Nematic Liquid Crystal Droplets Research Articles

Angular structure of radiation scattered by monolayer of polydisperse droplets of nematic liquid crystal

We considered light scattering by a polydisperse ensemble of droplets of a nematic liquid crystal. To model light scattering by a monolayer of polymer-dispersed spherical droplets of a nematic liquid crystal with cylindrical symmetry of its internal structure, we proposed a semianalytical modeling method. The method is based on interference approximation of the theory of multiple wave scattering, anomalous diffraction approximation, and effective-medium approximation. The method takes into account cooperative optical effects in concentrated, partially ordered layers and can be used to analyze the small-angle structure of the intensity of scattered radiation in relation to the concentration, size, polydispersity of liquid crystal droplets, orientation of their optical axes, and refractive indices of the liquid crystal and polymer. The obtained relations can be applied to solving direct and inverse problems of light scattering in composite liquid crystal materials using data of polarization measurements. We present graphical results of solving the direct problem for components of the polarization vector of scattered wave. These results illustrate the formation of an angular structure for monolayers with a high concentration of polydisperse droplets of the liquid crystal in the range of small scattering angles (0 < θs ≤ 8°).

Electro-optical characteristics of polymer-dispersed liquid crystal film controlled by ionic-surfactant method

The electro-optical characteristics of a composite film based on a polymer containing dispersed nematic liquid crystal (NLC) doped with an ionic surfactant have been studied. The threshold character of the reorientation of NLC droplets as well as the dependence of the modulation amplitude of light transmission on the electric field strength has been revealed for the effect of ionic modification of surface anchoring. The dynamical parameters of the optical response have been determined.

Laser induced rotation of trapped chiral and achiral nematic droplets

We study the response of optically trapped achiral and chiralized nematic liquid crystal droplets to linear as well as circular polarized light. We find that there is internal dissipation in rotating achiral nematic droplets trapped in glycerine. We also demonstrate that some chiralized droplets rotate under linearly polarized light. The best fit to our data on chiralized droplets indicates that rotational frequency of these droplets with radius R is approximately proportional to 1/R 2, rather than to 1/R 3.

Optical response of nematic droplets in a polymer matrix to a strong pulsed magnetic field

A change in the optical transmission of an ordered single-layer ensemble of nematic liquid crystal (NLC) droplets of 4-n-pentyl-4′-cyanobiphenyl (5CB) in a poly(vinyl butyral) (PVB) matrix in response to a magnetic field pulse with an amplitude of H = 9 × 106 A/m was observed. Since the leading pulse front duration (τonH = 5 ms) significantly exceeded the theoretically calculated NLC response time (τonLC = 0.8 ms), the dynamics of transformations of the NLC director orientation and the optical response were analogous to those in the case of a stationary or slowly varying magnetic field. In contrast, the field decay time (τoffH = 25 ms) was much shorter than the NLC relaxation time (τoffLC = 300 ms) and, hence, the trailing front of the optical response was not influenced by the oscillatory character of the magnetic field decay during capacitor discharge via an RL chain.

Electrically tunable liquid crystal optical microresonators

Whispering-gallery-mode resonators made of nematic liquid-crystal droplets offer a wavelength tunability approximately two orders of magnitude larger than that of conventional solid-state microresonators.

Light-Induced Nonlinear Rotations of Nematic Liquid Crystal Droplets Trapped in Laser Tweezers

We report on optically induced rotations of nematic liquid crystal droplets. We show experimentally that bipolar nematic liquid crystal droplets trapped in laser tweezers having circular polarization can exhibit nonlinear rotational motion, unlike optically trapped solid birefringent micro-plates. Such nonlinear rotations are retrieved by analyzing the polarization dynamics analysis of the light beam after it has passed through the droplet. The occurrence of complex dynamics is only found for large enough trapping power and droplet diameter. The analogy with optically induced nonlinear rotations in nematic liquid crystal films is briefly discussed.

