09/25/2002
Monte Carlo and theoretical studies of thin 3D films of biaxial and uniaxial nematics with tangential boundary conditions show distinct differences in structure and evolution of topological defects. In the uniaxial films, defects of strength k=+/1 are point defects that bear no bulk singularity and disappear by annihilation with each other. In the biaxial films, k=+/1 defects are true singular bulk disclinations that split into pairs of k=+/1/2 lines; the latter disappear by annihilation processes of the type +1/21/2=0. These observed differences are of relevance for the current debate on the existence of biaxial phases.
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08/19/2005
We demonstrate that the finite rate of dielectric relaxation in liquid crystals which has been ignored previously causes profound effects in the fast dielectric reorientation of the director. We propose a theory of dielectric response in which the electric displacement depends not only on the present (as in the standard theory) but also on the past values of electric field and director. We design an experiment with a dualfrequency nematic in which the standard "instantaneous" model and our model predict effects of opposite signs; the experimental data support the latter model.
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11/19/2012
Molecular shape is an important factor in determining the material properties of thermotropic liquid crystals (LCs). We synthesized and investigated several LC compounds formed by asymmetrically bent molecules with a rigid fourring core in the shape of the letter 'L'. We measured the temperature dependencies of dielectric permittivities, birefringence, splay K1 and bend K3 elastic constants, splay viscosity eta(splay) and flow viscosities eta(parallel to) and eta(perpendicular to). The bendsplay anisotropy delta K31  K3  K1 is negative, similar to the case of nematic LCs formed by symmetrically bent molecules of Vshape. The dielectric anisotropy Delta epsilon and birefringence are positive in the entire nematic range. The splay viscosity eta(splay) and the flow viscosities eta(parallel to) and eta(perpendicular to) are smaller than the viscosities measured for the symmetric Vshaped bentcore materials at similar temperatures. The ratio Gamma = eta(splay)/eta(parallel to,perpendicular to) is in the range 54 that is typical for rodlike LCs. The reported Lshaped bentcore nematic LCs combine the useful features of bentcore LCs (such as a negative delta K31, suitable for formulation of broadrange blue phases) with the relatively low viscosities, a property typical for rodlike LCs and beneficial for electrooptic switching applications.
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03/02/2007
We report on a thermodielectric bistability in dual frequency nematic liquid crystals (LCs) caused by the anisotropic nature of dielectric heating and director reorientation in an electric field. The bistability is a result of the positive feedback loop: director reorientation > anisotropic dielectric heating > dielectric anisotrophy > director reorientation. We demonstrate both experimentally and theoretically that two states with different temperature and director orientation, namely, a cold planar state and a hot homeotropic state coexist in a LC cell for a certain frequency and amplitude range of the applied voltage.
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07/12/1999
The popular “highelectricfield” technique to determine the polar anchoring coefficient W of a nematic–substrate interface requires the simultaneous measurement of the capacitance and optical phase retardation of a liquid crystal cell as a function of applied voltage. We develop a generalized model that makes it possible to eliminate the capacitance measurement. The new technique, called the RV (retardation versus voltage) technique, requires only themeasurement of retardation as a function of applied voltage, and allows for the determination ofW by a linear fit over a prescribed voltage window. The technique is not sensitive to uniformity of the cell thickness, does not require patterned electrodes, and allows for the local probe of thesurface. The value of W obtained by the RV technique is the same as W obtained by the traditional technique.
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06/13/2008
The dielectric dispersion in the uniaxial nematic liquid crystals affects the switching dynamics of the director, as the dielectric torque is determined by not only the present values of the electric field and director but also by their past values. We demonstrate that this “dielectric memory” leads to an unusual contribution to the dielectric torque that is linear in the present field and thus polarity sensitive. This torque can be used to accelerate the “switchoff” phase of director dynamics.
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01/24/2011
We design a nematic collider for controlled outofequilibrium anisotropic aggregation of spherical colloidal particles. The nematic surrounding imparts dipolar interactions among the spheres. A bidirectional backflow of the nematic liquid crystal in a periodic electric field forces the spheres to collide with each other. The inelastic collisions are of two types, head to tail and head to head. Headtotail collisions of dipoles result in longitudinal aggregation while headtohead collisions promote aggregation in the transversal direction. The frequency of headtohead collisions is set by the impact parameter that controls the resulting shape of aggregates, their anisotropy, and fractal dimension.
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09/21/2007
We study electricfieldinduced dynamics of colloids in a nematic cell, experimentally and by computer simulations. Solid particles in the nematic bulk create director distortions of dipolar type. Elastic repulsion from the walls keeps the particles in the middle of cell. The ac electric field reorients the dipoles and lifts them to top or bottom, depending on dipole orientation. Once near the walls, the colloids are carried along two antiparallel horizontal directions by nematic backflow. Computer simulations of the backflow agree with the experiment.
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10/15/1999
We analyze the highelectricfield technique designed by Yokoyama and van Sprang [J. Appl. Phys. 57, 4520 (1985)] to determine the polar anchoring coefficient W of a nematic liquid crystalsolid substrate. The technique implies simultaneous measurement of the optical phase retardation and capacitance as functions of the applied voltage well above the threshold of the Frederiks transition. We develop a generalized model that allows for the determination of W for tilted director orientation. Furthermore, the model results in a new highfield technique, (referred to as the RV technique), based on the measurement of retardation versus applied voltage. W is determined from a simple linear fit over a wellspecified voltage window. No capacitancemeasurements are needed to determine W when the dielectric constants of the liquid crystal are known. We analyze the validity of the Yokoyama–van Sprang (YvS) and RV techniques and show that experimental data in real cells often do not follow the theoretical curves. The reason is that the director distribution is inhomogeneous in the plane of the bounding plates, while the theory assumes that the director is not distorted in this plane. This discrepancy can greatly modify the fitted value of 1/W, and even change its sign, thus making the determination of W meaningless. We suggest a protocol that allows one to check if the cell can be used to measureW by the YvS or RV techniques. The protocol establishes new criteria that were absent in the original YvS procedure. The results are compared with other data on W, obtained by a thresholdfield technique for the same nematicsubstrate pair.
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08/01/2000
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.
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