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Maurizio Nobili - One of the best experts on this subject based on the ideXlab platform.
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Dynamics of Nematic Liquid Crystal Disclinations: The Role of the Backflow
Physical Review Letters, 2005Co-Authors: Christophe Blanc, D. Svenšek, S. Žumer, Maurizio NobiliAbstract:We measure the electric-field-driven annihilation of nematic Disclination pairs with strength ±1/2 in the 4−cyano−4′−n-pentylbiphenyl (5CB) liquid crystal. The use of a very weak azimuthal anchoring ensures a two-dimensional director field. The relaxation is governed by the formation of a π wall connecting the two opposite charge defects. The +1/2 Disclinations move almost twice as fast as the −1/2 Disclinations. The simple used geometry allows a quantitative comparison with numerical studies based on the hydrodynamics of the tensorial order parameter. The simulations show that in the π wall regime the symmetry breaking is due to the backflow and not to the elastic anisotropy.
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Electric-field-induced deformation dynamics of a single nematic Disclination
Physical Review E : Statistical Nonlinear and Soft Matter Physics, 2005Co-Authors: Angela Vella, Romuald Intartaglia, Christophe Blanc, Ivan I. Smalyukh, Oleg D. Lavrentovich, Maurizio NobiliAbstract:Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable Disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable Disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the Disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called π wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Léger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the Disclination line. At small voltages, the Disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the π walls. The π walls tend to shrink. When the voltage increases above EF, the Fréedericksz threshold, the BL walls appear and connect pairs of Disclinations along a path complementary to the π wall. At E>2EF, the BL walls store sufficient energy to prevent shrinking of the π walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the Disclinations are strongly bent in the region between the π and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface Disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.
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electric field induced deformation dynamics of a single nematic Disclination
Physical Review E, 2005Co-Authors: Angela Vella, Romuald Intartaglia, Christophe Blanc, Ivan I. Smalyukh, Oleg D. Lavrentovich, Maurizio NobiliAbstract:Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable Disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable Disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the Disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called pi wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Leger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the Disclination line. At small voltages, the Disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the pi walls. The pi walls tend to shrink. When the voltage increases above E(F), the Freedericksz threshold, the BL walls appear and connect pairs of Disclinations along a path complementary to the pi wall. At E>2 E(F), the BL walls store sufficient energy to prevent shrinking of the pi walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the Disclinations are strongly bent in the region between the pi and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface Disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.
Christophe Blanc - One of the best experts on this subject based on the ideXlab platform.
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Dynamics of Nematic Liquid Crystal Disclinations: The Role of the Backflow
Physical Review Letters, 2005Co-Authors: Christophe Blanc, D. Svenšek, S. Žumer, Maurizio NobiliAbstract:We measure the electric-field-driven annihilation of nematic Disclination pairs with strength ±1/2 in the 4−cyano−4′−n-pentylbiphenyl (5CB) liquid crystal. The use of a very weak azimuthal anchoring ensures a two-dimensional director field. The relaxation is governed by the formation of a π wall connecting the two opposite charge defects. The +1/2 Disclinations move almost twice as fast as the −1/2 Disclinations. The simple used geometry allows a quantitative comparison with numerical studies based on the hydrodynamics of the tensorial order parameter. The simulations show that in the π wall regime the symmetry breaking is due to the backflow and not to the elastic anisotropy.
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Electric-field-induced deformation dynamics of a single nematic Disclination
Physical Review E : Statistical Nonlinear and Soft Matter Physics, 2005Co-Authors: Angela Vella, Romuald Intartaglia, Christophe Blanc, Ivan I. Smalyukh, Oleg D. Lavrentovich, Maurizio NobiliAbstract:Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable Disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable Disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the Disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called π wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Léger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the Disclination line. At small voltages, the Disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the π walls. The π walls tend to shrink. When the voltage increases above EF, the Fréedericksz threshold, the BL walls appear and connect pairs of Disclinations along a path complementary to the π wall. At E>2EF, the BL walls store sufficient energy to prevent shrinking of the π walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the Disclinations are strongly bent in the region between the π and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface Disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.
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electric field induced deformation dynamics of a single nematic Disclination
Physical Review E, 2005Co-Authors: Angela Vella, Romuald Intartaglia, Christophe Blanc, Ivan I. Smalyukh, Oleg D. Lavrentovich, Maurizio NobiliAbstract:Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable Disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable Disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the Disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called pi wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Leger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the Disclination line. At small voltages, the Disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the pi walls. The pi walls tend to shrink. When the voltage increases above E(F), the Freedericksz threshold, the BL walls appear and connect pairs of Disclinations along a path complementary to the pi wall. At E>2 E(F), the BL walls store sufficient energy to prevent shrinking of the pi walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the Disclinations are strongly bent in the region between the pi and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface Disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.
Claude Fressengeas - One of the best experts on this subject based on the ideXlab platform.
