Calamitic Nematic Phases Research Articles

Structure and local order of lyotropic cholesteric calamitic phases: The effect of the chiral molecule

We investigate a lyotropic mixture presenting in the calamitic nematic phase (NC) and its corresponding calamitic cholesteric phase (ChC), where a small amount of the chiral agent (brucine sulfate) was added. Different experimental techniques (polarized optical microscopy and laser conoscopy) were used to characterize the phases. The main technique employed in the analysis of the structure and local ordering at nanoscale is the Small-Angle X-ray Scattering, where advanced modeling analysis were applied. The lyotropic nematic mixtures was composed of potassium laurate/potassium sulfate/dodecanol/water and the cholesteric phases were obtained from these mixtures, by adding the chiral molecule, brucine sulfate. From an advanced modeling analysis, we show that the micellar overall shape is not modified by the doping with brucine. However, the presence of the brucine between micelles in the ChC phase imposes a higher correlation between micelles along the direction of the pseudo-lamellar ordering. Finally, the order parameter P2 was calculated and these values for the phases NC and ChC are 0.8133(6) and 0.747(2), respectively, indicating a slightly higher orientational ordering in the NC phase.

Shear viscosity and rheology of ternary and quaternary lyotropic liquid crystals in discotic and calamitic nematic phases

We report rheological studies of micellar nematic phases of potassium laurate/ decanol/ water mixtures with and without addition of salt (K2SO4) at a fixed temperature of 20 ∘C. Using a cone-plate rheometer, we verified that the ternary mixture (i.e., without salt) exhibiting the discotic (ND) nematic phase showed only a slight pseudo-plastic (or shear-thinning) behavior. In contrast, a pronounced pseudo-plastic effect was observed in the ND phase of the quaternary samples. On the other hand, all the NC samples (salted or unsalted) showed a quasi-Newtonian behavior. In addition, we have made viscosity measurements using a (variation of a) rotating Couette flow viscometer. Finally, we also performed density measurements, using a density cell attached to the viscometer. Our results showed a contrasting behavior between the two types of nematic phases in two aspects. Firstly, regarding the ternary samples only, the apparent viscosity η at moderate shear rates was lower in the NC than in the ND phase. The second aspect, regarding both ternary and quaternary samples, is that η in the ND phase dramatically increased as the salt concentration increased, while in the NC phase η was quite insensitive to the addition of salt. In order to analyse the results, we have used two different theoretical frameworks, namely the nematodynamics in the first aspect and the classical theory of electrolyte solutions in the second one.

Biaxial coherence length in a nematic π-cell

In a highly frustrated calamitic nematic phase, the strain can be relaxed by lowering the nematic order: the starting uniaxial symmetry can be broken and it can be replaced locally with transient biaxial domains. Using simple optical retardation measurements, we estimate the length scale over which the biaxial disturbance decays in space within a π-cell submitted to a weak electric field. We also characterise the transition cascade from the uniaxial splay texture to a bend texture through slow defect motion.

Oscillating fronts produced by spinodal decomposition of metastable ordered phases

Mesophase–particle mixtures are emerging functional material systems whose structure, dynamics and formation through phase transitions are poorly understood. In this paper, novel phase transitions in a system of coupled conserved and non-conserved order parameters – a mixture of liquid crystals and nanoparticles – are studied via phase-field modelling, under conditions where an intermediate phase, which is unstable to composition fluctuations, can be formed between the initial supercooled phase and the new stable phase. This paper analyzes the formation, stability and evolution of this intermediate phase and how it impacts the transformation of an isotropic phase when it is quenched with single and double temperature jumps to form a calamitic nematic phase. It is found that the intermediate phase forms through a front-splitting mechanism and grows for some time, but eventually decays through a spinodal decomposition starting at the interface and propagating to the bulk, producing a moving oscillating front. It is found that this phenomenon can be triggered solely by the presence of the interface even in the absence of composition fluctuations in the bulk. Spinodal decomposition initiated by thermal fluctuations also generates a moving oscillating front (because the intermediate phase was formed by a moving front). As a consequence, the velocity of the moving front changes from a small value at short time (interface-induced spinodal), to a large value at some finite time (moving bulk spinodal). The effect of a double quench was also analyzed and it was found that the oscillating front can be confined to a region close to the interface (“mushy” region), and the isotropic phase (originally of infinite extension) can be confined to a region close to the oscillating front, generating a very atypical phase morphology.

Absorption coefficient and order parameter in a reentrant isotropic–calamitic nematic phase transition

The parallel ( k ∥) and perpendicular ( k ⊥) absorption coefficients are determined along the entire range of a calamitic nematic phase ( N C ) and near the N C –reentrant isotropic ( I RE ) and N C –isotropic ( I) phase transitions. We have made these measurements in a lyotropic mixture of potassium laurate, decanol and D 2O in the spectral range from 400 to 700 nm using a spectrophotometer equipped with sheet polarizers. Through the dichroic ratio ( R = k ∥/ k ⊥) the order parameter ( S) is obtained. The nature of these transitions is discussed as a consequence of experimental data.

