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Pozycja Dielectric and flexoelectric deformations induced by ac electric field in insulating nematic layers(Wydawnictwo Politechniki Łódzkiej, 2010) Derfel, Grzegorz; Buczkowska, Mariola; Guzowski, JakubPokaż więcej The deformations of homeotropic nematic layers induced by ac electric field of frequency f were studied numerically. Two kinds of nematic liquid crystals with small negative dielectric anisotropy, Δε < 0, were considered: (i) the non-flexoelectric nematic, (ii) the nematic characterised by the positive sum of flexoelearic coefficients e = e11 + e33 > 0. It was found that at sufficiently low frequencies, the deformations varied with time. The deformations of purely dielectric nature had the period l/(2f). The time period of flexoelectric distortions was equal to 1/f. The influence of rotational viscosity and of thickness of the layer on the dynamics of deformations was analysed.Pokaż więcej Pozycja Direct current electric field induced deformations of dielectrically isotropic flexoelectronic nematic layers(Wydawnictwo Politechniki Łódzkiej, 2011) Derfel, Grzegorz; Buczkowska, Mariola; Pietrzyk, BartłomiejPokaż więcej Deformations of nematic layers induced by d.c. electric field were studied numerically. The nematic liquid crystal under consideration possessed flexoelectric properties. Its dielectric anisotropy was zero, so the deformations were solely of flexoeleclric nature. The threshold voltages for deformations were calculated for nematics with very low, moderate and high ion content. In the case of weak flexoelectric properties, the predicted threshold voltage reached c. 100V in pure material. It decreased strongly with increasing ion content and reached values much lower than the theoretical values calculated for the perfectly insulating material. The director distributions in deformed stales were also determined. Four kinds of director profiles were distinguished.Pokaż więcej Pozycja Flexoelectric torque in uniformly lying helix structures of chiral nematic liquid crystal(Wydawnictwo Politechniki Łódzkiej, 2018) Buczkowska, MariolaPokaż więcej Electric field induced deformations of chiral nematic liquid crystal layers were studied numerically. Uniformly lying helix structure of short pitch flexoelectric mixtures was considered. The electro-optic effect due to rotation of optical axis around the normal to the layer was simulated. The flexoelectric torque arising under the action of bias voltage and responsible for this rotation was calculated. It was found that the prevailing torque occurs in close vicinity of the boundary plates. Nonlinearity of superposition of splay and bend contributions to the total flexoelectric torque was found.Pokaż więcej Pozycja Flexoelectro-optic effect with large rotation angle of optic axis(Wydawnictwo Politechniki Łódzkiej, 2020) Buczkowska, MariolaPokaż więcej The flexoelectro-optic effect in layers of chiral nematics was investigated numerically. The simulations concerned short pitch materials with the helix axis parallel to the layer plane. Configurations which allow to avoid accumulation of ions at the electrodes and are suitable for square-wave driving voltage were considered. They were related to large rotation angles of the optical axis of the layer about the normal to the layer. Influence of chosen parameters on efficiency of the electro-optic effect was studied.Pokaż więcej Pozycja Influence of chirality on two-dimensional deformations of twisted flexoelectric nematic layers(Lodz University of Technology Press, 2019) Buczkowska, MariolaPokaż więcej Two-dimensional deformations induced by electric field in twisted nematic cells filled with liquid crystal possessing flexoelectric properties were simulated numerically. The influence of chiral dopants on the occurrence and structure of the spatially periodic patterns was investigated. It was found that the chirality influences mainly the direction of the periodic patterns whereas the threshold voltage for deformation and the width of the stripes are weakly affected.Pokaż więcej Pozycja Influence of strong flaxoelectric properties on deformations of nematic liquid layers induced by DC electric field(Wydawnictwo Politechniki Łódzkiej, 2010) Buczkowska, Mariola; Machlowski, MichałPokaż więcej Deformations of nematic layers induced by electric field were studied numerically. The role of very strong flexoelectric properties was considered. The threshold voltages for deformations were calculated for nematics with very low, moderate and high ion contents and characterized by negative dielectric anisotropy. The director distributions were also determined. When the sum of flexoelectric coefficients reached large values, the threshold voltage significantly decreased. The strong flexoelectric properties caused that even for extremely pure materials the threshold was lower than the theoretical value calculated for perfectly insulating material.Pokaż więcej Pozycja Influence of tilted surface alignment on flexoelectric domains in twisted nematic layers(Wydawnictwo Politechniki Łódzkiej, 2017) Buczkowska, Mariola; Derfel, GrzegorzPokaż więcej The small deformations induced by electric field in twisted flexoelectric nematic layers were simulated numerically. The onedimensional (i.e. homogeneous over the whole area of the layer) and two-dimensional (i.e. spatially periodic flexoelectric domains) deformations were considered. It was shown that the periodic deformations do not arise if sufficiently high pretilt angle is imposed by the boundary conditions. The value of pretilt angle necessary for elimination of domains increases with the flexoelectric coefficient e33. This result is of practical meaning because the flexoelectric domains are undesirable from an applicative point of view since they destroy the homogeneous appearance of the area of an excited pixel of a display.Pokaż więcej Pozycja Macroscopic model formulae describing anisotropic anchoring of nematic liquid crystals on solid substrates.