Role of radius on prewetting behavior in nematic liquid-crystal droplets

The prewetting phenomena in a nematic liquid crystal confined to a droplet embedded in a spherical solid surface are discussed. This paper is based on Landau-de Gennes theory and Nobili-Durand surface energy. By using a Maxwell construction, we find that the first-order boundary-layer transition inside of droplet which vanishes completely below a critical radius Rc when bulk nematic isotropic transition temperature is approached from above. We obtain a narrow temperature interval above the bulk nematic-isotropic phase transition which corresponds to nematic boundary layer inside of droplet. The interval length depends on surface potential and droplet radius. We also find that there is no critical radius for boundary transition when the nematic-isotropic transition temperature is approached from below.

A method for calculating the light extinction coefficients of a polymer layer with small nematic liquid-crystal droplets

A method is proposed for analyzing radiation extinction coefficients of polymer films containing small nematic liquid-crystal droplets with cylindrical symmetry. This method is based on the Rayleigh-Gans approximation and the effective scattering matrix. The orientational ordering of both liquid-crystal molecules in droplets and the droplets in a layer is described by multilevel order parameters. The extinction of radiation in polymer films is analyzed for the cases of oblique and normal incidence of light. Simple and convenient expressions are derived for calculating and estimating the extinction coefficients for the case of normal incidence of light on a layer containing spherical droplets. The error of the method is estimated.

Studies of optical transmission properties of electron beam cured polymer/liquid crystal systems

Polymer dispersed liquid crystal (PDLC) films were obtained by a polymerization induced phase separation technique induced by electron beam (EB) curing. A binary system composed of a monomeric diacrylate and a nematic liquid crystal (LC) was employed as initial reactive blend prior to the polymerization/cross-linking processes. The detailed experimental investigation of the electro-optical response was conducted for a set of PDLC films. The analysis was accomplished by a proper morphology consideration and refractive index measurements, which depend on the solubility of LC in the cured polymer matrix. A known model describing the electro-optical response of PDLC films based on a minimization of the total electrical and elastic energy of the nematic LC droplets dispersed in the polymer matrix was applied to describe the obtained experimental results. This model is based on the assumption that the single scattering is implemented, the droplet structure is not altered by an application of the electrical field, and only the droplet director changes its orientation with the applied field. The electro-optical response of PDLC films was expressed via a hierarchy of order parameters. Experimental results of the refractive index measurements and the morphological and thermophysical investigations were used as input parameters to compare theoretical and experimental results. A good agreement between the experimental data and calculated transmittance/voltage dependencies was found, when the multiple scattering in the film is negligible.

Magneto-Optical Study of Friedericksz Threshold in Polymer Dispersed Nematic Liquid Crystals

A computer model of reorientation processes in the bipolar nematic droplets of ellipsoidal form with strong tangential anchoring has been developed. The approximate formula to estimate the Friedericksz threshold has been obtained when the operating field (electric or magnetic) is orthogonal to the bipolar axis. The dependence of the threshold field on the droplet form has been considered. The reorientation of the bipolar droplets of 5CB nematic liquid crystal dispersed in polyvinylbutyral has been studied by the magneto-optical method. The calculation data are in good agreement with measured values of the threshold field.

Continuously rotating chiral liquid crystal droplets in a linearly polarized laser trap

The transfer of optical angular momentum to birefringent particles via circularly polarized light is common. We report here on the unexpected, continuous rotation of chiral nematic liquid crystal droplets in a linearly polarized optical trap. The rotation is non-uniform, occurs over a timescale of seconds, and is observed only for very specific droplet sizes. Synchronized vertical motion of the droplet occurs during the rotation. The motion is the result of photo-induced molecular reorganization, providing a micron sized opto-mechanical transducer that twists and translates.

