Molecular Ordering of Nematics Between Concentric Cylinders: Results and Perspectives

Abstract

We present a short overview of recent investigations of the equilibrium tilt angle profile in a cell limited by two concentric cylinders filled with nematic liquid crystals (NLC), both in presence and in absence of an external electric field. The analyses are carried out by recalling some recent results obtained in the framework of the elastic continuum theory and Monte Carlo (MC) simulations, or suitable combinations of both approaches. These are very powerful tools to face the richness of the configurations of NLC in complex geometries. In particular, they are helpful in providing a comprehension of the molecular organisation of these systems for what concerns the spontaneous Fréedericksz-like transition, connected with the phenomenon of escaping into the third dimension. Some concluding remarks and perspectives points towards some remarkable features of confined systems in soft matter.     

The Nematic-Isotropic Transition

Abstract

We describe simple classroom experiments that can model the nematic phase, the first order nematicisotropic phase transition, and the effect external stimuli, such as surface treatment, have on the phase transition. These experiments are suitable for the high school level but can be easily adapted to fit the middle school mathematics and science curriculum. With even more simplification, it can also be used to fit the elementary level to qualitatively demonstrate the total loss of order as a liquid crystal melts into a liquid.     

Wavelength-tuneable laser emission from a dye-doped achiral nematic liquid crystal dispersed into a chiral polymer scaffold

ABSTRACT

We compare the emission characteristics of a thin-film liquid crystal (LC) laser created using a polymer-stabilized, dye-doped chiral nematic LC to that of an LC laser that was fabricated using an achiral, dye-doped nematic refilled into a chiral polymer scaffold that was templated from the same chiral nematic host. Both lasers exhibit wavelength tuning upon the application of an external electric field. However, for the templated sample, tuning is found to occur across a broader wavelength-range for the same electric field amplitude. We discuss the benefits of the templated approach and how it can be used to circumvent dye bleaching that may occur during photo-polymerisation.     

Acid mediated tunability of stimulated laser emission from dye doped chiral microdroplets

Abstract

In the last decade, the possibility to use liquid crystal droplets as optical micro-cavities and lasers has attracted much attention since it paves the way for many applications in the field of sensors or tunable photonics. Several techniques can be used to obtain small micro-resonators as, for example, dispersing a cholesteric liquid crystal inside an immiscible isotropic fluid to create an emulsion. Since liquid crystals are extremely sensitive to external factors as temperature or external fields, laser tuning can be easily achieved. Here, we report on the possibility to tune the laser emission from dye doped cholesteric liquid crystals microdroplets dispersed in a glycerol matrix in presence of nitric acid molecules in the emulsion. Using a fluorescent dye with pH dependent optical properties, the emitted laser wavelength can be tuned in a range of 60?nm. This effect could find applications for the development of spectroscopy based sensors.     

Magnetic properties and sensitized visible and NIR luminescence of DyIII and EuIII coordination polymers by energy transfer antenna ligands

Two coordination polymers, {[Ln(2-stp)(4,4′-bipy)(H₂O)].(H₂O)}, [Ln = Dy (1) and Eu (2), 2-stp = 2-sulfoterephthalate and 4,4′-bipy = 4,4′-bipyridine] have been characterized by solid state UV-vis, FTIR spectra, X-ray single crystal diffraction and solid state photoluminescence. Variable-temperature magnetic susceptibility and isothermal magnetization as function of external magnetic field is also studied for both complexes. After ligand-mediated excitation, both complexes show the characteristic visible and NIR luminescence of the corresponding Ln^III ions (Ln = Dy, Eu) which is due to efficient energy transfer from the ligands to the central Ln^III ions via an antenna effect. The indirect energy transfer in both complexes has been investigated and discussed in detail.     

