smectic-A Phase Research Articles

Nanosecond control and optical pulse shaping by stimulated emission depletion in a liquid crystal.

We study anisotropic Stimulated Emission Depletion (STED) from dye molecules, which are collectively ordered in a host liquid crystal. Due to the ordering of fluorescent emitters, the STED efficiency depends on the polarization of the depletion beam and time-delay of the STED pulse. The depletion efficiency is highest at lower temperatures in the highly ordered smectic-A phase and deteriorates in the higher temperature nematic and isotropic phases. We demonstrate by temporal tuning of STED that it is possible to generate an arbitrary sequence of nanosecond fluorescent pulses with variable width and variable delay. Our results show that the STED mechanism in principle allows for very fast (GHz) and efficient control of light by light, which could in the future be used for all-optical control of the flow of light in photonic microdevices based on liquid crystals. Using STED anisotropy and time-control, new modalities of STED imaging in liquid crystals could be developed.

Photopyroelectric Calorimetry for the Thermal and Optical Evaluations Over Phase Transitions in Liquid Crystals

We report on calorimetric and optical evaluations performed by Photopyroelectric calorimetry on two phenomena involving changes in average molecular alignment. We first study the possibility to obtain permanent homeotropic single domain alignment in the smecticA phase by applying a “vertical” strain field in the volume of samples of 4-decyl-4′-cyanobiphenyl and of 4-octyl-4′- cyanobiphenyl, liquid crystals. We then report on the effects produced by the illumination by UV light on 4-octyl-4′- cyanobiphenyl samples hosting photochromic molecules in terms of the change induced to the phase transitions characteristics and the average molecular alignment.

Tuning the mesomorphic properties of carbosilane-terminated and conventional 2-alkoxy-5-(4-alkoxyphenyl)-1,3,4-thiadiazole liquid crystals as binary mixtures with bimodal length distributions

The phase diagrams of binary mixtures of smectic C (SmC) mesogens – the carbosilane-terminated mesogen 2-(6-chlorohexyloxy)-5-(4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl)-1,3,4-thiadiazole (QL13-6) and one of nine conventional 2-alkoxy-5-(4-alkoxyphenyl)-1,3,4-thiadiazole mesogens (QL14-m/n) – with a molecular length ratio QL13-6/QL14-m/n ranging from 1.24 to 1.46 show the induction of a smectic A (SmA) phase in the mole fraction range 0.1 ≤ xQL13-6 ≤ 0.6. The results show that the temperature range of the induced SmA phase is remarkably sensitive to the molecular length ratio, and that this approach to tuning the mesomorphic properties of SmA–SmC liquid crystal mixtures may provide a powerful new tool in the formulation of mixtures for SSFLC and electroclinic applications.

On the calculation of the absolute grand potential of confined smectic-A phases

We determine the absolute grand potential Λ along a confined smectic-A branch of a calamitic liquid crystal system enclosed in a slit pore of transverse area A and width L, using the rod–rod Gay–Berne potential and a rod–wall potential favouring perpendicular orientation at the walls. For a confined phase with an integer number of smectic layers sandwiched between the opposite walls, we obtain the excess properties (excess grand potential Λexc, solvation force fs and adsorption Γ) with respect to the bulk phase at the same μ (chemical potential) and T (temperature) state point. While usual thermodynamic integration methods are used along the confined smectic branch to estimate the grand potential difference as μ is varied at fixed L, T, the absolute grand potential at one reference state point is obtained via the evaluation of the absolute Helmholtz free energy in the (N, L, A, T) canonical ensemble. It proceeds via a sequence of free energy difference estimations involving successively the cost of localising rods on layers and the switching on of a one-dimensional harmonic field to keep layers integrity coupled to the elimination of inter-layers and wall interactions. The absolute free energy of the resulting set of fully independent layers of interacting rods is finally estimated via the existing procedures. This work opens the way to the computer simulation study of phase transitions implying confined layered phases.

Phase transitions, optical, dielectric and viscoelastic properties of colloidal suspensions of BaTiO3 nanoparticles and cyanobiphenyl liquid crystals

We report experimental studies on the phase transitions and physical properties of colloidal suspensions of BaTiO3 nanoparticles and two cyanobiphenyl liquid crystals (4-pentyl-4ʹ-cyanobiphenyl and 4-octyl-4ʹ-cyanobiphenyl). From the differential scanning calorimetric measurements, we show that the nanoparticles have antagonistic effect on the isotropic to nematic and nematic to smectic-A phase transitions. The birefringence, dielectric anisotropy and splay elastic constant remain almost unchanged, whereas the bend elastic constant and rotational viscosity decrease considerably. The experimental results are discussed based on the possible contribution of BaTiO3 nanoparticles and free surfactant molecules in the suspensions.