Statics and dynamics of radial nematic liquid-crystal droplets manipulated by laser tweezers

Laser manipulation of trapped radial 4'-n-pentyl-4-cyanobiphenyl (5CB) nematic liquid-crystal droplets induced by molecular reordering is presented. We show experimentally that optical tweezers having linear, elliptical, or circular polarization can break the radial symmetry of the initial molecular organization inside a radial nematic droplet. Static distorted or twisted deformation modes and steady or unsteady nonlinear rotational dynamics are observed. Statics results are analyzed in terms of light-induced radial or left-right symmetry breaking effects associated with optical reorientation. The dynamical observations are compared with simulations from a slab analog model, for which orientational processes driven by optical nonlinearities can be accurately described. This study confirms that light-induced bulk reordering is an essential ingredient towards the understanding of the behavior of radial nematic liquid-crystal droplets in laser tweezers, as suggested by previous studies.

Rotatable liquid crystal waveplate

A photoresist well of size 60 × 60 × 15 μm3 has been used to confine a droplet of nematic liquid crystal to create a rotatable waveplate. The optical texture of the droplet between crossed polarisers is consistent with the nematic n-director running substantially along a diameter of the droplet and connecting two nematic defects on the curved edges of the droplet. Electric field induced azimuthal rotation of the axis of the nematic liquid crystal droplet has been demonstrated. At higher temperatures, 30 °C and above, the droplet is more circular in shape and can be switched to arbitrary rotation angles. At lower temperatures, 25 °C and below, the sides of the droplet are straightened by the interaction with the well walls and the switching tends to favour discrete orientations of the optic axis. The shape of the time–voltage switching response curve for rotation by an angle of 40° also depends on the temperature of the droplet. A switching time that is inversely proportional to the voltage squared results when the droplet is nearest to circular in shape.

Multistable liquid crystal waveplate

A multistable liquid crystal waveplate has been produced by confining a roughly disk-shaped nematic liquid crystal droplet of diameter 50μm between two photoresist coated surfaces separated by 15μm. Dimple features occur on opposite sides of the curve-free edge of the droplet, which may indicate the position of nematic disclinations. The diameter that connects these features coincides with an optic axis of the droplet. In-plane ac voltage bursts produce an optic axis rotation, controllable down to 1° steps, to other arbitrary, stable orientations. The rotation angle is proportional to the burst duration and the voltage squared.

Self-assembled block copolymers as matrix for multifunctional materials modified with low-molecular-weight liquid crystals

Meso/nanocomposite films made of low-molecular-weight nematic 4′-(hexyloxy)-4-biphenyl-carbonitrile (HOBC) embedded in poly(styrene- block-ethylene oxide) (PSEO) diblock copolymer have been prepared by using solvent-induced phase separation at room temperature. Their phase separation behavior has been investigated by morphological and viscoelastic analyses as a function of HOBC content and preparation conditions. Self-assembly of block copolymer in HOBC/PSEO systems containing 5, 10, 30 and 50 wt.% HOBC leads to submicron phase segregation of HOBC. Moreover, narrower droplet size and good distribution of nematic liquid crystal droplets in the self-assembled PSEO matrix have been observed. Thus solvent-induced microphase separation of poly(ethylene oxide)-block provokes segregation of the liquid crystal as a mesophase. Self-assembled HOBC/PSEO systems maintain generated droplet-like morphology of the HOBC phase even after 3 months, thus confirming the possibility of obtaining multifunctional materials with stable and well-defined morphologies based on block copolymer-dispersed liquid crystals as optically transparent matrices. This kind of material generates thermally reversible material for optoelectronics based on room temperature self-assembling block copolymers matrices.

