Author
B. R. Ratna
Bio: B. R. Ratna is an academic researcher from Raman Research Institute. The author has contributed to research in topics: Liquid crystal & Dielectric. The author has an hindex of 13, co-authored 27 publications receiving 676 citations.
Topics: Liquid crystal, Dielectric, Phase (matter), Bilayer, Multicritical point
Papers
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TL;DR: In this paper, optical studies have been carried out on a nematogenic copper complex, which incorporates the features of both rod-like and disk-like molecules, and conoscopic figures have been presented demonstrating the occurrence of a biaxial nematic phase in the pure complex, the uniaXial-biaxial (Na-Nb) transition in binary mixtures, and the temperature variation of the biaaxiality near this transition.
Abstract: Optical studies have been carried out on a nematogenic copper complex, which incorporates the features of both rod-like and disk-like molecules. Conoscopic figures are presented demonstrating (i) the occurrence of a biaxial nematic phase in the pure complex, (ii) the uniaxial-biaxial (Na-Nb) transition in binary mixtures, and (iii) the temperature variation of the biaxiality near this transition. The I-Na-Nb phase diagram has been studied for the binary system.
105 citations
TL;DR: The static dielectric constants of the following compounds have been measured as functions of temperature in their nematic and isotropic phases: (i) 4′-nalkyl-4-cyanobiphyenyls, (ii) trans-p-n-alkoxy-α-methyl [pacute]-cyanophenyl cinnamates and (iii) CBOOA.
Abstract: The static dielectric constants of the following compounds have been measured as functions of temperature in their nematic and isotropic phases: (i) 4′-n-alkyl-4-cyanobiphyenyls, (ii) trans-p-n-alkoxy-α-methyl [pacute]-cyanophenyl cinnamates and (iii) CBOOA. All these exhibit a very large positive dielectric anisotropy because of the C≡N end group. For all the compounds the average dielectric constant ϵ evaluated from the measured values of ϵ ∥ and ϵ∥ is, throughout the nematic phase, less than the extrapolated isotropic value. Further, ϵ decreases with decreasing temperature, the effect being greter in compounds whose nematic phase is preceded by smectic phase. These facts are in conformity wiht the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar. For the two homologous series (i) and (ii), the dielectric anistropy does not show an alternation with increasing chain length, but decreases continuously.
77 citations
TL;DR: The principal dielectric constants of the pentyl to octyl derivatives of 4′-nalkyl-4-cyanobiphenyl have been measured as functions of temperature in their nematic and isotropic phases as mentioned in this paper.
Abstract: The principal dielectric constants of the pentyl to octyl derivatives of 4′-n-alkyl-4-cyanobiphenyl have been measured as functions of temperature in their nematic and isotropic phases. All the compounds exhibit a strong positive dielectric anisotropy due to the presence of a large dipole moment along the major molecular axis. The principal dielectric constantse
‖ ande
⊥ as well as the anisotropy Δe decrease with increasing alkyl chain length. The experimental value of
$$\bar \in = \frac{1}{3}$$
(e‖+2e⊥) decreases with decreasing temperature, and is throughout less than the extrapolated isotropic value, in conformity with the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar.
71 citations
TL;DR: Presented at the 1st international symposium on ferroelectric liquid Crystals, held in 1987 at Archon, Bordeaux.
Abstract: Evidence of a first-order smectic-A--smectic-${C}^{\mathrm{*}}$ (SmA-Sm${C}^{\mathrm{*}}$) transition is presented. Detailed x-ray studies on 4-(3-methyl-2-chloropentanoyloxy)4'-heptyloxybiphenyl show that the SmA-Sm${C}^{\mathrm{*}}$ transition in this material is characterized by a discontinuous jump in the layer spacing (and hence in the tilt angle) and by a narrow two-phase region in which the density modulations due to the smectic-A and the smectic-${C}^{\mathrm{*}}$ phases coexist. Addition of a second compound drives the SmA-Sm${C}^{\mathrm{*}}$ transition towards second order.
65 citations
TL;DR: In this paper, a biaxial nematic liquid crystal was found in a simple thermotropic system, where the liquid crystal is a bao-nematic liquid.
Abstract: We report a biaxial nematic liquid crystal in a simple thermotropic system.
51 citations
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TL;DR: An overview on the recent developments in the field of liquid crystalline bent-core molecules (so-called banana liquid crystals) is given in this article, dealing with general aspects of the systematisation of the mesophases, development of polar order and chirality in this class of LC systems and explaining some general structure-property relationships.
Abstract: An overview on the recent developments in the field of liquid crystalline bent-core molecules (so-called banana liquid crystals) is given. After some basic issues, dealing with general aspects of the systematisation of the mesophases, development of polar order and chirality in this class of LC systems and explaining some general structure–property relationships, we focus on fascinating new developments in this field, such as modulated, undulated and columnar phases, so-called B7 phases, phase biaxiality, ferroelectric and antiferroelectric polar order in smectic and columnar phases, amplification and switching of chirality and the spontaneous formation of superstructural and supramolecular chirality.
753 citations
TL;DR: In this paper, a review of the field of coordination chemistry with transition metal liquid crystals is presented. But this review is limited to coordination chemistry who can use specific expertise of new transition metal liquids.
435 citations
TL;DR: A comprehensive overview of phase transition studies can be found in this article, where the authors identify the essential key concepts and points of difficulty associated with the study of phase transitions and discuss the most widely used experimental techniques for measuring these transition properties.
308 citations
TL;DR: An overview of the current state of research in the field of biaxial nematic liquid crystalline materials is given in this paper, where the major theoretical concepts are outlined, including the classification to different symmetries, the importance of cooperativity and cluster formation for the development of BN order and the conditions for the establishment of field induced and spontaneous BN in nematic phases.
Abstract: An overview over the current state of research in the field of biaxial nematic liquid crystalline materials is given. After a short introduction, providing some general aspects and summarizing the classical approaches, the main part focuses on recent developments of new concepts for designing biaxial nematics. First, the major theoretical concepts are outlined, including the classification to different symmetries, the importance of cooperativity and cluster formation for the development of biaxial order and the conditions for the establishment of field induced and spontaneous biaxiality in nematic phases. These new concepts also require the re-evaluation of the tools used for the identification of phase biaxiality, which are discussed briefly. In the second part, recent progress in the design of potential biaxial nematic materials, especially focussing on bent-core molecules with nematic phases, is reported and, finally, comparisons with phase biaxiality as observed in smectic liquid crystals are made.
299 citations
TL;DR: A review of the thermotropic materials for which a biaxial nematic has now been claimed can be found in this article, where it is suggested that deuterium NMR spectroscopy is a powerful method with which to determine this.
238 citations