shear at rates between 0.5 and 7.5 s-1, the nanorod assemblies acquired macroscopic orientational order in which the hexagonal planes were coincident with the flow-vorticity plane. The nanorods could be re-dispersed by strong shear but re-assembled following cessation of the shear. In the nematic phase of the micelles at higher surfactant concentration, the nanorods did not acquire hexagonal order but instead formed smectic-like layers in the gradient-vorticity plane under shear. Finally, at still higher surfactant concentration, where the micelles form a hexagonal phase, the nanorods showed no translational ordering but did acquire nematic-like order under shear due to alignment in the flow. Depletion forces mediated by the wormlike micelles are identified as the driving mechanism for this sequence of nanorod ordering behaviors, suggesting a novel mechanism for controlled, reconfigurable assembly of nanoparticles in solution.

Polymorphic regenerated silk fibers assembled through bioinspired spinning

A variety of artificial spinning methods have been applied to produce regenerated silk fibers; however, how to spin regenerated silk fibers that retain the advantages of natural silks in terms of structural hierarchy and mechanical properties remains challenging. Here, we show a bioinspired approach to spin regenerated silk fibers. First, we develop a nematic silk microfibril solution, highly viscous and stable, by partially dissolving silk fibers into microfibrils. This solution maintains the hierarchical structures in natural silks and serves as spinning dope. It is then spun into regenerated silk fibers by direct extrusion in the air, offering a useful route to generate polymorphic and hierarchical regenerated silk fibers with physical properties beyond natural fiber construction. The materials maintain the structural hierarchy and mechanical properties of natural silks, including a modulus of 11 ± 4 GPa, even higher than natural spider silk. It can further be functionalized with a conductive silk/carbon nanotube coating, responsive to changes in humidity and temperature.

Planar contraction flow of a nematic PPTA solution

Planar contraction flow of a nematic solution of poly-paraphenylene terephthalamide (PPTA) in sulphuric acid (H2SO4) was studied experimentally and compared with the flow of an isotropic model fluid, a poly-ethyleneglycol–poly-ethyleneoxide (PEG-PEO) Boger fluid, in the same geometry. The dimensions and deformation rates in our experiments are relevant for industrial polymer extrusion processes, such as fiber spinning. Under these conditions, viscoelastic lip vortices arise at the re-entrant corners in the nematic flow. The experimental results show that the vortex growth behavior in the nematic PPTA solution closely resembles that in isotropic viscoelastic fluids, such as the PEG-PEO Boger fluid that was used as a reference. The velocity field in the nematic flow is characteristic of Hele-Shaw flow of a shear-thinning power-law fluid, although locally, the influence of the nematic defect structure was observed, especially for the first minutes after starting the flow. Furthermore, the lateral movement of the extruded jet, downstream of the contraction, was shown to be coupled with velocity fluctuations in the upstream flow. Our experiments give detailed insight in the complex flow behavior of a nematic polymer solution under flow conditions that are relevant to practical applications. The results can be applied in the optimization of nematic polymer extrusion processes.

Two-point particle tracking microrheology of nematic complex fluids.

Many biological and technological complex fluids exhibit tight microstructural alignment that confers them nematic mechanical properties. Among these we count liquid crystals and biopolymer networks, which are often available in microscopic amounts. However, current microrheological methods cannot measure the directional viscoelastic coefficients that appear in the constitutive relation of nematic complex fluids. This article presents directional two-point particle-tracking microrheology (D2PTM) - a novel microrheology technique to determine these coefficients. We establish the theoretical foundation for D2PTM by analyzing the motion of a probing microscopic particle embedded in a nematic complex fluid, and the mutual hydrodynamic interactions between pairs of distant particles. From this analysis, we generalize the formulation of two-point particle tracking microrheology for nematic complex fluids, and demonstrate that the new formulation provides sufficient information to fully characterize the anisotropic viscoelastic coefficients of such materials. We test D2PTM by simulating the Brownian motion of particles in nematic viscoelastic fluids with prescribed directional frequency-dependent shear moduli, showing that D2PTM accurately recovers the prescribed shear moduli. Furthermore, we experimentally validate D2PTM by applying it to a lyotropic nematic liquid crystal, and demonstrate that this new microrheology method provides results in agreement with dynamic light scattering measurements. Lastly, we illustrate the experimental application of the new technique to characterize nematic F-actin solutions. These experiments constitute the first microrheological measurement of the directional viscoelastic coefficients of an anisotropic soft material.

