Biomedical subjects

Y Reznikov

Publications and source records attributed to Y Reznikov.

7 recordsLinked to original sources

PubMed2006

Colloidal particles at a nematic-isotropic interface: effects of confinement.

When captured by a flat nematic-isotropic interface, colloidal particles can be dragged by it. As a result spatially periodic structures may appear, with the period depending on particle mass, size, and interface velocity (J.L. West, A. Glushchenko, G.X. Liao, Y. Reznikov, D. Andrienko, M.P. Allen, Phys. Rev. E 66, 012702 (2002)). If liquid crystal is sandwiched between two substrates, the interface takes a wedge-like shape, accommodating the interface-substrate contact angle and minimizing the director distortions on its nematic side. Correspondingly, particles move along complex trajectories: they are first captured by the interface and then "glide" towards its vertex point. Our experiments quantify this scenario, and numerical minimization of the Landau-de Gennes free energy allows for a qualitative description of the interfacial structure and the drag force.

Anisotropy↗

PubMed2003

Surface reorientation induced by short light pulses in doped liquid crystals.

Fast surface reorientation induced by a single 4-ns low-energy laser pulse in dye-doped liquid crystals is reported. The reorientation is due to light-induced modification of the surface anisotropy, which affects the liquid crystal's director through the appearance of a preferred direction on the irradiated surface. The detected signals can be interpreted as being the result of light-induced desorption and adsorption of dye molecules.

Journal Article↗

PubMed2001

Photo-orientation of liquid crystals due to light-induced desorption and adsorption of dye molecules on an aligning surface.

We show that adsorption of dye molecules control the light-induced alignment of dye-doped nematic liquid crystal (LC) on a nonphotosensitive polymer surface. The dependencies of light-induced twist structures on exposure, thermal baking, thickness, and aging before irradiation of the LC cells allowed us to propose the following mechanism for the alignment. Before irradiation, the "dark"-adsorbed layer on the tested surface is formed from dye molecules predominantly aligned along the initial direction of the director. Irradiation of the cell with linearly polarized light produces an additional layer with different orientational ordering of dye molecules. The final easy axis is determined by the competition of "dark" and light-induced contributions to anchoring and is aligned between the "dark" easy axes and polarization of the light. For quantitative interpretation, we apply the tensor model of anchoring and assume that the photoalignment in the mesophase is a cumulative effect of the light-induced anchoring on the background of the already existing anisotropic "dark" dye layer.

Journal Article↗

PubMed2001

Hidden photoalignment of liquid crystals in the isotropic phase.

We found the effect of a hidden photoalignment of a dye-doped nematic liquid crystal (LC) on a nonphotosensitive polymer surface after polarized irradiation of the cell in the isotropic phase. We observed that irradiation resulted in a uniform planar orientation of the LC after cooling to the mesophase. The direction of a light-induced easy axis on the polymer can be either parallel or perpendicular to the polarization of the incident light, depending on the light intensity. We attribute this behavior to two mechanisms of photoalignment: light-induced adsorption of dye molecules on the substrate, and anisotropic desorption in a previously adsorbed dye layer. The experimental results on photoalignment of a LC on a thin dye film confirm our model.

Journal Article↗

PubMed2000

Nematic director slippage: role of the angular momentum of light.

We propose a theoretical model of the light-induced director slippage effect. In this effect the bulk director reorientation contributes to the surface director reorientation. It is found that the director and ellipticity profiles, obtained in the geometric optics approximation, are dependent on the ellipticity of the incident light wave. The director distribution is spatially modulated in linearly polarized light but grows monotonically in circularly polarized light. The surface director deviation has been examined, and comparison made with existing experimental data, which then permits the magnitude of the orientational nonlinearity coefficient to be calculated.

Journal Article↗

PubMed2000

Photoalignment of liquid crystals by liquid crystals

We observed light-induced alignment of the nematic liquid crystal, 4, 4(')-n-pentylcyanobiphenyl (5CB), on a fused quartz. Irradiation of the adsorbed layer with polarized ultraviolet light produced homogeneous alignment in a 5CB-filled liquid crystal cell with the easy orientation axis perpendicular to the polarization direction. The measured anchoring energy increases with illumination up to 10(-4) erg/cm (2). Phase retardation and pretilt measurements confirmed near homogeneous alignment of the LC in a cell, while the polarization dependence of second harmonic generation suggested a near normal alignment of the adsorbed layer. We believe that light-induced rearrangement or phototransformation of the adsorbed layer causes the observed phenomenon.

Journal Article↗

PubMed2000

Tensor and complex anchoring in liquid crystals

We propose a tensor description of surface anchoring of liquid crystals (LCs). The model allows one to consider both the homogeneous and inhomogeneous parts of LC anchoring and to calculate the cumulative effect of different treatments as a sum of corresponding tensors. For the planar alignment the tensor representation is reduced to the complex azimuthal anchoring coefficient, whose amplitude and phase determine, respectively, the strength of azimuthal anchoring and the azimuthal angle of the easy axis. We predict and experimentally confirm that two consecutive photoalignment treatments with beams of perpendicular polarizations can compensate each other and restore the initial anchoring.

Journal Article↗