Cholesteric Helix Research Articles

Do Lehmann cholesteric droplets subjected to a temperature gradient rotate as rigid bodies?

We performed a Fluorescence Recovery After Photobleaching (FRAP) experiment during the Lehmann rotation of cholesteric droplets in thermodynamic coexistence with the isotropic liquid and subjected to a temperature gradient. By creating and tracking bleached spots near the surface of banded droplets (in which the cholesteric helix is perpendicular to the gradient) and concentric circle droplets oriented by an electric field (in which the helix is parallel to the gradient), we found that neither type of droplet rotates as a solid. This result shows that the texture rotation is mainly due to the local director rotation.

Simulating the pitch sensitivity of twisted nematics of patchy rods

Stiff, elongated biomolecules such as filamentous viruses, DNA or cellulose nanocrystals are known to form liquid crystals often exhibiting a helical supramolecular organization. Little is known about the microscopic origin, size and handedness of the helical pitch in these, so-called cholesteric phases. Experimental observations in chiral lyotropics suggest that long-ranged chiral forces of electrostatic origin acting between the mesogens are responsible for such organization. Using large-scale computer simulation we study the sensitivity of the pitch imparted by soft microscopic helices and confirm that the helical sense is sensitive to a change of packing fraction, magnitude of the molecular pitch and amplitude of the chiral interactions. In particular, we find evidence that the cholesteric helix sense may change spontaneously upon variation of particle density, at fixed molecular chirality. These pitch inversions have been reported in recent theoretical studies but simulation evidence remains elusive. We rationalize these sudden changes in the supramolecular helical symmetry on the basis of detailed measurements of the mean-torque generated by the twisting of the helices. The simulation methodology employed does not require confining the twisted nematic in a slab geometry and allows for a simultaneous measurement of the pitch and the twist elastic constant. We find that the twist elastic constant increases almost linearly with density suggesting that twisted nematic shows no signs of anomalous stiffening due to pre-smectic fluctuations at higher packing fraction.

Synthesis and spatial structure of new chiral dopants from allobetuline series for cholesteric liquid-crystal compositions

New series of chiral dopants for cholesteric liquid-crystal compositions were synthesized on the base of 2-substituted allobetuline derivatives, and their steric structure was determined by X-ray analysis. The relationship between spatial structure of these compounds and their ability to induce cholesteric helix in 4-pentyl-4′-cyanobiphenyl nematic solvent was examined. The highest values of the helical twisting power |β| (71.38 ± 3.4) and (84.25 ± 3.7) mkm−1 mol·pats−1 showed (E)-2-(4-chlorophenylmethylidene)-allobetuline and (2R,3R)-3-(4′-chlorophenyl)-2,2′-spiro-oxyranoallobetuline correspondingly. How the value of |β| in this series of compounds varies depending on the spatial arrangement relative to the aryl moiety of the chiral core is shown.

Nanostructures on the Basis of Porous Alumina with Intercalated with Cholesteric Liquid Crystal

The introduction of liquid crystal materials into the porous material host will help us to get optically active nanocomposites. Nanocomposite based on aluminum oxide host with cholesteric liquid crystal is characterized by a shift of bandwidth wavelength minimum. In case of intercalation of cholesteric liquid crystals into the pores of aluminum oxide host the shift of transmittance minimum into the short wave region are observed. We analyzed the ways, which indicate the deformation of the pitch of the cholesteric helix and showed the role of capillary forces in the deformation of the pitch of cholesteric liquid crystal.