Binary and ternary mixtures of liquid crystals with CO2

Liquid crystals, elongated molecules with a structured liquid phase, may be used as new solvents for CO2 capture. However, no molecule has been found yet with optimal properties. Therefore, mixtures of two liquid crystals and CO2 are investigated. Also, the phase behavior of some binary subsystems of the investigated ternary systems is studied for comparison. In the mixtures investigated, 4,4′‐pentyloxycyanobiphenyl + 4,4′‐heptyloxycyanobiphenyl + CO2 and 4,4′‐propylcyclohexylbenzonitrile + 4,4′‐heptylcyclohexylbenzonitrile + CO2, the nematic phases form a nematic homogeneous solution and the solid phases form an eutectic system, leading to a material with improved properties for CO2 capture. Moreover, the ternary mixture of 4,4′‐propylcyclohexylbenzonitrile + 4,4′‐heptylcyclohexylbenzonitrile + CO2 showed an increased solubility of CO2 compared with the binary subsystems. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2977–2984, 2015

Detection of Significant Aprotic Solvent Effects on the Conformational Distribution of Methyl 4-Nitrophenyl Sulfoxide: From Gas-Phase Rotational to Liquid-Crystal NMR Spectroscopy.

The conformational equilibrium of methyl 4-nitrophenyl sulfoxide (MNPSO) was experimentally investigated in the gas phase by using microwave spectroscopy and in isotropic and nematic liquid-crystal solutions, in which the solvents are nonaqueous and aprotic, by using NMR spectroscopy; moreover, it was theoretically studied in vacuo and in solution at different levels of theory. The overall set of results indicates a significant dependence of the solute conformational distribution on the solvent dielectric permittivity constant: when dissolved in low-polarity media, the most stable conformation of MNPSO proved to be strongly twisted with respect to that in more polar solvents, in which the conformational distribution maximum essentially coincides with that obtained in the gas phase. We discuss a possible explanation of this behavior, which rests on electrostatic solute-solvent interactions and is supported by calculations of the solute electric dipole moment as a function of the torsional angle. This function shows that the least polar conformation of MNPSO is located at a twist angle close to that of the conformational distribution maximum found in less-polar solvents. This fact, associated with a relatively flat torsional potential, can justify the stabilization of the twisted conformation by the less-polar solvents.

Dynamic viscoelastic relaxations of nematic solutions of poly(p-phenylene terephthamide) in sulfuric acid after cessation of shearing

This rheological study investigated strain recovery and evolution of dynamic moduli (G″) of nematic poly(p-phenylene terephthamide)/H2SO4 solutions after cessation of shearing. Recoil experiment showed a large recoverable strain when the prestrain was smaller than 20 strain units. When the prestrain was larger than 50 strain units, the relative recoverable strain turned out to be small. After cessation of steady-state shearing, the variation of G″ with time was determined by the preshear rate (γ˙p). When γ˙p was low in Region I of the flow curve, G″ slowly increased with time after cessation of shearing. On the contrary, when γ˙p was high in Region III, G″ quickly decreased with the time. In a wide γ˙p range, G″ first increased and then decreased with the time of relaxation. After cessation of the steady-state shearing, scaling relationships of the time and γ˙p were observed for both G″ increasing and decreasing stages as well as the first stage of the strain recovery.

Colloidal aggregation and dynamics in anisotropic fluids.

