Thin Nematic Cell Research Articles

Colorimetry characterization of molecular reorientation transition in thin nematic cells.

The characterization of equilibria and their transition is fundamental in dynamic systems. Experimentally, the characterization of transitions is complex due to time scales separation, the effect of thermal fluctuations, and inherent experimental imperfections. Liquid crystal devices are derived from the manipulation of the molecular reorientation and transition between them by employing external electrical and magnetic fields. Here, we investigate and determine the Fréedericksz transition using hue measurements of the transmitted light in thin nematic liquid crystal cells. Based on birefringent retardation experienced by transmitted light due to molecular reorientation, the color adjustment of the nematic liquid crystal cells under white light illumination is characterized. By monitoring the hue of the transmitted light, the bifurcation diagram is determined. As a function of the voltage frequency, the critical transition voltage is characterized. The critical voltage increases with the applied frequency.

Thickness Induced Line-Defect Reconfigurations in Thin Nematic Cell

We studied the impact of the cell thickness on configurations of line disclinations within a plane-parallel nematic cell. The Lebwohl-Lasher semimicroscopic approach was used and (meta)stable nematic configurations were calculated using Brownian molecular dynamics. Defect patterns were enforced topologically via boundary conditions. We imposed periodic circular nematic surface fields at each confining surface. The resulting structures exhibit line defects which either connect the facing plates or remain confined within the layers near confining plates. The first structure is stable in relatively thin cells and the latter one in thick cells. We focused on structures at the threshold regime where both structures compete. We demonstrated that “history” of samples could have strong impact on resulting nematic configurations.

Capillary-induced giant elastic dipoles in thin nematic films

Directed and true self-assembly mechanisms in nematic liquid crystal colloids rely on specific interactions between microparticles and the topological defects of the matrix. Most ordered structures formed in thin nematic cells are thus based on elastic multipoles consisting of a particle and nearby defects. Here, we report, for the first time to our knowledge, the existence of giant elastic dipoles arising from particles dispersed in free nematic liquid crystal films. We discuss the role of capillarity and film thickness on the dimensions of the dipoles and explain their main features with a simple 2D model. Coupling of capillarity with nematic elasticity could offer ways to tune finely the spatial organization of complex colloidal systems.

Formation of vortex flows in thin nematic cells

The director field reorientation and the formation of steady vortex flows in doubly and obliquely oriented thin liquid-crystal cells under the temperature gradient have been theoretically described within the nonlinear generalization of the classical Ericksen-Leslie theory for describing liquid crystal hydrodynamics. This generalization allowing the consideration of thermomechanical contributions to the shear stress and to the entropy balance equation has made it possible to describe the formation of a steady two-vortex flow in doubly oriented liquid-crystal cells.

Computer Simulation of the Deuterium NMR Spectra Reflecting the Director Distribution for Multi-Domains in a Thin Nematic Cell

Deuterium NMR spectroscopy is widely employed to investigate the static and dynamic director alignment for low molar mass nematics contained in a thin sandwich cell. When the director is not uniformly aligned the appearance of the deuterium NMR spectra, recorded in the measurements, reflects the form of the director distribution. This occurs because the observed spectrum is a weighted sum of the spectra from all director orientations in the sample. In order to obtain the form of the director distribution function by simulating the spectra it is necessary to know how the spectral lineshape varys with the director orientation. As well as being of practical relevance, when the director distributes thermally with the average director orientation in multi domains, this contribution to the lineshape is of fundamental interest for nematic behaviour. Here we explore ways to vary the aveage director orientation using an electric field and employ this to measure the voltage dependence of the NMR lineshapes for the nematic phase of 5CB-d2. This approach is investigated by using a computer simulation of the NMR lineshapes.

Transport and crystallization of colloidal particles in a thin nematic cell

In a thin planar nematic cell, the application of an AC electric field induces a macroscopic transport of micrometer-sized colloidal particles along the nematic director. We have analyzed the dependence of particle velocities on the electric-field amplitude and frequency and found that it decreases exponentially with increasing frequency. Using specially designed electrodes we have observed that colloidal particles could be pumped and accelerated across the field-no-field interface, and measured the structural force and the corresponding potential, which is of the order of 10000 kBT for 4 microm particles. We demonstrate that spatially periodic close-packed crystalline colloidal structures can be obtained, which are thermodinamically metastable for many days after turning off the electric field and slowly decay into linear chains. Above the nematic-isotropic phase transition, such crystalline structures are non-stable and decay in few minutes.

Role of Surface Effects on the Formation and the Dynamics of Defects in a Nematic Cell with a Planar Anchoring

ABSTRACT We have studied the dynamics of defects (disclinations and walls) which appear in a thin nematic cell with planar anchoring, after a quench from the isotropic phase. The transient texture can be analyzed in terms of ±1/2 disclinations and surface π-walls (corresponding to a π-rotation of the director in the plane of the cell). We determine the structure of the walls and explain the main features of the dynamics. The relaxation of the texture is largely dominated by surface effects, which control the relaxation trajectories and velocities of the disclinations, as well as the dynamics of π-walls.

Optical Phase Conjugation and Wavefront Correction by Thin Nematic Liquid Crystal Cells

ABSTRACT We report a study of optical phase conjugation in thin nematic cells by using low intensity of the incident beams and without any external applied field. The reported effect exploits the colossal optical nonlinearity recently observed in thin nematic liquid crystals films doped with a small amount of methyl red. The high conjugated reflectivity values obtained with the analysed samples, allows successful correction of severely aberrated wavefronts of very weak light beams.

Theory of nematic–smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic–smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic–smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic–smectic interface which is perpendicular to the smectic layers.

Spatially Resolved Study of the Fréedericksz Transition in Thin Nematic Cells

Abstract We present a novel method for spatially resolved measurements of the amplitudes of thermal fluctuations and surface pretilt angles in thin cells filled with nematic liquid crystals (LC). We use the idea that the dynamical response of the director field during a switching process critically depends on the initial conditions of the director field which vary over the cell plane. The applied switching voltage acts as an amplifier which augments initial director deformations by orders of magnitude. By means of a CCD line scan camera combined with a polarizing microscope, we study the time dependence of the optical transmission curves. Their slopes give information on viscoelastic parameters whereas their delay can be related directly to the initial director angle.