Rheo-NMR study of water-based cellulose liquid crystal system at high shear rates

Since long ago cellulosic lyotropic liquid crystals were thought as potential materials to produce fibers competitive with spidersilk or Kevlar, yet the processing of high modulus materials from cellulose-based precursors was hampered by their complex rheological behavior. In this work, by using the Rheo-NMR technique, which combines deuterium NMR with rheology, we investigate the high shear rate regimes that may be of interest to the industrial processing of these materials. Whereas the low shear rate regimes were already investigated by this technique in different works [1–4], the high shear rates range is still lacking a detailed study. This work focuses on the orientational order in the system both under shear and subsequent relaxation process arising after shear cessation through the analysis of deuterium spectra from the deuterated solvent water. At the analyzed shear rates the cholesteric order is suppressed and a flow-aligned nematic is observed which for the higher shear rates develops after certain time periodic perturbations that transiently annihilate the order in the system. During relaxation the flow aligned nematic starts losing order due to the onset of the cholesteric helices leading to a period of very low order where cholesteric helices with different orientations are forming from the aligned nematic, followed in the final stage by an increase in order at long relaxation times corresponding to the development of aligned cholesteric domains. This study sheds light on the complex rheological behavior of chiral nematic cellulose-based systems and opens ways to improve its processing.

Dynamics of supramolecular structures induced in liquid crystals

A theory is experimentally validated for the first time, which describes (within nonequilibrium hydrodynamics) the properties of supramolecular spatially periodic structures in a planar layer of a cholesteric liquid crystal at ultrasound intensities exceeding the threshold of their formation in a frequency range where the wavelength is larger than the cholesteric helix pitch. Distortions in the layer texture in planar and wedgeshaped capillaries with planar boundary conditions at normal incidence of longitudinal waves have been observed for the frequency range of 1–4 MHz. The domain periods at the threshold of the effect and above it are determined for layers 20–100 μm thick at helix pitch values of 2–10 μm. The dependences of the domain period on the ultrasound intensity and the preliminary layer extension have been established. A model constructed with allowance for structural relaxation processes and nonlinear relaxation phenomena is tested. It is shown that a complete description of the effect, which is consistent with the experimental data, can be obtained only using approach.

Polarization holographic grating recording in the cholesteric azobenzene-containing films with the phototunable helix pitch

The recording of polarization gratings in films of a cholesteric liquid crystalline polymer with different helix pitch was studied in detail. For this purpose, the cholesteric mixture of the nematic azobenzene-containing copolymer with a chiral- photochromic dopant was prepared. The utilization of such mixture has made possible to realize dual optical photorecord- ing in one system, first due to the phototuning of the helix pitch by UV light and second the polarization grating recording process by exposure with polarized visible light. The diffraction efficiency strongly depends on the cholesteric helix pitch and films thickness: the increase of the confinement ratio d/p (where d, film thickness; p, helix pitch) results in growth of the diffraction efficiency. Comparison of the induction of polariza- tion gratings in cholesteric, nematic (copolymer without chiral dopant), and amorphous (nonannealed) cholesteric films revealed that only the cholesteric films were characterized by significant oscillations in the diffraction efficiency signal as well as by the presence of the maximum in the first-order dif- fraction efficiency in the initial stage of the grating recording process. It was found that in addition to the polarization gra- ting surface relief gratings (SRGs) were also formed in the studied systems, however, the amplitude of the SRG inscribed in the cholesteric films was lower (� 20 nm) compared to the grating amplitude obtained in nematic films (� 60 nm). More- over, increasing helix pitch resulted in a decrease of the SRG amplitude. The obtained experimental data demonstrate the great potential of cholesteric LC mixtures of such type for dif- ferent applications as photoactive materials for photonics. V C 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 773-781

Lehmann rotation of the cholesteric helix in droplets oriented by an electric field.

We study the Lehmann rotation of the cholesteric helix in droplets of the liquid crystal N-(p-methoxybenzilidene)-p-butylaniline doped with a small amount of the chiral molecule R811 when they are subjected to a temperature gradient. We show that the helix rotates much faster when it is parallel to the temperature gradient than when it is perpendicular to it. The first configuration is obtained by submitting the droplets to an ac electric field parallel to the temperature gradient, whereas the second one is observed at zero field. We show that the rotation velocity of the helix strongly depends on the droplet radius, even when the helix is parallel to the temperature gradient. This observation supports the idea that the Leslie thermomechanical coupling cannot explain alone the Lehmann effect.