(Wydawnictwo Politechniki Łódzkiej, 2015) Derfel, Grzegorz; Buczkowska, MariolaPokaż więcej Formulae used for description of anchoring energy of nematic liquid crystal aligned on solid substrates are reviewed. They are based on macroscopic approach considering the concepts of the easy axis and of the anchoring strength parameters. Properties of the modified formula proposed in our earlier article are illustrated by exemplary plots which allows for comparison with other formulae. The modified formula describes the dependence of energy on the azimuthal and polar deviation of director from the easy axis and is valid qualitatively for deviation of any magnitude.Pokaż więcej Pozycja Numerical investigation of deformations of planar flexoelectric nematic layers containing ions(Wydawnictwo Politechniki Łódzkiej, 2008) Buczkowska, Mariola; Wasilewski, GrzegorzPokaż więcej Deformations of planar nematic layers of conducting nematic liquid crystal possessing flexoelectric properties and subjected to external d.c. voltage were studied numerically. Various kinds of electrodes, from strongly blocking to well conducting, were assumed. The calculations were performed for nematic material characterized by positive dielectric anisotropy and by positive sum of flexoelectric coefficients. The threshold voltages for deformations were computed for various ion concentrations and for several rates of electrode processes. The enhancement as well as reduction of the threshold were observed. Well defined dependence of layer behaviour on the kind of electrodes and on the ion concentration was found. These effects are due to interactions between flexoelectric polarization and strong electric field of high gradient.Pokaż więcej Pozycja Numerical studies of significance of anchoring strength energy for behaviour of flexoelectric homeotropic nematic layer subjected to DC electric field(Wydawnictwo Politechniki Łódzkiej, 2009) Buczkowska, Mariola; Grzelczak, KatarzynaPokaż więcej Elastic deformations of homeotropic nematic liquid crystal layers subjected to a d.c. electric field were studied numerically. The flexoelectric properties of the nematic material and the presence of ionic space charge were taken into account. The influence of anchoring strength W was investigated. The calculations were performed for layers with negative dielectric anisotropy and negative sum of flexoelectric coefficients. Strongly blocking electrodes were assumed. The threshold voltages for deformations were computed for several anchoring strength values and for low, moderate and high ion content. Its value was nearly proportional to W for low anchoring strength and saturated at some high value of W. The director distributions were also determined. This task was complicated by the presence of space charge of ions. Remarkable change of deformation character was noticed when the ion content and anchoring strength were varied.Pokaż więcej Pozycja Role of ions mobility in flexoelectric deformations of conductive homeotropic nematic layers(Wydawnictwo Politechniki Łódzkiej, 2008) Buczkowska, Mariola; Derfel, GrzegorzPokaż więcej Numerical studies of d.c. electric field induced deformations of homeotropic layers of flexoelectric nematics containing ions are presented. It is shown that anomalous dependencies of threshold voltage for deformations on ion concentration, which were found in our previous papers, are due to difference between mobilities of cations μ + and anions μ -. The influence of this difference on the deformations is analyzed. The set of equations describing the director orientation, the electric potential and the ion concentrations as functions of the coordinate normal to the layer, is solved. The calculations are performed for four types of nematic materials characterized by both signs of dielectric anisotropy and of the sum of flexoelectric coefficients. Finite surface anchoring strength is assumed. It is shown that the anomalies concerning the threshold vanish if μ + becomes equal to μ - results are explained by means of qualitative analysis of the electric field distributions.Pokaż więcej Pozycja Temporal characteristics of DC electric field induced deformations of flexoelectric nematic layers(Wydawnictwo Politechniki Łódzkiej, 2012) Derfel, Grzegorz; Buczkowska, MariolaPokaż więcej Dynamics of DC electric field induced deformations of flexoelectric nematic layers were studied numerically. The characteristic response times were calculated. It was found that the cooperative effect of flexoelectricity and of ionic space charge may affect the dynamics of the deformations even in the case of rigid anchoring.Pokaż więcej Pozycja Two-dimensional deformations in twisted flexoelectric nematic cells.(Wydawnictwo Politechniki Łódzkiej, 2016) Derfel, Grzegorz; Buczkowska, MariolaPokaż więcej Electric field induced deformations occurring in twisted nematic cells filled with liquid crystalline material possessing flexoelectric properties were simulated numerically. The aim of computations was to compare the two-dimensional periodic deformations with one-dimensional distortions reported in our earlier paper occurring in the same cells. It was found that the periodically deformed structures have lower free energy counted per unit area of the layer than the one-dimensional deformations.Pokaż więcej