V. Yu. Reshetnyak

Harvesting single ferroelectric domain stressed nanoparticles for optical and ferroic applications

We describe techniques to selectively harvest single ferroelectric domain nanoparticles of BaTiO3 as small as 9 nm from a plethora of nanoparticles produced by mechanical grinding. High resolution transmission electron microscopy imaging shows the unidomain atomic structure of the nanoparticles and reveals compressive and tensile surface strains which are attributed to the preservation of ferroelectric behavior in these particles. We demonstrate the positive benefits of using harvested nanoparticles in disparate liquid crystal systems.

The Frederiks effect and related phenomena in ferronematic materials

Using continuum and statistical mechanical theories, we study the switching properties of a ferronematic in a nematic liquid crystal cell subject to homeotropic boundary conditions at the cell and particle walls. An external magnetic field normal to the cell plane is also imposed. At low fields we find thresholdless switching of the nematic director, consistent with experimental data. At higher fields, there are three regimes, depending on the strength of the anchoring interaction between the director and the ferroparticle orientation. For low anchoring strengths, there is an inverse Frederiks effect, and the nematic reorientation reduces and then disappears continuously at a critical magnetic field. At intermediate fields, the degree of reorientation reduces at high fields but remains finite. For high fields, however, the director switching saturates. The dimensionless temperature scale in the problem involves the temperature, the mean nematic elastic constant, the colloidal density, and the cell dimensi...

Fredericksz transition threshold in nematic liquid crystals filled with ferroelectric nano-particles

A key liquid crystalline property for electro-optic applications is the Fredericksz threshold electric field. There has been recent experimental interest in liquid crystal-based colloidal suspensions in which the colloidal nanoparticles both possess a permanent electric polarization and provide strong director anchoring on the particle surface. Such suspensions are sometimes known as Filled Liquid Crystals. Our calculations suggest, in qualitative agreement with experiment, that filling the nematic liquid crystal with ferroelectric nanoparticles can significantly decrease the electric Fredericksz transition threshold field.

Effective medium theory of light scattering in polymer dispersed liquid crystal films

We present calculations of the optical properties of PDLC films subjected to normally incident unpolarized light, in the long-wavelength regime under which the Rayleigh-Gans approximation applies. The calculations use an effective medium theory to take account of dependent scattering effects due to close packing of droplets, and use the Percus-Yevick approximation for hard spheres to account for interference effects. The director configurations inside the droplets are found using the effective medium theory which we have developed in earlier papers. Typical results are presented for radial and bipolar droplets. Detailed quantitative results have been derived for the optical characteristics in the presence of an applied field for the technologically important case of PDLC films containing partially ordered bipolar droplets.

Effective-medium theory of polymer dispersed liquid crystal droplet systems: I. Spherical droplets

We introduce an effective-medium theory for the local field and mean dielectric function of a polymer dispersed liquid crystal (PDLC) film consisting of identical spherical liquid crystal droplets dispersed in a polymer matrix. The theory takes account of the tensor nature of the dielectric response of the liquid crystals and the subsequent re-orientation of the liquid crystal tensor inside the PDLC droplets, as well as the presence of other droplets. We present explicit numerical results for droplets with radial and bipolar director boundary conditions as a function of external voltage.

Effective Medium Theory of Polymer Dispersed Liquid Crystal Droplet Systems II: Partially Oriented Bipolar Droplets

We construct a theory of the dielectric properties of a polymer dispersed liquid crystal droplet film containing bipolar droplets whose principal axes respond to an electric field. The theory involves a balance of an elastic energy term favouring an initial zero-field distribution of droplet axes and a dielectric energy term favouring droplet orientation parallel to an imposed field. We discuss (i) droplets initially orientationally ordered by an in-plane strain field and (ii) an initially random distribution of droplets. In case (i) we find a weakly droplet concentration dependent sharp threshold field at which droplet reorientation occurs. In case (ii) we find more gradual droplet reorientation. In the latter case the droplet axis distribution exhibits an unexpectedly rich structure.

Influence of the Aerosil Surface Modification on Electro-Optical Characteristics of Filled Liquid Crystals

Abstract We have studied the influence of surface state of aerosil on electro-optical characteristics of suspensions ‘aerosil-liquid crystal (LC)’. Modification of aerosil surface by hydrophobic fragments improves these characteristics. Also the dependence of a hydrophobic modificator structure on electro-optical characteristics was investigated. Explanation is given on the basis of the influence of the surface structure on the anchoring energy value of the LC-aerosil interface.

Kinetic Characteristics of Light Induced Anisotropy and Mechanisms of the Molecular Alignment in Azo Dye Containing Polymer Films

Abstract Induction and relaxation of optical anisotropy in azobenzene containing polymer films under the action of polarized UV-light irradiation are investigated by polarization UV and IR spectroscopy methods. The dependence of anisotropy relaxation on the exposure conditions is established. The results are interpreted supposing the partial ordering of main-chains fragments in irradiated films. A theoretical model explaining experimental results is developed.

Weak anchoring effects in ferronematic systems

We study the magnetically induced behaviour of a ferronematic cell with finite anchoring energy at the cell surfaces. We calculate the dependence of orientational and concentration profiles on the magnetic field, the director anchoring energy, and the cell thickness. We find a new intermediate thickness high field state, in which there is no ferroparticle segregation and a highly ordered director field. This contrasts with previous work in which only a restricted set of system parameters and strong anchoring at the cell surfaces were considered.

Two-Beam Energy Exchange in a Hybrid Photorefractive Inorganic-Cholesteric Cell

We develop a theoretical model to describe two-beam energy exchange in a hybrid photorefractive inorganic-cholesteric cell. A cholesteric LC cell is placed between two inorganic photorefractive windows. Weak and strong light beams are incident on the LC cell. The interfering light beams induce a periodic space-charge field in the photorefractive windows. This penetrates into the LC, inducing a diffraction grating written on the LC director. The theory calculates the energy gain of the weak beam, as a result of its interaction with the pump beam within the diffraction grating. In the theory, the flexoelectric mechanism for electric field-director coupling is a more important than the LC static dielectric anisotropy coupling. The flexoelectric polarization in the bulk LC follows from the initial director pretilt at the cell substrates and is the main physical mechanism governing the magnitude of the director grating and the two-beam coupling. The LC optics is described in the Bragg regime. Theoretical results for exponential gain coefficients have been compared with experimental results for hybrid cells filled with cholesteric mixtures TL205/CB15 and BL038/CB15. In order to reconcile theory and experiment, we require that (a) the magnitude of the director grating must be cubic rather than linear in the space-charge field, and (b) near the cell surface, nematic ordering must dominate. Within this paradigm, we are able to fit experimental data to theory for both cholesteric mixtures, subject to the use of some fitting parameters.