Disclination lines at homogeneous and heterogeneous colloids immersed in a chiral liquid crystal.

In the present work we perform Monte Carlo simulations in the isothermal-isobaric ensemble to study defect topologies formed in a cholesteric liquid crystal due to the presence of a spherical colloidal particle. Topological defects arise because of the competition between anchoring at the colloidal surface and the local director. We consider homogeneous colloids with either local homeotropic or planar anchoring to validate our model by comparison with earlier lattice Boltzmann studies. Furthermore, we perform simulations of a colloid in a twisted nematic cell and discuss the difference between induced and intrinsic chirality on the formation of topological defects. We present a simple geometrical argument capable of describing the complex three-dimensional topology of disclination lines evolving near the surface of the colloid. The presence of a Janus colloid in a cholesteric host fluid reveals a rich variety of defect structures. Using the Frank free energy we analyze these defects quantitatively indicating a preferred orientation of the Janus colloid relative to the cholesteric helix.

Induction of selective reflection bands by a pulsed electric field in layers of chiral liquid crystals

The effect of the induction and “triplet” splitting of intense selective reflection bands has been experimentally revealed in chiral liquid crystals in a pulsed electric field. The effect is associated with the strong deformation of the helicoid of a chiral liquid crystal in the pulsed field and with the conservation of its step even at field amplitudes exceeding the critical field of the untwisting of the cholesteric helix. Intense polarization-sensitive light reflection bands appear in the spectral ranges where higher orders of selective reflection are forbidden for the unperturbed chiral liquid crystal.

Unwinding of a cholesteric liquid crystal and bidirectional surface anchoring

We examine the influence of bidirectional anchoring on the unwinding of a planar cholesteric liquid crystal induced by the application of a magnetic field. We consider a liquid crystal layer confined between two plates with the helical axis perpendicular to the substrates. We fix the director twist on one boundary and allow for bidirectional anchoring on the other by introducing a high-order surface potential. By minimizing the total free energy for the system, we investigate the untwisting of the cholesteric helix as the liquid crystal attempts to align with the magnetic field. The transitions between metastable states occur as a series of pitchjumps as the helix expels quarter- or half-turn twists, depending on the relative sizes of the strength of the surface potential and the bidirectional anchoring. We show that secondary easy axis directions can play a significant role in the unwinding of the cholesteric in its transition towards a nematic, especially when the surface anchoring strength is large.

Specific features of the spectral properties of a cholesteric liquid crystal with a resonance defective nanocomposite layer

This paper reports on studying the spectral properties of a cholesteric liquid crystal with a defective layer of a nanocomposite consisting of metallic nanoballs dispersed in a transparent matrix and characterized by an effective resonance dielectric permittivity. The transmission, reflection, and absorption spectra of waves of both circular polarizations have been calculated, and the spectral splitting of the defective mode when its frequency coincides with the resonance frequency of the nanocomposite has been studied. An essential dependence of the splitting on the nanoball concentration in the defect has been established. It has been shown that, depending on the position of the resonance frequency with respect to the boundaries of the cholesteric band gap, an additional passband appears in the transmission spectrum, which corresponds to waves of the diffracting circular polarization, or an additional band gap for waves of both circular polarizations, which are substantially modified with variations of both the incidence angle of light and the cholesteric helix pitch.

Cholesteric liquid crystal–carbon nanotube composites with photo-settable reversible and memory electro-optic modes

The photoresponsive electro-optical composites based on cholesteric liquid crystal (CLC) with optically controlled chirality and a minute amount of carbon nanotubes (CNTs) are studied. In cells with homeotropic anchoring, these composites exhibit a transition from fingerprint texture to homeotropic nematic texture in the course of photoinduced unwinding of the cholesteric helix. Compared with the CLC counterpart, this transition is much delayed, because of the stabilization of cholesteric filamentary domains by CNTs. The CLC-CNT composites demonstrate dual-mode operation with optical switching between reversible and memory mode. It is found that the memory response is associated with the elastic network of filamentary cholesteric domains that stabilizes the planar CLC texture reached in an electric field. In turn, the reversible mode corresponds to the unwound cholesteric state. Potential applications of this effect are discussed.

