Nematic Host Phase Research Articles

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.

Statistical Theory of Helical Twisting in Nematic Liquid Crystals Doped with Chiral Nanoparticles

A molecular field theory of the cholesteric ordering in nematic nanocomposites doped with chiral nanoparticles was developed taking into consideration chiral dispersion interaction between rod-like nanoparticles. It was shown that the inverse pitch of the cholesteric helical structure is proportional to the anisotropy of the effective polarizability and the anisotropy of the effective gyration tensor of a nanoparticle in the nematic host. The theory enables one to predict the helical sense inversion induced by a change of the low-frequency dielectric susceptibility of the nematic host phase. The components of the high-frequency effective polarizability and the effective optical activity of a gold rod-like nanoparticle in a particular nematic solvent were calculated numerically.

Molecular-Theory of High Frequency Dielectric Susceptibility of Nematic Nanocomposites

A molecular-statistical theory of the high frequency dielectric susceptibility of the nematic nanocomposites has been developed and approximate analytical expressions for the susceptibility have been obtained in terms of the effective polarizability of a nanoparticle in the nematic host, volume fraction of the nanoparticles and the susceptibility of the pure nematic phase. A simple expression for the split of the plasmon resonance of the nanoparticles in the nematic host has been obtained and it has been shown that in the resonance frequency range the high frequency dielectric anisotropy of the nanocomposite may be significantly larger than that of the pure nematic host. As a result, all dielectric and optical properties of the nanocomposite related to the anisotropy are significantly enhanced which may be important for emerging applications. The components of the dielectric susceptibility have been calculated numerically for particular nematic nanocomposites with gold and silver nanoparicles as functions of the nanoparticle volume fraction and frequency. The splitting of the plasmon resonance has been observed together with the significant dependence on the nanoparticle volume fraction and the parameters of the nematic host phase.

Coarse-grained treatment of the self-assembly of colloids suspended in a nematic host phase.

The complex interplay of molecular scale effects, nonlinearities in the orientational field and long-range elastic forces makes liquid-crystal physics very challenging. A consistent way to extract information from the microscopic, molecular scale up to the meso- and macroscopic scale is still missing. Here, we develop a hybrid procedure that bridges this gap by combining extensive Monte Carlo (MC) simulations, a local Landau-de Gennes theory, classical density functional theory, and finite-size scaling theory. As a test case to demonstrate the power and validity of our novel approach we study the effective interaction among colloids with Boojum defect topology immersed in a nematic liquid crystal. In particular, at sufficiently small separations colloids attract each other if the angle between their center-of-mass distance vector and the far-field nematic director is about 30°. Using the effective potential in coarse-grained two-dimensional MC simulations we show that self-assembled structures formed by the colloids are in excellent agreement with experimental data.

Flow-induced deformation of closed disclination lines near a spherical colloid immersed in a nematic host phase

ABSTRACTWe present nonequilibrium molecular dynamics simulations of a spherical colloidal particle with a chemically homogeneous surface immersed in a nematic liquid-crystal host phase. This setup is then placed between planar and atomically structured substrate surfaces that serve to fix the nematic far-field director . The substrates are separated by a sufficiently large distance such that they do not interfere directly with the environment of the colloid. Because of a mismatch between and the local homeotropic anchoring of molecules of the liquid crystal (i.e., mesogens) at the surface of the colloid circular defect (Saturn) rings ℓ arise if the host is in thermodynamic equilibrium (i.e., in the absence of flow). The size of these rings depends on the range of the mesogen–colloid interactions which we model via an attractive Yukawa potential. As Poiseuille flow is initiated, ℓ is deformed. The degree of deformation is analysed quantitatively in terms of characteristic geometric parameters fitted to suitable projections of ℓ. Our results suggest that smaller ℓ are shifted downstream while approximately maintaining their circular shape, whereas larger ones exhibit an elastic deformation in addition. We provide a simple geometric argument to predict the downstream shift of smaller, circular ℓs in excellent agreement with the simulation data over the range of steady-state flows considered.

