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Georg H Mehl - One of the best experts on this subject based on the ideXlab platform.
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Director configuration in the twist-bend Nematic Phase of CB11CB
Journal of Materials Chemistry C, 2016Co-Authors: Isabella Miglioli, Georg H Mehl, Corrado Bacchiocchi, Alberto Arcioni, Alexandra Kohlmeier, Claudio ZannoniAbstract:The director distribution in the Nematic Phases exhibited by the 1′′,11′′-bis(4-cyanobiphenyl-4′-yl)undecane (CB11CB) liquid crystal has been studied in the bulk using the EPR spin probe technique. EPR spectra confirmed the presence of a higher temperature uniaxial Nematic Phase and of a lower temperature Nematic Phase in which the director distribution is not uniform. Spectra recorded in the lower temperature Nematic Phase were not fully compatible with theoretical EPR spectra calculated according to the recently proposed model for the twist-bend Phase in which the local domain director twists around an axis with a fixed tilt angle, θ0, but were well described by a “distributed-tilt” model in which the director has a relatively narrow distribution, centred at θ0.
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Nematic Nematic Phase transition in the liquid crystal dimer cbc9cb and its mixtures with 5cb a high resolution adiabatic scanning calorimetric study
Physical Review E, 2011Co-Authors: Chandra Shekhar Pati Tripathi, Georg H Mehl, Christ Glorieux, Alexandra Kohlmeier, Patricia Losadaperez, Mariagabriela Tamba, Jan LeysAbstract:: The Phase transition behavior of the liquid crystal dimer α,ω-bis(4,4'-cyanobiphenyl)nonane (CBC9CB), which has been reported to exhibit a Nematic-Nematic Phase transition, has been investigated by means of high-resolution adiabatic scanning calorimetry. This Nematic-Nematic Phase transition is weakly first-order with a latent heat of 0.24±0.01 kJ kg(-1). Mixtures up to 40 wt % with 4-pentyl-4'-cyanobiphenyl (5CB) liquid crystals have also been investigated, which also show this Nematic to Nematic Phase transition. The transition stays weakly first-order with a decreasing latent heat with increasing concentration of 5CB. For mixtures with more than 40 wt % uniaxial Nematic-unknown Nematic Phase transition was not observed.
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High-resolution calorimetric study of a liquid crystalline organo-siloxane tetrapode with a biaxial Nematic Phase.
Physical review. E Statistical nonlinear and soft matter physics, 2008Co-Authors: George Cordoyiannis, Daniela Apreutesei, Georg H Mehl, Christ Glorieux, Jan ThoenAbstract:High-resolution adiabatic scanning calorimetry and differential scanning calorimetry have been employed to study the thermal behavior of an organo-siloxane tetrapode reported to exhibit a biaxial Nematic Phase. No signature of the uniaxial to biaxial Nematic Phase transition could be retraced in sequential heating and cooling runs under different scanning rates, within the experimental resolution. The results obtained reveal that an extremely small heat should be involved in the uniaxial to biaxial Nematic Phase transition. The isotropic to uniaxial Nematic transition at 318+/-0.01 K is very stable, and it is weakly first order with a rather small latent heat of 0.20+/-0.02 J/g .
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High-resolution calorimetric study of a liquid crystalline organo-siloxane tetrapode with a biaxial Nematic Phase.
Physical Review E, 2008Co-Authors: George Cordoyiannis, Daniela Apreutesei, Georg H Mehl, Christ Glorieux, Jan ThoenAbstract:High-resolution adiabatic scanning calorimetry and differential scanning calorimetry have been employed to study the thermal behavior of an organo-siloxane tetrapode reported to exhibit a biaxial Nematic Phase. No signature of the uniaxial to biaxial Nematic Phase transition could be retraced in sequential heating and cooling runs under different scanning rates, within the experimental resolution. The results obtained reveal that an extremely small heat should be involved in the uniaxial to biaxial Nematic Phase transition. The isotropic to uniaxial Nematic transition at $318\ifmmode\pm\else\textpm\fi{}0.01\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is very stable, and it is weakly first order with a rather small latent heat of $0.20\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕\mathrm{g}$.
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Deuterium NMR investigation of the biaxial Nematic Phase in an organosiloxane tetrapode.
Physical review letters, 2005Co-Authors: João L. Figueirinhas, Carlos Cruz, Daniela Filip, Gabriel Feio, A. C. Ribeiro, Y. Frère, T. Meyer, Georg H MehlAbstract:Deuterium NMR is used to examine the molecular order exhibited by an organosiloxane tetrapode giving the first experimental evidence, using a bulk sample, for the existence of a biaxial Nematic Phase in this type of compounds. The temperature dependence of the averaged quadrupolar coupling constant and asymmetry parameter was determined in the compound's Nematic Phase. Two distinct regimes could be identified, one with a vanishing asymmetry parameter corresponding to a uniaxial Nematic Phase and another with a significant temperature dependent asymmetry parameter, corresponding to a biaxial Nematic Phase. The high values obtained for the asymmetry parameter at the lower end of the Nematic range are well above experimental error and constitute a definite proof of the biaxial nature of the Nematic Phase exhibited by the studied compound for those temperatures.