We present experiments and numerical simulations to investigate the collective behavior of submicrometer-sized particles immersed in a nematic micellar solution. We use latex spheres with diameters ranging from 190 to 780 nm and study their aggregation properties due to the interplay of the various colloidal forces at work in the system. We found that the morphology of aggregates strongly depends on the particle size, with evidence for two distinct regimes: the biggest inclusions clump together within minutes into either compact clusters or V-like structures that are completely consistent with attractive elastic interactions. On the contrary, the smallest particles form chains elongated along the nematic axis, within comparable timescales. In this regime, Monte Carlo simulations, based on a modified diffusion-limited cluster aggregation model, strongly suggest that the anisotropic rotational Brownian motion of the clusters combined with short-range depletion interactions dominate the system coarsening; elastic interactions no longer prevail. The simulations reproduce the sharp transition between the two regimes on increasing the particle size. We provide reasonable estimates to interpret our data and propose a likely scenario for colloidal aggregation. These results emphasize the growing importance of the diffusion of species at suboptical-wavelength scales and raise a number of fundamental issues.

Comparative Analysis of Fluorine-Containing Mesogenic Derivatives of Carborane, Bicyclo[2.2.2]octane, Cyclohexane, and Benzene using the Maier–Meier Theory

Two series of related three-ring nematoges with Δε > 0 and containing 12-vetex carborane (A), 10-vertex carborane (B), bicyclo[2.2.2]octane (C), cyclohexane (D) and benzene (E) were prepared and investigated as additives to 6-CHBT nematic host and in the pure form (series 2). Dielectric results were analyzed with the Maier-Meier relationship to gain an understanding of behavior of additives in nematic solutions. Molecular parameters for each nematogen were obtained at the B3LYP/6-31G(d,p) level of theory in the host's dielectric medium, and dielectric data was used as the only experimental parameter to calculate apparent order parameter Sapp and the Kirkwood factor g. The results demonstrated that compounds in series 1 stabilize the nematic phase (high Sapp) of the host more than additives in series 2 (low Sapp), and carbocycles C and D are more effective (higher Sapp) than carborane analogues A and B (lower Sapp). The method provides insight into behavior of additives in nematic solutions and is useful for comparative analysis of a series of compounds or a series of hosts.

Alignment of nematic and bundled semiflexible polymers in cell-sized confinement

The finite size of cells poses severe spatial constraints on the network of semiflexible filaments called the cytoskeleton, a main determinant of cell shape. At the same time, the high packing density of cytoskeletal filaments poses mutual packing constraints. Here we investigate the competition between excluded volume interactions in the bulk and surface packing constraints on the orientational ordering of confined actin filaments as a function of filament density and the presence of crosslinks. We grow fluorescently labeled actin filaments in shallow (thickness dz 3 μm), rectangular microchambers with a systematically varied length (dy between 5 and 100 μm) and in-plane aspect ratio (dx/dy between 1 and 10). We determine the nematic director field by image analysis of fluorescence confocal images. We find that high-density (nematic) solutions respond sensitively to changes in the size and aspect ratio of the chambers. In small chambers (dy ≤ 20 μm), filaments align parallel to the long walls as soon as the aspect ratio is ≥1.5, indicating that surface-induced ordering dominates. In larger chambers, the filaments instead align along the chamber diagonal, indicating that bulk packing constraints dominate. The nematic order parameter is maximal in small and highly anisometric chambers. In contrast to the nematic solutions, low-density (isotropic) solutions are rather insensitive to confinement. Bundled actin solutions behave similarly to nematic solutions, but are less well-ordered. Our observations imply that the orientational order of actin filaments in flat confining geometries is primarily determined by a balance between bulk and surface packing constraints with a minimal effect of the enthalpic cost of filament bending. Our assay provides an interesting platform for the future reconstitution of more complex, active cytoskeletal systems with actively treadmilling filaments or molecular motors.