Study of the Cholesteric-to-Cholesteric Phase Transitions on the Lyotropic Mixture of KL/K2SO4/1-Undecanol/Water/Brucine Presenting the Cholesteric Biaxial Phase

Lyotropic cholesteric liquid crystalline phases were prepared by doping the quaternary mixture of potassium laurate (KL)/potassium sulfate (K2SO4)/1-undecanol (UndeOH)/water with brucine. The phase diagram was constructed as a function of the brucine concentration. Three cholesteric phases were identified: cholesteric biaxial, calamitic uniaxial, and discotic uniaxial. It was observed that there is a critical brucine concentration (X* b ) to cholesterize the nematic host phase when the mixture is confined in a thin sample holder, below which the cholesteric helical arrangement is not achieved. The helical twisting power of brucine was calculated as 12.12 ± 0.40 μm−1.

Phase Behavior of Nanoparticles in a Thermotropic Liquid Crystal

In this work, we describe the outstanding behavior of a nanocomposite system composed of the thermotropic liquid crystal 5CB doped with nanoparticles of the magnetic iron oxide maghemite (gamma-Fe(2)O(3)). We show that the I-N transition is associated with a reversible gathering of nanoparticles inside droplets of the ferronematic phase coexisting with a nonmagnetic nematic host phase.

Molecular Structure and Helical Twisting Power

The helical structure of the chiral nematic phases induced by chiral dopants in nematic solvents provides macroscopic information about the molecular chirality of the dopant. The surface model, which combines the order parameters of the dopant in nematic host phase and chirality parameters of the dopant, is a simple method to characterise the molecular properties and the induced chiral nematic phases. In this model, the representation of the molecular surface accesible from the nematic solvent plays an important role. The rolling sphere method, in common use for calculation of the molecular surface, rolls over a set of the van der Waals spheres representing constituent atoms of the molecule. In this paper we will present the effect of changing the radius of the rolling sphere on the values of the surface chirality indices. Here chiral binaphthyl molecules have been choosen since their twisting ability has a relatively strong dependence on the molecular structure. We also discuss a range of chirality indices and their dependence on the ordering strength of the solvent.

Structures and Properties of Induced Lyotropic Cholesteric Phases

Phase chirality in disk-like lyotropic cholesteric phases (ChD) was investigated, which were induced by center and axial chiral dopants to achiral lyotropic nematic host phases (ND). In a lyotropic nematic matrix of the ND phase in the ternary system hexadecyldimethylethylammoniumbromid (C16Me2EABr)/water/n-decanol, a disk-like lyotropic cholesteric phase ChD was induced by addition of the axial optically active compound R(−)-1,1′-binaphthalene-2,2′-diyl-hydrogenphosphate (BDP). The HTP value of the BDP is generally lower than the HTP value of inducing substances with center chirality as cholesterol, prednisolon etc. At constant composition of the ND phase, the helix lengths were determined in dependence on the BDP concentration by means of evaluation of the “spaghetti-like texture” using polarizing microscopy. The reciprocal helix-lengths are changing linearly with rising BDP concentration. Furthermore selected results of X-ray scattering and diffraction on the structure formation in the ChD phase were discussed.

Sugar Amphiphiles as Revealing Dopants for Induced Chiral Nematic Lyotropic Liquid Crystals

The existence of phase chirality in lyotropic liquid crystals still raises questions. The mechanisms behind the transfer of chirality throughout the long-range orientational order are not yet obvious. Guest/host systems with chiral dopants in achiral host phases offer the capability of systematic investigations. We demonstrate that the large amount of accessible sugar amphiphiles exhibits remarkable structure/property relations. Their helical twisting power HTP increases strongly with the number of sugar units of a dopant molecule. The spatial range of the chirality information reaching from a chirally doped micelle to adjacent aggregates is essential for the development of phase chirality. The induced twist of the lyotropic nematic host phase is highly sensitive to small changes of the sugar type (e.g., galacto- to glucopyranose). Depending on the nature of the host phase, either the α- or the β-linkage of the sugar to the hydrophobic moiety of the sugar dopant results in larger HTP values. We propose that our amphiphilic sugar derivatives act like antennae to transfer chirality information. Their effectiveness as chiral dopants is due to a hydrophobic anchoring within the micelles and an extension of their chiral moiety far into the intermicellar region. The chirality transfer works especially well if the hydrophilic and chiral sugar moieties are oriented toward a neighboring micelle in the direction of the helix axis.