Lech Longa - One of the best experts on this subject based on the ideXlab platform.
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Stability of biaxial Nematic Phase for systems with variable molecular shape anisotropy.
Physical Review E, 2007Co-Authors: Lech Longa, Grzegorz Pajak, Thomas WydroAbstract:We study the influence of fluctuations in molecular shape on the stability of the biaxial Nematic Phase by generalizing the mean field model of Mulder and Ruijgrok [Physica A 113, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems - a mixture of potassium laurate, 1-decanol, and D2O, where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the biaxial Nematic Phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimeric- or tetrapod-like molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the Phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the biaxial Nematic Phase can be either enhanced or depressed, relative to the uniaxial Nematic Phases. In the former case the splitting of the Landau point into two triple points with a direct Phase transition line from isotropic to biaxial Phase is observed.
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Stability of biaxial Nematic Phase for systems with variable molecular shape anisotropy.
Physical review. E Statistical nonlinear and soft matter physics, 2007Co-Authors: Lech Longa, Grzegorz Pajak, Thomas WydroAbstract:We study the influence of fluctuations in molecular shape on the stability of the biaxial Nematic Phase by generalizing the mean-field model of Mulder and Ruijgrok [Physica A 113, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems--a mixture of potassium laurate, 1-decanol, and D2O , where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the biaxial Nematic Phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimericlike or tetrapodlike molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the Phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the biaxial Nematic Phase can be either enhanced or depressed, relative to the uniaxial Nematic Phases. In the former case the splitting of the Landau point into two triple points with a direct Phase transition line from isotropic to biaxial Phase is observed.
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Luckhurst–Romano model of thermotropic biaxial Nematic Phase
Liquid Crystals, 2005Co-Authors: Lech Longa, Grzegorz PająkAbstract:Recent experiments show that the long looked for thermotropic biaxial Nematic Phase is finally stabilized in a low mass liquid crystalline system. Inspired by this experimental observation we concentrate on some theoretical issues concerned with this Phase. In particular we show that the simplest Lebwohl–Lasher biaxial model, as introduced by Luckhurst and Romano, is consistent with the minimal coupling Landau–de Gennes phenomenological approach. The model shows a rich spectrum of possibilities, in particular a direct isotropic–biaxial Nematic Phase transition. A possible bridge between molecular and phenomenological approaches, in particular an interpretation of the alignment tensor, is discussed.
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luckhurst romano model of thermotropic biaxial Nematic Phase
Liquid Crystals, 2005Co-Authors: Lech Longa, Grzegorz PająkAbstract:Recent experiments show that the long looked for thermotropic biaxial Nematic Phase is finally stabilized in a low mass liquid crystalline system. Inspired by this experimental observation we concentrate on some theoretical issues concerned with this Phase. In particular we show that the simplest Lebwohl–Lasher biaxial model, as introduced by Luckhurst and Romano, is consistent with the minimal coupling Landau–de Gennes phenomenological approach. The model shows a rich spectrum of possibilities, in particular a direct isotropic–biaxial Nematic Phase transition. A possible bridge between molecular and phenomenological approaches, in particular an interpretation of the alignment tensor, is discussed.
Thomas Wydro - One of the best experts on this subject based on the ideXlab platform.
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Stability of biaxial Nematic Phase for systems with variable molecular shape anisotropy.
Physical Review E, 2007Co-Authors: Lech Longa, Grzegorz Pajak, Thomas WydroAbstract:We study the influence of fluctuations in molecular shape on the stability of the biaxial Nematic Phase by generalizing the mean field model of Mulder and Ruijgrok [Physica A 113, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems - a mixture of potassium laurate, 1-decanol, and D2O, where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the biaxial Nematic Phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimeric- or tetrapod-like molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the Phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the biaxial Nematic Phase can be either enhanced or depressed, relative to the uniaxial Nematic Phases. In the former case the splitting of the Landau point into two triple points with a direct Phase transition line from isotropic to biaxial Phase is observed.
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Stability of biaxial Nematic Phase for systems with variable molecular shape anisotropy.
Physical review. E Statistical nonlinear and soft matter physics, 2007Co-Authors: Lech Longa, Grzegorz Pajak, Thomas WydroAbstract:We study the influence of fluctuations in molecular shape on the stability of the biaxial Nematic Phase by generalizing the mean-field model of Mulder and Ruijgrok [Physica A 113, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems--a mixture of potassium laurate, 1-decanol, and D2O , where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the biaxial Nematic Phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimericlike or tetrapodlike molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the Phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the biaxial Nematic Phase can be either enhanced or depressed, relative to the uniaxial Nematic Phases. In the former case the splitting of the Landau point into two triple points with a direct Phase transition line from isotropic to biaxial Phase is observed.
Prabir K. Mukherjee - One of the best experts on this subject based on the ideXlab platform.
