Bipolar Nematic Droplets Research Articles

Electro-optic response of bipolar nematic liquid crystal confined in oblate spheroid

Electro-optic response of liquid crystals (LCs) relies on the molecular reorientation of LCs under external electric field and is important for a wide spectrum of applications. Here, we uncover an interesting electro-optic response of 5CB nematic LC confined in an oblate spheroid and subjected to external electric field. Under the planar anchoring, the nematic LC spheroid adopts a bipolar structure with the bipolar axis laid in the horizontal film plane. When a threshold electric field EF, is applied, the bipolar structure reorients from the horizontal configuration (LC molecules align along long axis direction) to the vertical configuration (LC molecules align along short axis direction), involving the competition of elastic energy, surface anchoring energy and electric field energy. In contrast to bipolar nematic LC droplets, the vertical configuration does not relax to the low-energy horizontal configuration after removing E; we argue that is due to the oblate shape of the nematic LC spheroid, which traps the bipolar structure in a local energy minimum. We use continuum simulation to demonstrate the detailed response and the reorientation dynamics of bipolar nematic spheroids under E field, showing consistent results with the experiments and confirming the proposed switching mechanism. Nevertheless, the vertical configuration of the bipolar structure could relax to the low-energy horizontal configuration by thermal cycling. Our studies provide clear experimental results that show the characteristics of the electro-optic response of oblate LC spheroids, which have both fundamental and practical implications.

The structural transformations of polymer dispersed liquid crystalline films in an external deformation field

The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence the polymer matrix were studied. It is shown that the orientation ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix.

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.

Topological Changes in Bipolar Nematic Droplets under Flow

Bipolar liquid crystal drops moving inside microchannels exhibit periodic director field transformations due to induced circulating flows inside them. These modifications are characterized by changes in the type of point surface disclinations; they periodically change from splay to bend disclinations, implying the drop changes between bipolar and escaped concentric configurations. Upon stopping the flow, this structure does not relax to the lower energy bipolar configuration; we argue this is due to drop flattening inside the channels.

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.

Texture formation under phase ordering and phase separation in polymer-liquid crystal mixtures

Computational modeling of texture formation in coupled phase separation-phase ordering processes in polymer/liquid crystal mixtures is performed using a unified model based on the nematic tensor order parameter and gradient orientation elasticity. The computational methods are able to resolve defect nucleation, defect-defect interactions, and defect-particle interactions, as well as global and local morphological features in the concentration and order parameter spatiotemporal behavior. Biphasic structures corresponding to polymer dispersed liquid crystals (PDLCs), crystalline filled nematic (CFNs), and random filled nematics (RFNs) are captured and analyzed using liquid crystal defect physics and structure factors. Under spinodal decomposition due to concentration fluctuations, the PDLC structure emerges, and the nucleation and repulsive interaction of defects within nematic droplets leads to bipolar nematic droplets. Under spinodal decomposition due to ordering fluctuations, the CFNs structure emerges, and the stable polymer droplet crystal is pinned by a lattice of topological defects. For intermediate cases, where the mixture is unstable to both concentration and nematic order fluctuations, the RFN structure emerges, and polymer droplets and fibrils are pinned by a defect network, whose density increases with the curvature of the polymer-liquid crystal interface. The simulations provide an information of the role of topological defects on phase separation-phase ordering processes in polymer-liquid crystal mixtures.

Transformation of director configuration upon changing boundary conditions in droplets of nematic liquid crystal

The transformation of the director configuration upon changing boundary conditions from planar to homeotropic for bipolar nematic droplets dispersed in a polymeric matrix was studied. The characteristic textural patterns are presented for droplets with different concentrations of homeotropic surfactants, and their orientational structure is identified. The scenario predicted earlier by G.E. Volovik and O.D. Lavrentovich (ZhETF 85, 1997 (1983)) for the transformation of orientational structure of nematic droplets from bipolar to radial, without the formation of additional disclinations, is revealed. It is shown that, by using the computational method of minimization of the director elastic-strain energy in the droplet bulk and by introducing the inhomogeneous boundary conditions, one can obtain orientational structures that are analogous to the observed ones.