Influence of Lyotropic Nematic Host Phases on the Twisting Power of Chiral Dopants

The twisting power of several chiral dopants in two host phase classes is investigated under the viewpoint of its dependence on chain length and size of the headgroups of the surfactants. The results are explained qualitatively, using a simple model of intramicellar chiral induction, which is sensitive to flexibilities and lengths of the surfactant chains. Additional hints for this model are given by the temperature dependence of the pitch. It is shown that p(T) of “solid” micelles can be described by a model/formula valid for thermotropic liquid crystals and that deviations become larger if the micelles become more flexible. To interpret the observed temperature dependences, it is speculated that a change of dominance occurs, from sterical chiral interactions to dispersive ones.

Helical twisting power of laterally aryl substituted chiral mesogens

Abstract The relationship between the helical twisting power (HTP) of cholesteric liquid crystals and the molecular structure of the chiral mesogens has been investigated. Rod-like mesogens are compared with analogues bearing a bulky lateral branch. Additionally, the HTP of induced cholesteric phases formed by chiral guest molecules in nematic host phases has been studied in terms of different molecular structures. The paper gives information on the influence of bulky lateral groups in mesogens on the HTP.

An X-ray diffraction study of a mixture of azo dyes with 4-n-pentyl-4′-cyano-p-terphenyl

Abstract The intensities of the X-ray diffraction pattern of the aligned nematic phase of T15-azo dye mixtures have been measured as a function of temperature. Intermolecular distances as well as the monomer and associate lengths of the T15 molecules in the presence of the dyes were determined. The dimer fraction in the liquid crystal-dye mixtures were estimated and it was found that dyes with nitro groups in the para position influence particularly the monomer to dimer ratio in the liquid crystal range. The orientational distribution function, fd(β) and the order parameter H and H were calculated from the wide angle region. The perturbation of the nematic host phase by the dyes is discussed.

Calorific Effect of the Magnetically Induced Phase Transition in Induced Chiral Nematic Systems

Abstract The study of the calorific effect of the magnetically-induced phase transition in doped nematic phases is described. The systems examined consisted of PCB as a nematic host phase and S-N-1-phenylethyl-4n-alkylbenzamides as non-mesogenic optically active dopants. The calorific effect was estimated indirectly from the temperature dependence of the magnetic-field-threshold of the helical unwinding and of the reverse process; i.e. the helical winding. It appears that it is the ratio of the length of an optically active dopant and that of the nematic host which determines the temperature dependencies of such parameters as the helical pitch, microscopic twisting power, and latent heat. A comparison with the magnetocaloric effect in nematic liquid crystals1 is also made.

Infrared Linear Dichroism of Nematic Solutions II. Correlation of Biaxiality with Molecular Properties

Abstract The biaxial ordering of some monosubstituted benzenes was determined from the infrared linear dichroism of solutions in the nematic host phase ZLI 1695. A correlation was found between the biaxiality and the anisotropy of the molecular shape. The orientational order of the solutes in this solvent appears to be determined entirely by dispersion forces but in other nematic solvents additional types of intermolecular forces are involved.

Odd-even effect in the helical twisting power of chiral molecules in induced cholesteric mesophases

A chiral compound is dissolved in a series of nematic liquid crystalline solvents of varying chain length to produce induced cholesteric phases. Using infrared techniques, it is found that the helical twisting power of the two chiral compounds investigated show an odd-even effect. This can be rationalized as the consequence of the odd-even effect in the order parameter of the nematic host phases.

Temperature Dependence of the Intrinsic Pitch in Induced Cholesteric Systems

AbstractBased on the molecular‐statistical theory of Goossens a model has been developed to explain the temperature dependence of the intrinsic pitch in cholesteric systems induced in nematic host phases by chiral guest molecules. In the case of a rotational symmetric distribution function around the long axis of the chiral molecule the helix inducing dipole‐quadrupole interaction forces level out. Consequently, only a hindered long‐axis rotation causes the helical structure whose pitch increases with increasing temperature. The theoretically predicted value of the relative temperature gradient of reciprocal pitch 1/p−1(dp−1/dT) are in good agreement with the presented experimental data. The influence of structure of the chiral guest molecules on the helical twisting power as well as that of the nematic host phases also can be understood in terms of the proposed model.