Effect of various precipitants on the nucleation rate of tetragonal hen egg-white lysozyme crystals in an AC external electric field

It was observed that the effect of an external electric field on the nucleation rate of tetragonal hen egg-white lysozyme crystals varied depending on the precipitant used (NaCl, NiCl₂ or YbCl₃) and that the electric double layer (EDL) played an important role in generating an external electric field of the necessary strength to control the nucleation rate. This phenomenon depended on the ionic strength of the precipitant used; that is, a precipitant of greater ionic strength resulted in a thinner EDL and increased the effect of the external electric field as the driving force for nucleation. The dependence of the nucleation rate on the precipitant was attributed to the magnitudes of the external electric fields generated in EDLs of varying thickness which were formed in the presence of different precipitants.     

Electrical elements of the optical systems based on hydrogel - electrochromic polymer composites

Abstract

The results of studies of the electrically controlled elements of optical systems based on hydrogel - electrochromic polymer with conjugated π-electron system are shown. One important feature of conjugated polymers is the ability to change their optical characteristics under an external electric field. The electrochromic effect of these polymers may find applications in optical gating and filtering systems, optical sensors, ‘‘smart windows’’, optical memories, IR-switching, and electrochromic displays. Such electrochromic polymers were embedded in the matrix of a macroporous polymer hydrogel by the oxidative chemical polymerization method. The obtained samples based on hydrogel - polyaniline and hydrogel - polyorthotoluidine composites demonstrate the electrochromic behavior. In these composites, the electrolyte, as well as the electrochromic polymer, are located directly in the volume of hydrogel.     

Electric-field induced micromagnetic structural elements motion in thin ferrite garnet films

Abstract

We report direct magneto-optical measurements in epitaxial films of bismuth-substituted ferrite garnets that shown the magnetic domain walls can be reversibly changed under an electric field due to the magneto-electric effect. Electric field-induced bending or elongation of domain walls local areas was observed by us using the method of optical polarimetry. Upon removal of the electric field, the domain walls of the magnetic film return to their original configuration.     

Molecular Theory of Photogeneration

Abstract

The elucidation of the time evolution of optically excited states in condensed matter is a fundamental challenge to both a refined scientific probe of materials and to the design of a wide variety of photoelectronic devices. In low mobility materials important aspects of this evolution can be mapped onto a time-dependent random walk (RW) process involving change transfer (CD steps of an electron and cation between molecular sites. We use this KW process to calculate the dissociation efficiency η(E) of these carriers as a function of applied electric field E [1]. The Onsager theory of η(E) is formulated in terms of at least two parameters, r_o the initial separation of the electron-cation pair and η_o the quantum yield for the initial production of the pair. The Onsager approach is shown to be based on an approximation (continuum limit) of the KW process, which is often not valid, especially at small r_o and large E. The emphasis of the talk will be on the importance for η(E) of the competition between CT rates and excitation decay rates (R) and how key molecular properties, concentration, temperature, and E affect this competition. The difficulties of Interpreting experimental fits for r_o, η_o will be discussed in this context. Detailed features of η(E) (including activation energy A and high E-saturation) will be illustrated as a function of molecular parameters and system geometry in both the large and small R regimes (R→ ∞ corresponds to the Onsager model boundary condition of an infinite sink at the origin) as well as recent results of a calculation of the slope-to-intercept ratio for this model [2] and a comparison to simulations.     

Dynamics of the Reorientation of a Ferroelectric Liquid Crystal Under an Electric Field Studied by Time-Resolved Optical Waveguide Spectroscopy

Abstract

The dynamic structure and molecular reorientation of a ferroelectric liquid crystal induced by an alternating electric field have been investigated by time-resolved optical waveguide spectroscopy (TROWS) in which the transient waveguide mode patterns were recorded as a function of both time and angle of incidence of a probe beam. A ferroelectric liquid crystal, 4-(2S, 3S)-[2-chloro-3-methylpentanoyloxy]-4′-octyloxybiphenyl (3M2CPOOB) was utilized, which showed a fast response to an external electric field in the order of a microsecond. Under a high electric field, it forms the so-called bookshelf structure in which the director of 3M2CPOOB is tilted by 22.5° away from the smectic layer normal in the SmC* phase. Time-resolved waveguide mode patterns were successfully obtained with a time-resolution of 1.0 μs which enabled us to determine the transient features of dielectric tensor of 3M2CPOOB molecules in three dimensional coordinate system. A model that the molecular reorientation coupled with the layer buckling and the rotation around the cone was proposed from the quantitative analysis of the TROWS data.     