Self-Alignment of Dye Molecules in Micelles and Lamellae for Three-Dimensional Imaging of Lyotropic Liquid Crystals

We report alignment of anisotropic amphiphilic dye molecules within oblate and prolate anisotropic micelles and lamellae, the basic building blocks of surfactant-based lyotropic liquid crystals. Absorption and fluorescence transition dipole moments of these dye molecules orient either parallel or orthogonal to the liquid crystal director. This alignment enables three-dimensional visualization of director structures and defects in different lyotropic mesophases by means of fluorescence confocal polarizing microscopy and two-photon excitation fluorescence polarizing microscopy. The studied structures include nematic tactoids, Schlieren texture with disclinations in the calamitic nematic phase, oily streaks in the lamellar phase, developable domains in the columnar hexagonal phase, and various types of line defects in the discotic cholesteric phase. Orientational three-dimensional imaging of structures in the lyotropic cholesterics reveals large Burgers vector dislocations in cholesteric layering with singular disclinations in the dislocation cores that are not common for their thermotropic counterparts.

The heat conductivity of liquid crystal phases of a soft ellipsoid string-fluid evaluated by molecular dynamics simulation

We have applied a nonequilibrium molecular dynamics heat flow algorithm to calculate the heat conductivity of a molecular model system, which forms uniaxial and biaxial nematic liquid crystals. The model system consists of a soft ellipsoid string-fluid where the ellipsoids interact according to a repulsive version of the Gay-Berne potential. On compression, this system forms discotic or calamitic uniaxial nematic phases depending on the dimensions of the molecules, and on further compression a biaxial nematic phase is formed. In the discotic nematic phase, the heat conductivity has two components, one parallel and one perpendicular to the director, where the last mentioned component is the largest one. This order of magnitudes is reversed in the calamitic nematic phase. In the biaxial nematic phase there are three components of the heat conductivity, one in the direction around which the long axes of the molecules are oriented, this is the largest component, another one in the direction around which the normals of the broadsides of the molecules are oriented, this is the smallest component, and one in the direction perpendicular to these two directions with a magnitude in between those of the first mentioned components. The relative magnitudes of the components of the heat conductivity span a fairly wide interval so it should be possible to use the model to parameterise experimental data.

Thermal diffusivity and the conformal transformation on nematic liquid crystals

In this paper, we will study theoretically and experimentally the anisotropy of the thermal diffusivity in nematic liquid crystals. We will show that the Baalss-Hess conformal transformation [D. Baalss and S. Hess, Phys. Rev. Lett. 57, 86 (1986)], can be used to obtain a free of adjustable parameters equation that describes the anisotropy of the heat diffusion in these materials. The results of this theory will be compared with experimental data. This study will partially confirm the widely known experimental evidences that indicate that the thermal diffusivity is larger in the director direction than in the one perpendicular to it. For calamitic nematic phases this is true; for discotic nematic ones the reverse situation would be found; the diffusivity would become larger in the direction perpendicular to the director. We will also present experimental data supporting this theoretical prediction. The data comprehend the calamitic and discotic nematic lyotropic phases and a nematic thermotropic phase.

Nonlinear refractive index measurements of discotic and calamitic nematic lyotropic phases

In this work, through the Z-scan technique, we report on measurements of the nonlinear refractive index (n{2}) in discotic and calamitic nematic phases at room temperature in lyotropic mixtures of potassium laurate, decanol and D(2)O . This technique presents high sensitivity when compared to conventional interferometry. The nonlinear optical birefringence (Deltan{2}) of these nematic phases was also determined. The sign and absolute value of this relevant nonlinear parameter are discussed in terms of structural changes in the micellar configuration which takes place in each nematic lyotropic phase.

Bend and splay elastic constants at a reentrant isotropic–calamitic-nematic phase transition

By measuring the ratio between the major and minor axes of closed elliptical loops created by a magnetic field, the ratio between the bend (K33) and splay (K11) elastic constants of a lyotropic mixture of potassium laurate, decanol, and water in the calamitic nematic phase is measured as a function of temperature. Since these systems present two nematic-isotropic phase transitions--the usual one, at high temperatures, and a reentrant one, at low temperatures--such measurements are used to compare the behavior of these elastic constants at the neighborhoods of these two distinct regions, which have in common the vanishing of the order parameter. The experimental data have revealed the existence of two elastic constant branches, characterizing a distinct low- and high-temperature behavior. To explain such asymmetry we support that, because of the lyotropic nature of the compound, the micellar variation, which is mainly responsible for the reentrant phase, plays a distinct role at these two phase transitions.