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A Fast Fourier Transform-based approach for Generalized Disclination Mechanics within a Couple Stress theory
2016Co-Authors: Stéphane Berbenni, Vincent Taupin, Claude Fressengeas, L. CapolungoAbstract:Recently, a small-distortion theory of coupled plasticity and phase transformation accounting for the kinematics and thermodynamics of generalized defects called generalized Disclinations (abbreviated g- Disclinations) has been proposed. Then, a first numerical spectral approach has been developed to solve the elasto-static equations of field dislocation and g-Disclination mechanics set out in this theory for periodic media and for linear elastic media using the classic Hooke’s law. Here, given a spatial distribution of generalized Disclination density tensors in a homogenous linear higher order elastic media described, a couple stress theory with elastic incompatibilities of first and second orders is developed. The incompatible and compatible elastic second and first distortions are obtained from the solution of Poisson and Navier-type equations in the Fourier space. The efficient Fast Fourier Transform (FFT) algorithm is used based on intrinsic Discrete Fourier Transforms (DFT) that are well adapted to the discrete grid to compute higher order partial derivatives in the Fourier space. Therefore, stress and couple stress fields can be calculated using the inverse FFT. The numerical examples are given for straight wedge Disclinations and associated wedge Disclination dipoles which are of importance to geometrically describe tilt grain boundaries at fine scales in polycrystalline solids.
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Continuous description of the atomic structure of grain boundaries using dislocation and generalized-Disclination density fields
International Journal of Plasticity, 2016Co-Authors: Xiao-yu Sun, Vincent Taupin, Claude Fressengeas, Patrick CordierAbstract:An atomistic-to-continuum method is developed to derive dislocation, generalized-Disclination density fields and the associated elastic strain, rotation, curvature and second-distortion fields from the atomic structure of grain boundaries. From the relaxed and un-relaxed atomic positions, calculation of the transformation gradient feeds a mechanical framework, where discontinuities of the lattice elastic displacement and distortion (rotation and strain) are captured by smooth incompatible strain and second-distortion fields associated with the dislocation and generalized-Disclination density fields, respectively. The method is applied to a copper symmetrical tilt boundary as obtained from molecular dynamics simulations. The core structure of the boundary is found to contain edge dislocations and dipoles of generalized-Disclinations, including standard wedge-Disclination dipoles. The latter reflect in particular localized shear and stretch discontinuities across the interface, in addition to the overall rotation discontinuity.
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Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle
Nature, 2014Co-Authors: Patrick Cordier, Vincent Taupin, Benoit Beausir, Sylvie Demouchy, Fabrice Barou, Claude FressengeasAbstract:Mantle flow involves large strains of polymineral aggregates. The strongly anisotropic plastic response of each individual grain in the aggregate results from the interactions between neighbouring grains and the continuity of material displacement across the grain boundaries. Orthorhombic olivine, which is the dominant mineral phase of the Earth's upper mantle, does not exhibit enough slip systems to accommodate a general deformation state by intracrystalline slip without inducing damage. Here we show that a more general description of the deformation process that includes the motion of rotational defects referred to as Disclinations can solve the olivine deformation paradox. We use high-resolution electron backscattering diffraction (EBSD) maps of deformed olivine aggregates to resolve the Disclinations. The Disclinations are found to decorate grain boundaries in olivine samples deformed experimentally and in nature. We present a Disclination-based model of a high-angle tilt boundary in olivine, which demonstrates that an applied shear induces grain-boundary migration through Disclination motion. This new approach clarifies grain-boundary-mediated plasticity in polycrystalline aggregates. By providing the missing mechanism for describing plastic flow in olivine, this work will permit multiscale modelling of the rheology of the upper mantle, from the atomic scale to the scale of the flow.
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Grain boundary modeling using an elasto-plastic theory of dislocation and Disclination fields
Journal of the Mechanics and Physics of Solids, 2013Co-Authors: Vincent Taupin, L. Capolungo, Claude Fressengeas, A. Das, M. UpadhyayAbstract:Using a recent elasto-plastic theory of dislocation and Disclination fields, a continuous representation of grain boundaries is introduced. Periodic arrays of wedge Disclination dipoles, including those defined in the Disclination Structural Unit Model, are set-up as initial configurations in a dynamic model for symmetric tilt boundaries. These configurations are found to be unstable when the transport of Disclinations is allowed. Driven by their self couple-stress field, the motion of Disclinations leads to relaxation of the initial elastic curvature and stress fields and to nucleation and transport of relaxation dislocations, until an equilibrium configuration of lower energy is reached. Most of the residual elastic energy of grain boundaries is localized in a non-singular nanometric layer. This energy arises from alternative dilatation and contraction of the lattice around Disclinations, and from lattice curvature and shear between Disclination dipoles. By virtue of its continuous and dynamic character, the present theory allows modeling absolute misorientations and leads to energy density levels comparable to molecular statics findings.