Computer simulation of elongated bipolar nematic droplets II. External field aligned normal to the droplet axis of symmetry

Numerical results from the modelling and computer simulation of the magnetic-induced director reorientation dynamics in elongated bipolar nematic droplets are presented in this paper. The magnetic field is applied normally to the droplet axis-of-symmetry direction, which is one possible scenario found in applications of polymer dispersed liquid crystal (PDLC) films. This case has not yet been studied numerically, and its understanding is far from complete. The model is composed of the Leslie-Ericksen and Frank continuum theories and is solved in two dimensions since bipolar nematic droplets exhibit mirror symmetry in certain planes. The numerical results replicate frequently reported experimental observations on the performance of PDLC films. These observations include the ubiquitous exponential increase followed by saturation in light transmittance as the external applied field increases, and the exponential increase (decrease) followed by saturation as time increases in the on (off)-state. Furthermore, in contrast to current understanding for both the on- and off-states, the model predicts that the directors in the centre (surface) region of the droplet exhibit a dead time (no dead time) before reorientation. The numerical results presented in this paper provide a better understanding of the director reorientation dynamics in elongated bipolar nematic droplets; this can be used to optimize the design and performance of devices using PDLC films.

Interference quenching of light transmitted through a monolayer film of polymer-dispersed nematic liquid crystal

A theoretical study of light transmission through a polymer with a monolayer ensemble of bipolar nematic droplets under applied electric film is performed. The possibility of interference quenching of monochromatic coherent light directly transmitted through the film is predicted theoretically and realized in the experiment.

Computer simulation of elongated bipolar nematic droplets 1. External field aligned parallel to the droplet axis of symmetry

The magnetically-induced transient nematic director reorientation dynamics, confined in elongated bipolar droplets, is studied in this paper. Numerical results are obtained by solving the Leslie-Ericksen continuum theory in ellipses. The aspect ratio is varied to determine the effect of droplet shape on director reorientation dynamics. The magnetic field is restricted to the droplet axis of symmetry direction, which has not yet been studied but is fundamentally important in polymer dispersed liquid crystal (PDLC) film operation. The numerical results replicate frequently-reported experimental observations on the performance of PDLC films. These observations include the familiar exponential increases followed by saturation in light transmittance as the external applied field increases and the exponential increase (decrease) followed by saturation as time increases in the on- (off-) state. In addition, the experimental observation that switching field strength increases while decay time decreases as the droplet becomes more elongated, are also exhibited by the numerical results.

Friedericksz Threshold in Bipolar Nematic Droplets with Rigidly Fixed Poles

We have studied experimentally a problem of the threshold behavior of Friedericksz transition in bipolar nematic droplets with rigidly fixed poles. The dependence of the light transmission of the PDLC film on the applied voltage has obviously revealed a threshold for the straightforward passing beam, confirming the threshold behavior of reorientation process in a central part of the droplet, which was theoretically predicted in [5]. In agreement with a simulation [5] the dependencies of scattering light intensity versus voltage have showed the nonthreshold behavior. We have checked-up also the interference origin of the oscillations of volt-contrast curves for these PDLC films. A number of oscillations and their amplitude depend on wavelength and films thickness.

Bipolar Nematic Droplets with Rigidly Fixed Poles in the Electric Field

We have studied theoretically the reorientation process induced by the electric field inside a bipolar nematic droplet with rigidly fixed poles. Computer simulation has revealed the threshold director reorientation only in that part of a droplet, where the LC director is orthogonal to the applied field, and the nonthreshold reorientation in the rest of the droplet volume. A formula has been proposed to estimate the threshold field. The data obtained have allowed us to calculate a PDLC film transmittance in the framework of the anomalous diffraction approach and to interpret the oscillating behavior of the volt-contrast curve observed experimentally.