FLEXOELECTRIC INSTABILITY IN NEMATIC LIQUID CRYSTAL BETWEEN COAXIAL CYLINDERS

Abstract

The stability of the equilibrium configurations of a nematic liquid crystal confined between two coaxial cylinders is analysed when a radial electric field is applied and the flexoelectric effect is taken into account. The threshold for perturbations depending only on the radius r in the cylindrical coordinate system and strong boundary conditions is studied. A new type of orientational transition caused by pure flexoelectric effect is found.     

Degradation of a nano-cutting tool: An MD simulation

We present the results of molecular-dynamics simulations of repeated interactions of a realistically-shaped cutting edge with a model, infinitely hard grain. The model edge is composed of several hundred thousand atoms of an fcc metal treated with the Sutton–Chen potential, moving with a constant speed of 20 m s^?1. Plastic deformations appearing upon contact with the rigid obstacle are observed and described by means of temperature fields and the atomic slip vector. Preferred slip planes are identified and the normal force experienced by the tool is investigated. The effect of periodic boundary conditions along the z (in-plane) direction is assessed by comparing the results of two simulations, differing only by the simulation box’s length along the z-axis.     

COMPUTER SIMULATION OF THE HOMEOTROPIC TO FOCAL CONIC TRANSITION IN CHOLESTERICS

Abstract

Previous reports have proposed mechanisms for the homeotropic to focal conic transition in cholesterics. In this report, we present quantitative numerical modeling of this transition. We will show that the simulation shows the existence of the transient planar state, agreeing with previous experimental findings. We will also demonstrate that the simulations never show energy densities high enough to form isotropic regions, demonstrating that this transition is a continuous process. Instead of introducing isotropic regions, the system transforms through an undulation distortion which strongly resembles a Helfrich-Hurault distortion. This simulation is compared with two experimental samples: one with parallel boundary conditions and one with perpendicular boundary conditions. The simulations are compared with dynamical capacitive measurements and microscope photographs, and found to agree very well. We will present detailed drawings of the director configuration during this transition. The agreement of this simulation with the experimental data shows that the mechanism of this transition is now well understood     

A Chevron Model of the Electroclinic Effect across the SA*-SC* Phase Transition in a SSFLC

Abstract

We here present a thorough model of the electroclinic effect across the S_A*-S_C* phase transition, being the first to incorporate the vertical chevron structure. This structure has previously been observed in the S_C* phase of surface-stabilised FLC (SSFLC) devices, and some experimental evidence suggests that it may be present in the electroclinic effect [1–3]. Excellent agreement is achieved when comparing this model with previous experimental results of optical tilt through the A-C* transition for a SSFLC cell with homogeneous alignment.     

Interplay between the magnetic field and the dipolar interaction on a magnetic nanoparticle system: A Monte Carlo study

We have studied the influence of the applied magnetic field on the blocking temperature (T_B) of a fine magnetic particle system. By means of a Monte Carlo technique we have simulated zero field cooling (ZFC) curves under different applied fields, obtaining the respective T_B as a function of H. We have focused our study on the limit H → H_K (where H_K is the anisotropy field), since the results found in the literature usually lack a detailed study of this range. The simulations were done at different sample concentration of the nanoparticles, with the purpose of observing how the magnetic dipolar interaction affects the field dependence of T_B. The classical expression predicts T_B to disappear for H ? H_K, independently of the dipolar interaction strength. Our simulations show that at strong interacting conditions T_B exists even for fields H > H_K.     

