Nematic-Isotropic Transition: Thermal Hysteresis and Magnetic Field Effects
TL;DR: Using neutron scattering, the effect of an applied magnetic field on the nematic-isotropic transition temperature in para-azoxy-anisole (PAA) is studied and data indicate that in zero field the transition may be of second order.
Abstract: Using neutron scattering, we have studied the effect of an applied magnetic field on the nematic-isotropic transition temperature ${T}_{c}$ in para-azoxy-anisole (PAA). We have, for the first time, been able to measure a thermal hysteresis width, $({T}_{+}^{*}{\ensuremath{-}T}_{\ensuremath{-}}^{*}){/T}_{c}$, and find a value $3.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ for a field-aligned sample, much lower than any previous estimate, and decreasing to zero for weaker fields. Also ${T}_{c}$ decreases with the field, as does the step ${Q}_{c}$ in the order parameter. The data at the lowest fields indicate that in zero field the transition may be of second order, and we correlate this with an observed increase of the director fluctuations.
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TL;DR: Monte Carlo simulations for a simple model liquid crystal showing the influence of flat walls on the phase behavior is analyzed for two different anchoring mechanisms, one favoring homeotropic alignment and one simulating a twisted nematic cell without external fields.
Abstract: Monte Carlo simulations for a simple model liquid crystal are presented. The influence of flat walls on the phase behavior is analyzed for two different anchoring mechanisms, one favoring homeotropic alignment and one simulating a twisted nematic cell without external fields, e.g., two walls with different homogeneous planar alignment. The simulations are performed in the constant pressure ensemble. The box volume may change in the directions perpendicular to the wall normal. The isotropic-nematic phase transition in the bulk system is first studied for different isobars. For the weak first order transition we do not observe any hysteresis down to a temperature accuracy of deltaT=0.001. The isotherm T=1 is then studied in the bulk as well as in the confined geometries. The walls stabilize the positional order in the systems due to the formation of layers. The orientational order is weakly stabilized.
52 citations
TL;DR: In this paper, Monte Carlo simulations for a simple model for liquid crystals in the bulk are presented, where the model potential describes the anisotropy of the particles in the attractive part of the Lennard Jones-like pair interaction.
Abstract: Monte Carlo simulations for a simple model for liquid crystals in the bulk are presented. The model potential describes the anisotropy of the particles in the attractive part of the Lennard Jones-like pair interaction. Isotherms were calculated with constant-volume and constant-pressure simulations. Comparison with previously presented analytical calculations show a good agreement for high temperatures. Isochores for two different densities were calculated. A nematic phase is observed, often metastable. By decreasing the temperature further, we get a phase transition to a layered phase where the system shows a crystalline structure. In the heating process the nematic phase is very narrow, so we have a nearly monotropic nematic phase.
29 citations
Journal Article•
TL;DR: In this article, the authors used fringe-field-interdigit-dielectric spectroscopy (FFIDS) to record and correlate mass (m) changes of population of rice starch micro-granules and their effective dielectric permittivity, as well as X-ray diffraction (XRD) changes observed in this system during humidification.
Abstract: Purpose: The purpose of this paper was to record and correlate mass (m) changes of population of rice starch micro-granules and their effective dielectric permittivity(e′), as well as X-ray diffraction (XRD) changes observed in this system during humidification. Design/methodology/approach: Changes of mass of bio-polymeric-granular sample occurring during its exposition on saturated water vapour at room temperature, was recorded in the time. The e′ evolution was recorded by means of fringe-field-interdigit-dielectric spectroscopy (FFIDS) method. The temperature and relative humidity (RH %) of ambient atmosphere were controlled. Microstructure changes induced by water absorption were recorded by means of XRD diffractometer. Findings: The FFIDS method turned out to be sensitive technique to follow details of humidification process. Correlation between changes of e′ with simultaneously occurring mass increase can be a way to describe the humidification and drying processes of micro-granular bio-polymeric sample. The changes observed by means of XRD should enable to point the regions within granules structures where water molecules effectively interact with internal granules physical organisation on macromolecular level. Research limitations/implications: The time length of m(t) record was limited to ~11000 s in case of humidification by the nature of the process. The whole range of measurements was limited to max ~23 % of water uptake in order to prevent the molecular structure irreversible changes. Practical implications: The e′ monitoring of humidification turned out to be much more selective than only gravitational measurement of mass change. The correlation of both is giving new possibilities of modelling approach. The XRD observed changes within physical structures of rice starch granules seems to be of great importance for modelling of water behaviour in starch. Originality/value: For the first time humidification process was monitored in statu nascendi, without disturbing geometry of granules starch by means of e’ evolution record. It was enabled by application of interdigit comb capacitor as sensing unit. The high quality of XRD records enables a new insight into details of reversible swelling process of rise starch granules.