Topological defect transformation and structural transition of two-dimensional colloidal crystals across the nematic to smectic-A phase transition.

We observe that topological defects in nematic colloids are strongly influenced by the elasticity and onset of smectic layering across the nematic (N) to smectic-A(SmA) phase transition. When approaching the SmA phase from above, the nematic hyperbolic hedgehog defect that accompanies a spherical colloidal inclusion is transformed into a focal conic line in the SmA phase. This phase transformation has a strong influence on the pairwise colloidal interaction and is responsible for a structural transition of two-dimensional colloidal crystals. The pretransitional behavior of the point defect is supported by Landau-de Gennes Q-tensor modeling accounting for the increasing elastic anisotropy.

Calorimetric study of phase transitions in nanocomposites of quantum dots and a liquid crystal

The complex specific heat is measured over a wide temperature range for the liquid crystal (LC) 4-cyano-4-octylbiphenyl (8CB) and cadmium sulfate quantum dots (QDs) composites as a function of QD concentration. The thermal scans were performed under near-equilibrium conditions for all samples having QDs weight percent (φw) from 0 to 3wt% over a wide range of temperature well above and below the two transitions in pure 8CB. Isotropic (I) to nematic (N) and nematic to smectic-A (SmA) phase transitions evolve in character and their transition temperatures offset by (∼2.3 to 2.6 K) lower for all composite samples as compared to that in pure 8CB. The enthalpy change associated with I–N phase transitions shows slightly different behavior on heating and cooling and it also shows crossover behavior at lower and higher QD content. The enthalpy change associated with N–SmA phase transitions is independent of QD loading and thermal treatment. Given the homogeneous and random distribution of QD in these nanocomposites, we interpret that these results as arising that the nematic phase imposes self-assembly on QDs to form one-dimensional arrays leading to QDs and induces net local disordering effect in LC media.

Novel biphenyl-substituted 1,2,4-oxadiazole ferroelectric liquid crystals: synthesis and characterization.

Two novel series of unsymmetrically substituted 1,2,4-oxadiazole viz., R.Ox.C*Cn compounds are synthesized and characterized. An optically active, (S)-(+)-methyl 3-hydroxy-2-methylpropionate is used to introduce a chiral center in the molecule. A biphenyl moiety prepared by Suzuki coupling reaction is directly attached to the oxadiazole core at C-5 position. Investigations for the phase behavior revealed that the series with a benzyl group on one end of the oxadiazole core exhibits an 1D orthogonal smectic-A phase while the second series with dodecyl flexible end chain shows orthogonal smectic-A and tilted chiral smectic-C (SmC*) phases over a wide range of temperatures. The smectic-C phase exhibits ferroelectric (FE) polarization switching. The mesomorphic thermal stabilities of these compounds are discussed in the domain of the symmetry and the flexibility of the alkyloxy end chain length attached to the chiral center.

Detection of phase transitions from the study of whispering gallery mode resonance in liquid crystal droplets

We report studies on the whispering gallery mode (WGM) resonance of liquid crystal (LC) droplets across the smectic-A (SmA) to nematic phase transition. The quality factor (Q) in the SmA phase decreases rapidly with a characteristic slope change at the SmA-N transition. In the SmA phase, Q-factor is linearly proportional to the birefringence (Δn). We discuss the effect of topological defect transformation on the WGM resonance. The study of WGM resonance is expected to be very useful for detecting subtle phase transitions among LC mesophases.

Induced Polymorphism of Smectic Phases in Binary Mixture of Non-Mesogenic Compounds

We report the results of our studies on the optical and thermal properties of the mixture of sodium dodecyl sulfate (SDS) and orthophosphoric acid (H3PO4), which exhibits very interesting induced polymorphic smectic phases such as smectic-A, smectic-D, smectic-B, and smectic-E phases, sequentially when the specimen is cooled from its isotropic liquid phase. Different liquid crystalline phases observed in the mixture were studied using differential scanning calorimetry (DSC), X-ray, and optical microscopic techniques. The temperature variation of optical anisotropy has also been discussed. Helfrich potential and elastic moduli have been estimated in the smectic phase using Helfrich model.

Calculation of the absolute free energy of a smectic-A phase.