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Isotropic to biaxial Nematic Phase transition in an external magnetic field
Chemical Physics, 2013Co-Authors: Prabir K. Mukherjee, Muklesur RahmanAbstract:The first theoretical observation of the tricritical point for the isotropic to biaxial Nematic Phase transition of biaxial Nematic liquid crystals in the presence of an external field is reported. The influence of an external magnetic field on the isotropic to biaxial Nematic Phase transition have been studied using Landau phenomenological theory. Topological classification of Phase diagrams in the field – temperature coordinates is performed. It is shown that for a particular value of the magnetic field, the first order isotropic to biaxial Nematic Phase transition becomes second order Phase transition at a tricritical point.
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Influence of Pressure on Smectic A-Nematic Phase Transition
Modern Physics Letters B, 1999Co-Authors: Kisor Mukhopadhyay, Prabir K. MukherjeeAbstract:We propose a Landau-de Gennes phenomenological model to describe the pressure induced smectic A-Nematic Phase transition. The influence of pressure on smectic A-Nematic Phase transitions are discussed for varying coupling between orientational and translational order parameter with pressure. Increasing the pressure, the first order Nematic-smectic A transition becomes second order at a tricritical point which agrees fairly well with available experimental results.
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Critical behavior of uniaxial–biaxial Nematic Phase transition
The Journal of Chemical Physics, 1998Co-Authors: Prabir K. MukherjeeAbstract:A model considering both rodlike molecules and platelike molecules is presented in this paper. The model is treated in the framework of a Landau mean-field theory and renormalization group theory. Mean-field theory predicts the transition between the isotropic, rod Nematic, plate Nematic, and biaxial Nematic Phases. The renormalization group analysis shows that the transition exhibits a multiPhase bicritical point for n (dimensionality of the order parameter) ⩽2. However, for n>2, the transition exhibits a tetracritical point and an intermediate biaxial Nematic Phase can exist. The critical exponents are calculated to first order in e (=4−d).A model considering both rodlike molecules and platelike molecules is presented in this paper. The model is treated in the framework of a Landau mean-field theory and renormalization group theory. Mean-field theory predicts the transition between the isotropic, rod Nematic, plate Nematic, and biaxial Nematic Phases. The renormalization group analysis shows that the transition exhibits a multiPhase bicritical point for n (dimensionality of the order parameter) ⩽2. However, for n>2, the transition exhibits a tetracritical point and an intermediate biaxial Nematic Phase can exist. The critical exponents are calculated to first order in e (=4−d).
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critical behavior of uniaxial biaxial Nematic Phase transition
Journal of Chemical Physics, 1998Co-Authors: Prabir K. MukherjeeAbstract:A model considering both rodlike molecules and platelike molecules is presented in this paper. The model is treated in the framework of a Landau mean-field theory and renormalization group theory. Mean-field theory predicts the transition between the isotropic, rod Nematic, plate Nematic, and biaxial Nematic Phases. The renormalization group analysis shows that the transition exhibits a multiPhase bicritical point for n (dimensionality of the order parameter) ⩽2. However, for n>2, the transition exhibits a tetracritical point and an intermediate biaxial Nematic Phase can exist. The critical exponents are calculated to first order in e (=4−d).A model considering both rodlike molecules and platelike molecules is presented in this paper. The model is treated in the framework of a Landau mean-field theory and renormalization group theory. Mean-field theory predicts the transition between the isotropic, rod Nematic, plate Nematic, and biaxial Nematic Phases. The renormalization group analysis shows that the transition exhibits a multiPhase bicritical point for n (dimensionality of the order parameter) ⩽2. However, for n>2, the transition exhibits a tetracritical point and an intermediate biaxial Nematic Phase can exist. The critical exponents are calculated to first order in e (=4−d).
Grzegorz Pająk - One of the best experts on this subject based on the ideXlab platform.
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Luckhurst–Romano model of thermotropic biaxial Nematic Phase
Liquid Crystals, 2005Co-Authors: Lech Longa, Grzegorz PająkAbstract:Recent experiments show that the long looked for thermotropic biaxial Nematic Phase is finally stabilized in a low mass liquid crystalline system. Inspired by this experimental observation we concentrate on some theoretical issues concerned with this Phase. In particular we show that the simplest Lebwohl–Lasher biaxial model, as introduced by Luckhurst and Romano, is consistent with the minimal coupling Landau–de Gennes phenomenological approach. The model shows a rich spectrum of possibilities, in particular a direct isotropic–biaxial Nematic Phase transition. A possible bridge between molecular and phenomenological approaches, in particular an interpretation of the alignment tensor, is discussed.
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luckhurst romano model of thermotropic biaxial Nematic Phase
Liquid Crystals, 2005Co-Authors: Lech Longa, Grzegorz PająkAbstract:Recent experiments show that the long looked for thermotropic biaxial Nematic Phase is finally stabilized in a low mass liquid crystalline system. Inspired by this experimental observation we concentrate on some theoretical issues concerned with this Phase. In particular we show that the simplest Lebwohl–Lasher biaxial model, as introduced by Luckhurst and Romano, is consistent with the minimal coupling Landau–de Gennes phenomenological approach. The model shows a rich spectrum of possibilities, in particular a direct isotropic–biaxial Nematic Phase transition. A possible bridge between molecular and phenomenological approaches, in particular an interpretation of the alignment tensor, is discussed.