3 citations
Journal Article•
TL;DR: In this paper, the authors present how the external magnetic field can be employed in order to precise control the kink width in the long Josephson junction, which can find applications in future electronic devices.
Abstract: Purpose: The purpose of this report is to present how the external magnetic field can be employed in order to precise control the kink width in the long Josephson junction. Design/methodology/approach: In this paper we concentrate on construction of the analytical kink solutions of the sine-Gordon model with the profile modified by the external magnetic field. Findings: The main findings of this article are exact solutions of the sine-Gordon model which describe the squeezed or stretched kinks. Research limitations/implications: The paper is limited to the description of the dynamics of the long Josephson junctions which are one dimensional systems with stable kink structures. Practical implications: It is expected that the possibility of control the width of the kink will find applications in future electronic devices. Originality/value: The main idea of the paper is to use some special magnetic field configurations to modulate (precisely) the properties of the Josephson junction.
3 citations
TL;DR: In this paper, the temperature dependence of the order parameter dQ/dT|Tc has been calculated by means of Tao, Sheng and Lin's generized molecular field theory.
Abstract: By means of Tao, Sheng and Lin's generized molecular field theory, the temperature dependence of the order parameter dQ/dT|Tc has been calculated. All the parameters in the calculation are fixed by experiments like the previous paper. There is no adjustable parameter. Our calculated value of dQ/dT|Tc ~ 1%K-1 agree well with experiments. The new puzzle of small value of dQ/dT|Tc does not existence.
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932 citations
TL;DR: In this article, the authors assume that the local state of order in the isotropic phase is a symmetric traceless tensor Qαβ, proportional to the anisotropic part of a tensor property such as the magnetic susceptibility.
Abstract: We assume that (1) the local state of order in the isotropic phase is a symmetric traceless tensor Qαβ, proportional to the anisotropic part of a tensor property such as the magnetic susceptibility; (2) the free energy may be expanded in powers of Qαβ and of its gradients. This allows a unified description covering the anomalous magnetic birefringence, the intensity of light scattering, and the properties of the nematic/isotropic interface. For a cholesteric, although the optical rotation is huge in the ordered phase, we predict that it should not be anomalous just above the transition point Tc. We also investigate the dynamics of fluctuations of Qαβ, and discuss the flow birefringence, the frequency width of the Rayleigh scattering, and the attenuation of ultrasonic shear waves, in terms of 3 viscosity coefficients.
766 citations
TL;DR: In this article, the temperature of mesomorphic-isotropic phase changes and of transitions between mesophases may be markedly shifted by manageable electric fields, and experiments demonstrating such a shift in the nematicisotropic transition of $p$-ethoxybenzylidene-${p}^{\ensuremath{'}}$-aminobenzonitrile are reported.
Abstract: It is proposed that the temperature of mesomorphic-isotropic phase changes and of transitions between mesophases may be markedly shifted by manageable electric fields. First experiments demonstrating such a shift in the nematic-isotropic transition of $p$-ethoxybenzylidene-${p}^{\ensuremath{'}}$-aminobenzonitrile are reported.
75 citations
TL;DR: The nature of the orientational phase transition in the three-dimensional Lebwohl-Lasher model of liquid crystals has been studied by computer simulation using reweighting techniques and finite-size scaling analysis and unambiguous numerical evidence is found in favor of a weak first-order transition.
Abstract: The nature of the orientational phase transition in the three-dimensional Lebwohl-Lasher model of liquid crystals has been studied by computer simulation using reweighting techniques and finite-size scaling analysis. Unambiguous numerical evidence is found in favor of a weak first-order transition and the presence of pseudospinodal points, ${\mathit{T}}_{\ifmmode\pm\else\textpm\fi{}}^{\mathrm{*}}$, which are extremely close to the equilibrium transition temperature, \ensuremath{\Vert}${\mathit{T}}_{\mathit{c}}$-${\mathit{T}}_{\ifmmode\pm\else\textpm\fi{}}^{\mathrm{*}}$\ensuremath{\Vert}/${\mathit{T}}_{\mathit{c}}$\ensuremath{\lesssim}0.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$, in good agreement with experimental data for the nematic-isotropic transition.
75 citations