In this paper, we provide a scheme to compute the absolute free energy of a smectic-A phase via the "indirect method." The state of interest is connected through a three-step reversible path to a reference state. This state consists of a low-density layer of rods coupled to two external fields maintaining these rods close to the layer's plane and oriented preferably normal to the layer. The low-density free energy of the reference state can be computed on the basis of the relevant second virial coefficients between two rods coupled to the two external fields. We apply this technique to the Gay-Berne potential for calamitics with a parameter set leading to stable isotropic (I), nematic (N), smectic-A (SmA), and crystal (Cr) phases. We locate the I-SmA phase transition at low pressure and the sequence of phase transitions I-N-SmA along higher-pressure isobars and we establish the location of the I-N-SmA triple point. Close to this triple point, we show that the N-SmA transition is clearly first order. Our results are compared to the coexistence lines of the approximate phase diagram elucidated by de Miguel et al. [J. Chem. Phys. 121, 11183 (2004)] established through the direct observation of the sequence of phase transitions occurring along isobars under heating or cooling sequences of runs. Finally, we discuss the potential of our technique in studying similar transitions observed on layered phases under confinement.

Induced Reentrant Nematic and Smectic— A Phase in Mixture of Mesogenic and Nonmesogenic Compounds

We report the results of our studies on the optical and thermal properties of the mixture of 4-cyano-4′-octyloxybiphenyl (80CB) and N-Cetyl-N, N, N, trimethyl ammonium bromide (CTAB) + Glacial acetic acid (GAA), which exhibits a very interesting lower temperature reentrant nematic and smectic-A phases, sequentially when the specimen is cooled from its isotropic liquid phase. The different phases of these mixtures are identified using optical and microscopic techniques. The temperature variations of optical anisotropy, Ultrasonic velocity, molar compressibility, and adiabatic compressibility have been also discussed.

Preparation and thermo-optical characteristics of a smart polymer-stabilized liquid crystal thin film based on smectic A–chiral nematic phase transition

A smart polymer stabilized liquid crystal (PSLC) thin film with temperature-controllable light transmittance was prepared based on a smectic-A (SmA)–chiral nematic (N*) phase transition, and then the effect of the composition and the preparation condition of the PSLC film on its thermo-optical (T-O) characteristics has been investigated in detail. Within the temperature range of the SmA phase, the PSLC shows a strong opaque state due to the focal conic alignment of liquid crystal (LC) molecules, while the film exhibits a transparent state result from the parallel alignment of N* phase LC molecules at a higher temperature. Importantly, the PSLC films with different temperature of phase transition and contrast ratio can be prepared by changing the composition of photo-polymerizable monomer/LC/chiral dopant. According to the competition between the polymerization of the curable monomers and the diffusion of LC molecules, the ultraviolet (UV) curing surrounding temperature and the intensity of UV irradiation play a critical role in tuning the size of the polymer network meshes, which in turn influence the contrast ratio and the switching speed of the film. Our observations are expected to pave the way for preparing smart PSLC thin films for applications in areas of smart windows, thermo-detectors and other information recording devices.

Study on optical characterization of TGB and reentrant smectic-A phases in binary mixture of two liquid crystals

In this work, our investigation is to study the optical and thermal properties of the binary mixture of cholesteric and nematic compounds, namely, cholesteryl nonanoate and p-methoxybenzylidene-p-ethylaniline, which exhibits a very interesting liquid crystalline twisted grain boundary (TGB) phase and reentrant smectic-A phase. The chiral liquid crystalline TGB phases and reentrant smectic-A phases have been observed at different concentrations and at different temperatures. The existence of TGB and reentrant smectic-A phases is confirmed by differential scanning calorimetry and optical microscopic studies. The variation of optical anisotropy has been discussed. The helical pitch of the cholesteric phase has also been discussed.

Manipulating ordered structure of ionic liquid crystalline polymers through tuning the alkyl spacer length

The fabrication of ordered structure is very important for the applications of ionic liquid crystalline polymers (ILCPs) or polymerized (ionic liquid crystal) (PILC). In this paper, we reported a facile approach to manipulate the ordered structure of ILCPs through adjusting the length of alkyl spacer. We designed and synthesized a series of ILCPs contained imidazolium, poly(2,5-bis{[m-(4-butoxy-4′-imidazolium phenyl) m-alkyl] oxy carbonyl} styrene bis (fluoroborate) salts) (denoted as P4-m−BF4, m represents the number of carbon in the alkyl spacers and m = 2, 4, 6, 10) via radical polymerization. Combined differential scanning calorimetry (DSC), polarized light microscopy (PLM), X-ray scattering, and two-dimensional wide-angle X-ray diffraction (2D WAXD), we found that the ordered structure of this ILCPs can transfer from smectic A phase (SmA) to hexagonal columnar (ΦH) phase with the increase of spacer. Furthermore, these results were confirmed by reconstructed relative electron density map using fast fourier transform algorithm (FFT). The result revealed that the increase of alkyl spacer would affect the interaction between ions and side chain, and induce the packing of the side chains. It was evident that the alkyl spacer played an important role in the constructing of ordered structure and offered a new method to fabricate different ordered structure of ILCPs.