Light transmission, linear dichroism and birefringence of nematic/polymer dispersions

A theoretical study of light transmission, linear dichroism and birefringence of partially ordered dispersions of bipolar nematic droplets in a polymer matrix is presented. The treatment rests on the single scattering approach to an ensemble of uncorrelated and noninteracting anisotropic particles. Theoretical evaluations of the extinction cross sections and phase functions are performed in the anomalous diffraction approximation. Four basic model systems are analysed: PDLC and NCAP films in an external electric field, and PDLC and NCAP films under uniaxial mechanical deformation. The calculated dependences of the generalized parameters characterizing the light transmission, dichroism and birefringence on the angle of incidence of the probing light, droplet size and shape, refractive indices of the LC and polymer, and parameters of the external field are presented graphically and discussed in detail.

Simulation of reorientation dynamics in bipolar nematic droplets

A two-dimensional model composed of a synthesis of the Leslie-Ericksen continuumtheory of nematics and the Euler-Lagrange equation for surface director motion is used to study the magnetic-induced director reorientation dynamics confined in spherical bipolar droplets with viscoelastic surfaces. The magnetic field is restricted to the droplet axis of symmetry direction. The numerical results indicate that the surface viscosity and anchoring strength must be taken into account to describe accurately director reorientation dynamics in droplets. In addition, the numerical results replicate frequently reported experimental observations on the performance of polymer dispersed liquid crystal films. These observations include the familiar exponential increase followed by saturation in light transmittance as the external applied field increases, and the exponential increase (decrease) followed by saturation as time increases in the on (off) state. Furthermore, this model is able to predict precisely the relationships between the rise and decay times and the external applied field strength, and the fact that the switching field strength is inversely proportional to droplet size.

Orientational Ordering of Bipolar Nematic Droplets in a Stretched PVA Matrix

Abstract The structural modifications and orientational ordering of nematic (SCB) droplets with the bipolar director-field configuration, embedded in a uniaxially stretched polyvinyl alcohol (PVA) matrix, are studied using polarized optical microscopy. It has been found that the unidirectional ordering of droplets with stretching takes place in two stages. At small matrix deformations, each bipolar structure is captured by the surface of the droplet cavity, so that the bipolar axis makes an angle with the long axis of the cavity and therewith is gradually rotated toward the stretch direction in accordance with the affine transformation of matrix dimensions. As the droplet non-sphericity further increases, there comes a point where the elastic deformation energy of the captured structure overcomes the surface interactions and, as a result, the droplet abruptly reorients parallel to the long cavity axis. The measurements of the orientational distribution function have testified the pronounced threshold natu...

Small Angle Light Scattering from Bipolar Nematic Droplets

Abstract Theoretical and experimental studies are made of small angle light scattering (SALS) from the nematic droplets with a bipolar director configuration. Serial expressions for the scattered amplitudes from a single bipolar anisotropic sphere such as a bipolar nematic droplet are obtained using Rayleigh-Gans (RG) theory. Analytical solutions are also derived under reasonable approximations. All of the formulae are suitable for an arbitrary orientation of the droplet symmetry axis. Series of calculated Hv and Vv scattering patterns for the bipolar nematic droplets with typical orientations of droplet axes are presented. Some theoretical patterns have been confirmed in experiment with polymer dispersed liquid crystal (PDLC) films. As a result, SALS is proven useful to distinguish the director configurations of nematic droplets, and further to discriminate the orientations of the droplet axes. Moreover, the average size of the bipolar nematic droplets can be estimated by Hv scattering with the formulae ...

Optical properties of stretched polymer dispersed liquid crystal films

Abstract The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analysed in terms of a proposed theoretical model. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence of the polymer matrix were studied. It is shown that the orientational ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix. The ‘anomalous’ nematic director orientation is also observed in a portion of elongated droplets where the axes of bipolar configurations do not coincide with the major axes of the droplet cavities due to the presence of inclusions at the cavity walls. The effect of alternation of droplet size and shape upon stretching and the influence of optical anisotropy of the polymer matrix on film transmittance are analysed. On the basis of the results obtained, simple criteria for optimization of main PDLC polarizer performance are formulated.