Rheology–Structure Interrelationships of Hydroxypropylcellulose Liquid Crystal Solutions and Their Nanocomposites under Flow

A new approach to study the flow-induced orientation in LC polymer-based nanocomposites has been developed. This approach is a combination of optical, mechanical, and X-ray methods that allowed us to “visualize” physical processes taking place in these systems under flow. The HPC/water solution (matrices) concentrations were selected based on the accurate phase equilibrium lines determined with the optical interferometry method. Flow curves and the concentration dependence of the viscosity provide additional information on the phase state and structure of the samples. Rheo-X-ray data were obtained using a specially developed micro-Couette geometry consisting of a coaxial arrangement of two standard X-ray capillaries (the inner one rotates at different speeds). The orientation development of HPC in two-phase and full anisotropic solutions, as well as of the clay and HPC in the nanocomposites were measured (separately) as a function of shear rate. In addition, the decay of the orientation parameter with time after cessation of flow was analyzed to reveal several stages related to the disorientation and reformation of the cholesteric helix in HPC solutions. Crystalline nanoclay in flowing LC solutions also formed a mesophase structure. Specifically, the columnar mesophase of clay formed under certain conditions could be completely or partially converted to a discotic phase. Comparison of these results with optical images of the regular (circle-like) morphology observed under strong flow suggested that the elasticity is the general driving force for development of the regular instability in length scales of optical and X-ray scattering measurements.

A real time optical strain sensor based on a cholesteric liquid crystal network

A flexible, strain sensor for monitoring uniaxial deformations in oriented polymer films is demonstrated. The sensor consists of a crosslinked cholesteric liquid crystal layer which is spray-coated on a uniaxially oriented polyamide 6 substrate. When no deformation is applied, reflectivity measurements showed a well-defined reflection band. Upon uniaxial extension, the reflection band shifted towards lower wavelengths. A 40 nm shift was recorded for 13% of strain. This wavelength shift is attributed to a decrease in the cholesteric helix's pitch which is induced by the change in thickness of the CLC coating due to Poisson's contraction. As a result the colour of the film changed from orange to green with increasing strain. The optical response was found to follow the mechanical behaviour of the polymer substrate. Cycling loading showed a time dependent optical response of the sensor fitting with the viscoplastic mechanical response of the substrate giving real time information on the deformation.

Helical twisting in nemato-cholesteric systems based on cholesterol derivatives and photosensitive azoxy compounds

For cholesteric liquid crystal systems containing photosensitive nematic ZhK�440 and a mixture of cholesterol derivatives, changes in helical twisting induced by UV radiation were studied. The UVinduced shift of selective reflection maximum λ max was shown to depend upon concentration of the nematic compo� nent. For low concentrations of ZhK�440, λmax increases, which correlates with corresponding changes with increasing temperature. For higher concentrations, λmax decreases, regardless of the temperature behavior of the system. A theoretical description of the available experimental data is proposed on the basis of develop� ment of molecular models of helical twisting, including an assumed possibility of ordered orientation of short molecular axes of cisisomers formed as a result of UV irradiatio n, which is determined by the sense of the cholesteric helix already present in the system.

Rheology of cubic blue phases

We study the behaviour of cubic blue phases under shear flow via lattice Boltzmann simulations. We focus on the two experimentally observed phases, Blue Phase I (BPI) and Blue Phase II (BPII). The disclination network of Blue Phase II continuously breaks and reforms under steady shear, leading to an oscillatory stress response in time. For larger shear rates, the structure breaks up into a Grandjean texture with a cholesteric helix lying along the flow gradient direction. Blue Phase I leads to a very different response. Here, oscillations are only possible for intermediate shear rates -- very slow flow causes a transition of the initially ordered structure into an amorphous network with an apparent yield stress. Larger shear rates lead to another amorphous state with different structure of the defect network. For even larger flow rates the same break-up into a Grandjean texture as for Blue Phase II is observed. At the highest imposed flow rates both cubic blue phases adopt a flow-aligned nematic state. Our results provide the first theoretical investigation of sheared blue phases in large systems, and are relevant to understanding the bulk rheology of these materials.