Effect of bending flexibility on the phase behavior and dynamics of rods.

We study by means of molecular and Brownian dynamics simulations the influence of bending flexibility on the phase behavior and dynamics of monodisperse hard filamentous particles with an aspect ratio of 8 and persistence lengths equal to 3 and 11 times the particle length. Although our particles are much shorter, the latter corresponds to the values for wild-type and mutant fd virus particles that have been subject of a recent experimental study, where the diffusion of these particles in the nematic and smectic-A phase was investigated by means of video fluorescence microscopy [E. Pouget, E. Grelet, and M. P. Lettinga, Phys. Rev. E 84, 041704 (2011)]. In agreement with theoretical predictions and simulations, we find that for the more flexible particles (shorter persistence length) the nematic (N) to smectic-A (Sm-A) phase transition shifts to larger values of the particle density. Interestingly, we find that for the more rigid particles (larger persistence length), the smectic layer-to-layer distance decreases monotonically with increasing density, whereas for the more flexible ones, it first increases, reaches a maximum and then decreases. For our more flexible particles, we find a smectic-B phase at sufficiently high densities. Moreover, in line with experimental observations and theoretical predictions, we find heterogeneous dynamics in the Sm-A phase, in which particles hop between the smectic layers. We compare the diffusion of our two types of particle at identical values of smectic order parameter, and find that flexibility does not change the diffusive behavior of particles along the director yet significantly slows down the diffusion perpendicular to it. In our simulations, the ratio of diffusion constants along and perpendicular to the director decreases just beyond the N-Sm-A phase transition for both our stiff and more flexible particles.

From Microscopic Theory to Macroscopic Theory: a Systematic Study on Modeling for Liquid Crystals

In this paper, we propose a systematic way of liquid crystal modeling to build connection between microscopic theory and macroscopic theory. A new Q-tensor theory based on Onsager's molecular theory which leads to liquid crystals with certain shape has been proposed. Making uniaxial assumption, we can recover the Oseen-Frank theory from the derived $Q$-tensor theory, and the Oseen-Frank model coefficients can be examined. In addition, the smectic-A phase can also be characterized by the derived macroscopic model.

Nanoparticle-induced twist-grain boundary phase.

By means of high-resolution ac calorimetry and polarizing optical microscopy, it is demonstrated that surface-functionalized spherical CdSSe nanoparticles induce a twist-grain boundary phase when dispersed in a chiral liquid crystal. These nanoparticles can effectively stabilize the one-dimensional lattice of screw dislocations, thus establishing the twist-grain boundary order between the cholesteric and the smectic-A phases. A Landau-de Gennes-Ginzburg model is used to analyze the impact of nanoparticles on widening the temperature range of molecular organizations possessing a lattice of screw dislocations. We show that in addition to the defect-core-replacement mechanism, the saddle-splay elasticity may also play a significant role.

Dielectric Response of the Orthogonal Ferroelectric Smectic Phase in Thin Planar Cells in Low DC Bias Electric Field

The dielectric response of the ferroelectric smectic-A phase made of bent-core molecules in thin planar cells in low bias field is studied theoretically. The dependence of the analytically calculated phase mode frequency on the cell thicknesses and magnitude of the bias field is compared to the dependencies obtained by numerical calculations and recently reported experimental measurements, which show that in thin planar cells the phase mode frequency decreases with increasing bias field.

Reentrant nematic and tricritical behavior of the nematic to smectic-A phase transition in liquid crystal mixture

We present an improved analysis of the reentrant nematic and tricritical behavior of the nematic to smectic A phase transition in liquid crystal mixture within phenomenological theory. To describe the nematic–smectic A–reentrant nematic phase sequence in binary mixture, nematic and smectic order parameters have been coupled with the concentration. We show that reentrant nematic and the tricritical point on the N–SmA phase transition line can be achieved in the same phase diagram under certain conditions. We predict two tricritical points. One corresponds to nematic to smectic A phase transition and other corresponds to reentrant nematic to smectic A phase transition. A comparison of the results with experimental phase diagrams is described in great details. The predictions of the theory are in good agreement with available experimental data.