Analysis of the surface order parameter of liquid crystal on a polymer surface using the phase transition droplet method

The essence of the anisotropic alignment of liquid crystal (LC) molecules at the substrate surface is derived from the distribution of the molecular direction of the LC, which is best described in terms of the order parameter. The order parameter of LC molecules at a polymer surface can be determined by measuring the contact angle of a nematic LC droplet at the nematic-isotropic phase transition temperature. In this study, we quantified the relationship between the surface order parameter and contact angle of the droplet for a homogeneous alignment LC cell. Based on the results, we propose a model for a droplet on a polymer surface. Its validity was confirmed by measuring the polarized infrared absorption.

The electro‐optical response of nematic emulsions

Switchable nematic emulsions are micron‐sized droplets of nematic liquid crystal, floating in isotropic fluid matrices. Such droplets can be switched from an opaque (off) to a transparent (on) state by application of very low electric fields. It is known that the electro‐optical properties of liquid crystal dispersions are affected by several parameters, including the liquid crystal loading. The electro‐optical response of nematic emulsions has been investigated as a function of liquid crystal weight percentage. Almost transparent films with a reduced contrast ratio are obtained with lower liquid crystal contents. A macroscopic phase separation is observed when liquid crystal content exceeds 45 wt %. On the contrary, large contrast ratios and very low switching fields can be obtained if liquid crystal ranges from 25 to 35 wt %. Consequently, nematic emulsions prepared in this liquid crystal range can be used as promising systems for electro‐optical applications. In addition to technological developments, these results can help computational and basic studies of phase separation in novel multiphase liquid crystalline materials.

Friedericksz threshold field in bipolar nematic droplets with strong surface anchoring

A numerical method has been developed for calculating the director configuration in ellipsoidal droplets of a nematic liquid crystal with strong tangential anchoring in a uniform magnetic field of an arbitrary orientation. A relation has been obtained for determining the Friedericksz threshold corresponding to the beginning of the reorientation of the central region of a droplet when the field is orthogonal to the biopolar axis. The effect of the breaking of the orthogonal condition on the threshold character of the orientation process is considered. The reorientation of the ensemble of bipolar droplets of the 5CB nematic liquid crystal dispersed in polyvinyl butyral has been studied by the magneto-optical method. Comparative analysis of calculation data and measured values of the threshold field has been performed.

Focusing of light by polymer-dispersed liquid-crystal films with nanosized droplets

An analysis is presented of polarization-independent electrically tunable light focusing by polymerdispersed liquid-crystal films with nanosized liquid-crystal droplets. Polymer-dispersed liquid-crystal films with axially symmetric distributions of liquid-crystal droplet concentration and layers with axially symmetric thickness profiles are considered. The paraxial, Rayleigh, and Rayleigh-Gans approximations, as well as the Foldy-Twersky equation, are used to examine the dependence of focal length on lens geometry, droplet size, concentration of nematic liquid-crystal droplets, and applied field. The tunable focusing ranges are evaluated for both lens types considered in the study. Dependence of the transmittance of polymer-dispersed liquid-crystal film on its characteristics is analyzed. The results obtained are compared with those available from the literature.

New cellulose derivatives composites for electro-optical sensors

In this work we used hydroxypropylcellulose (HPC) and acetoxypropylcellulose (APC) to produce free standing solid films (∼60 μm) for electro-optical devices. Cellulose derivatives films were prepared from crosslinked both HPC and APC isotropic and liquid crystalline solutions without and with a low molecular weight nematic liquid crystal mixture (LC) E7. Films prepared from anisotropic solutions presented a band texture and E7 droplets of micron and submicron size were found to coexist with the band texture in the films prepared with E7. The optical cells were composed by the cellulose derivatives films covered on both free surfaces by a layer of the LC E7 and placed between two transparent conducting substrates. All the electro-optical cells prepared showed switching times in the range of some milliseconds to hundreds of milliseconds. The on-voltage ( V on) is lowered for cells prepared with the ester derivative film obtained from isotropic solutions. The maximum transmission values show a clearly tendency to decrease for the cellulosic matrix prepared from anisotropic solutions with and without nematic liquid crystal droplets.