Attractive-repulsive dynamics on light-responsive chiral microparticles induced by polarized tweezers

Multifunctional colloidal micro and nano-particles with controlled architectures have very promising properties for applications in bio and nanotechnologies. Here we report on the unique dichotomous dynamical behaviour of chiral spherical microparticles, either fluid or solid, manipulated by polarized optical tweezers. The particles are created using a reactive mesogen mixed with a chiral dopant to form cholesteric liquid crystal droplets in water emulsion. The photopolymerization enables the chiral supramolecular configurations to be frozen in solid particles. Different internal architectures in the supramolecular structures, guided by the interfacial chemistry, enable optically isotropic or anisotropic spherical objects to be obtained. For particles having radial configuration of the cholesteric helices, we show that light can exert either a repulsive or attractive force depending on the handedness of its circular polarization, due to the unique selective reflection property of the cholesteric phase. On the other hand, very exotic dynamics is observed in the case of anisotropic chiral particles. Depending on the light handedness, they behave like Janus spherical particles with dissimilar optical properties, meaning that the surface of the dielectric particles is partly transparent and partly reflecting. We foresee interesting potential applications in micro and optofluidics, microphotonics and materials science.

Highly circularly polarized white light using a combination of white polymer light-emitting diode and wideband cholesteric liquid crystal reflector

We present a simple and intriguing device that generates highly circularly polarized white light. It comprises white polymer light-emitting diodes (WPLEDs) attached to a wideband cholesteric liquid crystal (CLC) reflector with a wide photonic bandgap (PBG) covering the visible range. The degree of circular polarization realized is very high over the visible range. The wide PBG was realized by introducing a gradient in pitch of the cholesteric helix by controlling the twisting power within the CLC medium. WPLEDs fabricated using a ternary (red, green, and blue) fluorescent polymer blend with the same moiety showed a low turn-on voltage, high brightness, high efficiency, and good color stability.

Influence of a Change in Helical Twisting Power of Photoresponsive Chiral Dopants on Rotational Manipulation of Micro‐Objects on the Surface of Chiral Nematic Liquid Crystalline Films

Herein we report a group of five planar chiral molecules as photon-mode chiral switches for the reversible control of the self-assembled superstructures of doped chiral nematic liquid crystals. The chiral switches are composed of an asymmetrically substituted aromatic moiety and a photoisomerizing azobenzene unit connected in a cyclic manner through methylene spacers of varying lengths. All the molecules show conformational restriction in the rotation of the asymmetrically substituted aromatic moiety in both the E and Z states of the azobenzene units resulting in planar chirality with separable enantiomers. Our newly synthesized compounds in pure enantiomeric form show high helical twisting power (HTP) in addition to an improved change in HTP between the E and Z states. The molecule with a diphenylnaphthalene unit shows the highest ever known initial helical twisting power among chiral dopants with planar chirality. In addition to the reversible tuning of reflection colors, we employed the enantiomers of these five compounds in combination with four nematic liquid crystalline hosts to study their properties as molecular machines; the change in HTP of the chiral dopant upon photoisomerization induces rotation of the texture of the liquid crystal surfaces. Importantly, this study has revealed a linear dependence of the ratio of the difference between HTPs before and after irradiation against the absolute value of the initial HTP, not the absolute value of the change in helical twisting power between two states, on the angle of rotation of micro-objects on chiral nematic liquid crystalline films. This study has also revealed that a change in irradiation intensity does not affect the maximum angle of rotation, but it does affect the speed of rotational reorganization of the cholesteric helix.