Compact Permanent Magnetic Circuit with Periodic Magnetic Field Designed for Studies of Liquid Crystals

Abstract

We have developed a high-performance compact magnetic circuit to investigate orientational phase behaviors of biological and synthetic macromolecular systems by using spectroscopic methods such as X-ray scattering and light scattering. This magnetic circuit can apply a periodic magnetic field and gradient to samples. The maximum values of the magnetic field strength and gradient are varied from 1.5 tesla to 0.55 tesla and from 2.4 × 10² tesla/m to 7.3 × 10¹ tesla/m. Owing to the high periodicity of the magnetic field strength and gradient macromolecules are subjected to both rotational and translational forces from the magnetic field. We show some prominent features of this magnetic circuit and its first application to the synchrotron radiation small-angle X-ray scattering (SR-SAXS) studies of supermacromolecular liquid crystals.     

What is the Shape of a Polymer Chain near the Theta Point?

We address the question of the conformation of a polymer near the θ point. To this end, we present an argument that the statistics of polymer rings at the θ point in two dimensions is exactly given by the statistics of the external perimeter (“hull”) of a percolation cluster. As a consequence, the fractal dimension d_f (θ) of a polymer chain at the θ point coincides with that of the hull of the percolating cluster, d_f (θ) - d_H . We also perform extensive simulations of the conventional θ point model—the interacting self-avoiding walk (ISAW)—and the smart kinetic walk (SKW). We demonstrate that the SKW predicts a higher order critical point, termed the θ′ point, which has the same critical behavior as the ISA W. We conclude that the SKW is a well-defined walk that gives the conformation of a polymer near the θ point in two dimensions. In particular, we report accurate calculations of all three tricritical point exponents that are needed to fully describe the θ point.     

The dynamics of wet granular matter

We present a numerical study of a shear-induced solid–fluid transition in wet granular matter in order to show the self-organized critical behavior close to the transition point. The continuous time simulation is based on a simple model considering both the cohesive forces induced by the adsorbed liquid amount and the repulsive forces due to the hard core interaction of the granules. Dissipation is assumed to be entirely due to the hysteretic character of the cohesive forces. The aim of our analysis concerns the crossover from a solid like behavior to a mobile ergodic state under the influence of an external force field F exceeding the critical force F_c. Diffusion coefficients, dissipation and kinetic order parameters can be expressed as characteristic scaling laws.     

ON THE HYSTERESIS-FREE RESPONSE OF ANTIFERROELECTRIC SMECTICS—THE CASE OF MESOPHASES WITH LONG PERIOD -

Abstract

In order to elucidate the hysteresis-free response (V-shaped response) observed at a transmittance of light in antiferroelectric smectic materials, the dynamical responses of the order parameters to an electric field are studied on the basis of an extended ANNNI model with J ₃ < 0. It is shown that the V-shaped responses appear at mesophases with long period near the ferroelectric phase in low frequency range.     

Spectral Characteristics of C60-Conducting Polymer Junctions: Various Molecular D-A Type Photocells

Abstract

We have found recently that thin film layered heterojunction between C₆₀ and conducting polymer, like poly(3-alkylthiophene) (PAT) shows a photovoltaic effect due to photoinduced charge transfer at the interface. Here we describe two other examples of such photocells in RO-PPV/C₆₀ and PPP/C₆₀ heterojunctions and study their spectral characteristics. Contrary to conventional inorganic semiconductor p-n junction photocells, the C₆₀-PAT junction can be rather viewed as molecular donor-acceptor (D-A) type photocell in which the processes of photogeneration and separation of charge carriers are quite distinct. In a p-n junction free electron-hole pairs are known to be primarily photogenerated at interband transition and then separated in the internal electric field of the barrier, while in D-A molecular photocell mainly neutral excitons are first created by light, with charges being primarily separated at the narrow interface region due to D-A type intermolecular charge transfer interactions. We analyze how this processes depend on the wavelength of the pumping light and on the polarity and magnitude of the applied voltage.