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Author

V. Swaminathan

Other affiliations: University College Dublin
Bio: V. Swaminathan is an academic researcher from Trinity College, Dublin. The author has contributed to research in topics: Liquid crystal & Phase transition. The author has an hindex of 5, co-authored 12 publications receiving 63 citations. Previous affiliations of V. Swaminathan include University College Dublin.

Papers
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Journal ArticleDOI
TL;DR: The Debye-Langevin model is found to be extremely effective in confirming some of the conclusions of the de Vries chiral smectics and gives additional results on the order parameter and the dichroic ratio as a function of the field across the cell.
Abstract: Infrared and dielectric spectroscopic techniques are used to investigate the characteristics of two chiral smectics, namely, 1,1,3,3,5,5,5-heptamethyltrisiloxane 1-[${4}^{\ensuremath{'}}$-(undecyl-1-oxy)-4-biphenyl(S,S)-2-chloro-3-methylpentanoate] ($\mathrm{MS}{\mathrm{i}}_{3}\mathrm{M}{\mathrm{R}}_{11}$) and tricarbosilane-hexyloxy-benzoic acid (S)-4\ensuremath{'}-(1-methyl-hexyloxy)-3\ensuremath{'}-nitro-biphenyl-4-yl ester (W599). The orientational features and the field dependencies of the apparent tilt angle and the dichroic ratio for homogeneous planar-aligned samples were calculated from the absorbance profiles obtained at different temperatures especially in the smectic-A* phase of these liquid crystals. The dichroic ratios of the C-C phenyl ring stretching vibrations were considered for the determination of the tilt angle at different temperatures and different voltages. The low values of the order parameter obtained with and without an electric field applied across the cell in the $\mathrm{Sm}\ensuremath{-}{A}^{*}$ phase for both smectics are consistent with the de Vries concept. The generalized Langevin-Debye model introduced in the literature for explaining the electro-optical response has been applied to the results from infrared spectroscopy. The results show that the dipole moment of the tilt-correlated domain diverges as the transition temperature from $\mathrm{Sm}\ensuremath{-}{A}^{*}$ to $\mathrm{Sm}\ensuremath{-}{C}^{*}$ is approached. The Debye-Langevin model is found to be extremely effective in confirming some of the conclusions of the de Vries chiral smectics and gives additional results on the order parameter and the dichroic ratio as a function of the field across the cell. Dielectric spectroscopy finds large dipolar fluctuations in the $\mathrm{Sm}\ensuremath{-}{A}^{*}$ phase for both compounds and again these confirm their de Vries behavior.

13 citations

Journal ArticleDOI
TL;DR: A three-parameter mean-field model is introduced for the orientational distribution function (ODF) to reproduce the electro-optic properties and explains the experimental results and leads to the ODF, which exhibits a crossover from the sugar-loaf to diffuse-cone ODF some 3 °C above T_{AC}.
Abstract: Calamitic liquid crystals based on 5-phenyl-pyrimidine derivatives have been designed, synthesized, and characterized. The 5-phenyl pyrimidine core was functionalized with a chiral (R,R)-2,3-epoxyhexoxy chain on one side and either siloxane or perfluoro terminated chains on the opposite side. The one involving a perfluorinated chain shows $\mathrm{Sm}{A}^{*}$ phase over a wide temperature range of 82 \ifmmode^\circ\else\textdegree\fi{}C, whereas the siloxane analog exhibits both $\mathrm{Sm}{A}^{*}$ and $\mathrm{Sm}{C}^{*}$ phases over a broad range of temperatures, and a weak first-order $\mathrm{Sm}{A}^{*}\text{\ensuremath{-}}\mathrm{Sm}{C}^{*}$ transition is observed. For the siloxane analog, the reduction factor for the layer shrinkage R (relative to its thickness at the $\mathrm{Sm}{A}^{*}\text{\ensuremath{-}}\mathrm{Sm}{C}^{*}$ transition temperature, ${T}_{\mathrm{AC}}$) is $\ensuremath{\sim}0.373$, and layer shrinkage is 1.7% at a temperature of 13 \ifmmode^\circ\else\textdegree\fi{}C below the ${T}_{\mathrm{AC}}$. This compound is considered to have ``de Vries smectic'' characteristics with the de Vries coefficient ${C}_{\mathrm{deVries}}$ of \ensuremath{\sim}0.86 on the scale of zero (maximum-layer shrinkage) to 1 (zero-layer shrinkage). A three-parameter mean-field model is introduced for the orientational distribution function (ODF) to reproduce the electro-optic properties. This model explains the experimental results and leads to the ODF, which exhibits a crossover from the sugar-loaf to diffuse-cone ODF some 3 \ifmmode^\circ\else\textdegree\fi{}C above ${T}_{\mathrm{AC}}$.