Molar Refractivity and Thermal Expansion Coefficient of a Lyotropic Nematic Phase

ABSTRACT Measurements of the extraordinary/ordinary molar refractivity and thermal expansion coefficient as a function of temperature are presented for the lyotropic mixture of potassium laurate, decanol and water. The measurements cover the range of the calamitic nematic phase (N C ) and the vicinity of the reentrant isotropic (I RE )–N C –isotropic (I) phase transitions. Thermal expansion coefficient (β) was obtained from density data while that the refractivity parameters were determined through of the Vuks's equation together with refractive indices and density values. These parameters are discussed in terms of structural changes in the micellar configuration at the phase transitions.

Temperature dependence of the refractive index near the reentrant-isotropic--calamitic-nematic phase transition.

The laser-induced nonlinear optical response of a lyotropic liquid crystal system in the reentrant-isotropic and calamitic-nematic phases is investigated by the use of the thermal lens technique. The occurrence of an inversion in the temperature coefficient of the ordinary refractive index, dn(radially)/dT, near the reentrant-isotropic-calamitic-nematic phase transition, is discussed. This effect is attributed to the behavior of the electronic polarizability due to the change in micelle shape near the isotropic-nematic transition, and correlated with the results obtained near the nematic-isotropic transition, previously reported.

Biaxial Micelles in a Uniaxial Nematic Phase. An Angular-Dependent 23Na NMR Study

The angular dependence of longitudinal 23Na NMR spin relaxation in a lyotropic calamitic nematic phase is determined using a simple rotor technique. Analyzing the director-frame spectral densities, (ω), n = 0, 1, 2, and the quadrupole splitting shows that the experimental data cannot be accounted for in terms of the prolate micellar aggregates being uniaxial. Rather, the results indicate the micelles to be biaxial, with approximate axial ratios of ρ- ≈ 1.5 and ρ+ ≈ 3−5. Comparison with previous investigations of the nematic region in the sodium dodecyl sulfate−decanol−water system shows the interfacial curvature of the micelles to vary with composition in a way similar to that observed in the lamellar phase.

Reentrant Uniaxial-Biaxial Transition in Poly-Ethylene-Glycol Doped Lyotropic Liquid Crystals

Abstract Interferometric measurements of the effect of a polymer gradativelly added in a lyotropic liquid crystal is reported here. Studies of the phase diagram, birefringence and critical exponents as a function of the polymer concentration are presented. Measurements of the optical birefringence as a function of temperature are performed with the micellar system potassium laurate, 1-decanol, and water doped with variable quantities of the polymer poly-ethylene-glycol (PEG). The transitions from the calamitic nematic phase to a reentrant discotic-biaxial-discotic phase were observed when increasing the PEG molar concentration. Our results confirm the critical properties of the uniaxial-to-biaxial nematic transition. The critical exponent for the order parameter (β = 0.38) obtained for the sample without PEG is in god agreement with the value caculated for the XY model. Some results with higher PEG doping concentration indicate that β is higher than 0.38 leading to the mean-field value.

Micellar fluctuations in a calamitic nematic phase of a lyotropic liquid crystal studied by photon correlation

Abstract Using a Rayleigh scattering light beating technique, we have made new measurements on the slow polarized fluctuations, previously identified as being of micellar origin, for a lyotropic system consisting of K-laurate, decanol, D2O and presenting only the uniaxial calamitic nematic (Nc) phase (from among the three known nematic types: uniaxial disk-like (ND), calamitic (Nc) and biaxial (NB) phases). An elaborated analysis of these data is made in order to clarify the relationship between the light scattering signals coming from the micellar fluctuations and those from (classical) orientational fluctuations. The main conclusions of this analysis are that the micellar mode is independent of the temperature and wave-vector, and that its relaxation frequency is substantially lower than that found previously for the same lyotropic components, but presenting the biaxial nematic phase. Finally, some considerations are made of the possible relationships between the micellar fluctuations and the structural properties of lyotropic nematics.

A Mean-Field Theory of Nematic Ring Polymers

Abstract A mean-field calculation is presented for cyclic polymers with attached rod like mesogenic side groups. The mean-field includes energy contributions for ring-ring, rod-rod and ring-rod interactions. The uniaxial solution for this model yields two ordered phases: a conventional calamitic nematic phase in which the rods tend to align parallel and a discotic nematic phase in which it is the rings that tend to align parallel. These phases are shown to be dependent upon the relative strengths of the three energy contributions, the number of mesogens per ring and temperature. In certain cases polymorphism is observed with an isotropic phase cooling to the nematic discotic phase and then on further cooling to the calamitic nematic phase.