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Disclination densities from EBSD orientation mapping
International Journal of Solids and Structures, 2013Co-Authors: Benoit Beausir, Claude FressengeasAbstract:The aim of the paper is to show experimental evidence of the rotational defects referred to as Disclinations in polycrystalline aggregates. Using orientation maps obtained from electron backscattered diffraction or transmission electron microscopy, a method for the recovery of components of the Disclination density tensor is presented and applied to various polycrystalline materials. Mapping the Disclination densities reveals their extensive presence at intra-granular low-angle boundaries, low and high-angle grain boundaries and triple junctions, irrespective of the material symmetry and grain size. A significant level of rotational incompatibility, with dipolar distribution of the Disclinations, is detected in all cases investigated. Since high-angle rotational incompatibility cannot be accounted for consistently by dislocation-based models, the present results support considering Disclinations in addition to dislocations in the interpretation of grain boundaries and triple junctions.
Angela Vella - One of the best experts on this subject based on the ideXlab platform.
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Electric-field-induced deformation dynamics of a single nematic Disclination
Physical Review E : Statistical Nonlinear and Soft Matter Physics, 2005Co-Authors: Angela Vella, Romuald Intartaglia, Christophe Blanc, Ivan I. Smalyukh, Oleg D. Lavrentovich, Maurizio NobiliAbstract:Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable Disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable Disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the Disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called π wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Léger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the Disclination line. At small voltages, the Disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the π walls. The π walls tend to shrink. When the voltage increases above EF, the Fréedericksz threshold, the BL walls appear and connect pairs of Disclinations along a path complementary to the π wall. At E>2EF, the BL walls store sufficient energy to prevent shrinking of the π walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the Disclinations are strongly bent in the region between the π and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface Disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.
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electric field induced deformation dynamics of a single nematic Disclination
Physical Review E, 2005Co-Authors: Angela Vella, Romuald Intartaglia, Christophe Blanc, Ivan I. Smalyukh, Oleg D. Lavrentovich, Maurizio NobiliAbstract:Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable Disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable Disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the Disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called pi wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Leger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the Disclination line. At small voltages, the Disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the pi walls. The pi walls tend to shrink. When the voltage increases above E(F), the Freedericksz threshold, the BL walls appear and connect pairs of Disclinations along a path complementary to the pi wall. At E>2 E(F), the BL walls store sufficient energy to prevent shrinking of the pi walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the Disclinations are strongly bent in the region between the pi and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface Disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.
Alan H Windle - One of the best experts on this subject based on the ideXlab platform.
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effect of the elastic constant anisotropy on Disclination interaction in the nematic polymers
Journal of Physical Chemistry B, 2005Co-Authors: Wenhui Song, G Goldbeckwood, Alan H WindleAbstract:In this work, Disclination interaction behavior in relation to Frank elastic constant anisotropy in nematics has been studied. A large number of (+(1/2), -(1/2)) Disclination pairs are revealed by spontaneous band texture in a semiflexible copolyester. The pairs show no preferential relative orientation, with the intervening fields showing intermediate patterns. A two-dimensional tensor lattice model considering unequal elastic constants is applied to simulate the interaction behavior and patterns of Disclination pairs in the presence of elastic anisotropy. Scaling laws for Disclination density rho(t) as a function of time step t with different elastic anisotropy are obtained as t(-nu). The value of the exponent nu decreases as elastic anisotropy is increased. Obviously, elastic anisotropy slows the texture coarsening. The simulations also show that angular forces arise in the presence of elastic anisotropy and change the patterns of pairs during the texture coarsening. When Disclination density is considerably decreased, some +(1/2) Disclinations start to rotate to the energetically favored patterns depending on the sign of the elastic anisotropy. As a result of the Disclination rotation, the distribution of patterns of pairs continues to change during the annihilation. However, Disclination pairs are influenced not only by elastic anisotropy but also by Disclination interaction during the whole annihilation. Therefore, in a real system, the dependence of pairs on elastic anisotropy is not as strong as the theoretical prediction for an isolated pair, and the full pattern range of Disclination pairs can be observed.
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elastic constant anisotropy and Disclination interaction in nematic polymers ii effect of Disclination interaction
Liquid Crystals, 2003Co-Authors: Wenhui Song, G Goldbeckwood, Alan H WindleAbstract:A new tensorial approach introducing unequal elastic constants in a two-dimensional lattice model is applied to simulate the influence of Disclination interaction on the structures of half-wedge Disclinations, for the understanding of the variation of apparent elastic anisotropy ϵ {\rm _{a}} measured from different Disclinations in a texture (see preceding paper, part I [1]). The apparently random variation of ϵ {\rm _{a}} implies that Disclination interaction has a strong effect on the structure of Disclinations; as elastic anisotropy increases, its impact becomes overpowering. Nevertheless the Disclination interaction still acts in the same way as in the case of equal elastic constants. Analysis of the free energy of Disclination pairs shows that the structures of both +1/2 and −1/2 Disclinations are changed under the influence of a neighbouring Disclination, but in different ways. For +1/2 Disclination, the splay and bend distortions vary with the relative orientation of its −1/2 neighbour. Meanwhile, ...