Azimuthal and polar anchoring energies of aligning layers structured by nonlinear laser lithography

In spite of the fact that there are different techniques in the creation of the high-quality liquid crystals (LCs) alignment by means of various surfaces, the azimuthal and polar anchoring energies as well as the pre-tilt angle are important parameters to all of them. Here, the modified by a certain manner aligning layers, previously formed by nonlinear laser lithography (NLL), having high-quality nano-periodic grooves on Ti surfaces, recently proposed for LC alignment was studied. The change of the scanning speed of NLL in the process of nano-structured Ti surfaces and their further modification by means of ITO-coating, and deposition of polyimide film has enabled different aligning layers, whose main characteristics, namely azimuthal and polar anchoring energies, were measured. For the modified aligning layers, the dependencies of the twist and pre-tilt angles for LC cells filled by nematic E7 ({\Delta}{\epsilon} > 0) and MLC-6609 ({\Delta}{\epsilon} < 0) were obtained. Also the contact angle for droplets of isotropic liquid (glycerol), and nematic LCs was measured for the various values of the scanning speed during the laser processing.Comment: 49 pages, 18 figure

Rotation of single crystals of chiral dopants at the top of a nematic droplet: Analogy with Lehmann effect

Detailed studies are reported of the Lehmann-type effect of rotation of small single crystals of chiral substances during their dissolution at the top of a nematic droplet, which was first noted in our previous works. The rotation effect is shown to be essentially dependent on the dopant chirality. The angular velocity of the rotation of the chiral dopant crystals is experimentally shown to be dependent on the crystal dimensions, helical-twisting power of the dopant (determined in independent measurements) and viscosity of the nematic solvent, whereas no rotation was observed for non-chiral dopants or in isotropic solvents under the same conditions. For systematic series of dopants (e.g., cholesterol esters), the angular velocity was proportional to the helical-twisting power; with different nematic matrices, the observed angular velocity was proportional to the rotational-viscosity coefficient γ1 ; also, in a certain size range, it was inversely proportional to the characteristic linear dimension of the dissolved crystal. Basing on unit dimensions arguments, a semi-empirical equation is proposed giving a common quantitative description of the whole set of experimental data obtained. It is argued that the obtained results a) suggest a novel method for the evaluation of the helical-twisting power, especially useful for dopants of weak chirality, and b) that they are a direct evidence of transformation of the energy of chiral interactions into the energy of the molecular movement

Light-induced rewiring and winding of Saturn ring defects in photosensitive chiral nematic colloids

We study the winding and unwinding of Saturn ring defects around silica microspheres with homeotropic surface anchoring in a cholesteric liquid crystal with a variable pitch. We use mixtures of a nematic liquid crystal 5CB and various photoresponsive chiral dopants to vary the helical pitch and sense of the helical winding by illuminating the mixtures with UV or visible light. Upon illumination, we observe motion of the Grandjean-Cano disclination lines in wedge-like cells. When the line touches the colloidal particle, we observe topological reconstruction of the Grandjean-Cano line and the Saturn ring. The result of this topological reconstruction is either an increase or decrease of the degree of winding of the Saturn ring around the colloidal particle. This phenomenon is similar to topological rewiring of −1/2 disclination lines, observed recently in chiral nematic colloids

High-quality alignment of nematic liquid crystals using periodic nanostructures created by nonlinear laser lithography

It is well known that today two main and well studied methods for alignment of liquid crystals has been used, namely: rubbing and photoalignment technologies, that lead to the change of anisotropic properties of aligning layers and long-range interaction of the liquid crystal molecules in a mesophase. In this manuscript, we use the nonlinear laser lithography technique, which was recently presented as a fast, relatively low-cost method for a large area micro and nanogrooves fabrication based on laser-induced periodic surface structuring, as a new perspective method of the alignment of nematic liquid crystals. 920 nm periodic grooves were formed on a Ti layer processed by means of the nonlinear laser lithography and studied as an aligning layer. Aligning properties of the periodic structures of Ti layers were examined by using a combined twist LC cell. In addition, the layer of the nanostructured Ti was coated with an oxidianiline-polyimide film with annealing of the polymer film followed without any further processing. The dependence of the twist angle of LC cells on a scanning speed and power of laser beam during processing of the Ti layer was studied. The azimuthal anchoring energy of Ti layers with a periodic nanostructure was calculated. The maximum azimuthal anchoring energy for the nanostructured Ti layer was about 4.6 x 10(-5) J/m(2), which is comparable to the photoalignment technology. It was found that after the deposition of a polyimide film on the periodic nanostructured Ti layer, the gain effect of the azimuthal anchoring energy to -1 x 10(-4) J/m(2) is observed. Also, AFM study of aligning surfaces was carried out