13 citations

Journal ArticleDOI
TL;DR: Observations of a large electroclinic effect, a large increase in the birefringence with electric field, a low shrinkage in the layer thickness, and low values of the reduction factor suggest that the SmA^{*} phase in this material is of the de Vries type.
Abstract: A heptamethyltrisiloxane liquid crystal (LC) exhibiting I-SmA^{*}-SmC^{*} phases has been characterized by calorimetry, polarizing microscopy, x-ray diffraction, electro-optics, and dielectric spectroscopy. Observations of a large electroclinic effect, a large increase in the birefringence (Δn) with electric field, a low shrinkage in the layer thickness (∼1.75%) at 20 °C below the SmA^{*}-SmC^{*} transition, and low values of the reduction factor (∼0.40) suggest that the SmA^{*} phase in this material is of the de Vries type. The reduction factor is a measure of the layer shrinkage in the SmC^{*} phase and it should be zero for an ideal de Vries. Moreover, a decrease in the magnitude of Δn with decreasing temperature indicates the presence of the temperature-dependent tilt angle in the SmA^{*} phase. The electro-optic behavior is explained by the generalized Langevin-Debye model as given by Shen et al. [Y. Shen et al., Phys. Rev. E 88, 062504 (2013)10.1103/PhysRevE.88.062504]. The soft-mode dielectric relaxation strength shows a critical behavior when the system goes from the SmA^{*} to the SmC^{*} phase.

12 citations

Journal ArticleDOI
TL;DR: In this paper, an addition of chiral dopant to two achiral smectic liquid crystals from a homologous series, by varying weight percentages with known low values of layer shrinkage, leads to a chiral Smectic-C* phase with a finite value of spontaneous polarisation.
Abstract: An addition of chiral dopant to two achiral smectic liquid crystals from a homologous series, by varying weight percentages with known low values of layer shrinkage, leads to chiral smectic-C* phase with a finite value of the spontaneous polarisation. The electro-optical response arising from changes in the induced apparent tilt angle brought about by a weak electric field in the SmA* phase gives rise to power law dependency on the reduced temperature. The critical exponent γ of the power law depends on the dopant concentration but its value is found to be greater than the typical value of 1.32. This implies that the short-range correlation extends from two dimensions to three dimensions in these materials in the SmA phase. The layer thickness of smectic layers in the guest–host system remains unaltered up to the 15 wt % addition of the chiral dopant to two achiral smectics. The system thus retains the low layer shrinkage of the achiral smectic as evidenced by measurements of the layer thickness f...

11 citations

Journal ArticleDOI
TL;DR: It is concluded that the "cone-shaped" (or volcano-shaped) de Vries type of distribution can most preferably describe SmA* where "a first-order phase transition fromSmA* to SmC*" and a low layer shrinkage can both be easily explained.
Abstract: Two approaches exist in the literature for describing the orientational distribution function (ODF) of the molecular directors in SmA* phase of liquid crystals, though several models are recently proposed in the literature for explaining the de Vries behaviour. These ODFs correspond to either the conventional unimodal arrangements of molecular directors arising from the mean field theory that leads to the broad or sugar-loaf like distribution or to the "diffuse-cone-shaped" type distribution proposed by de Vries. The hypothesis by de Vries provides for a realistic explanation as to how at a molecular level, a first-order SmA* to SmC* transition can occur where the uniform molecular director azimuthal distributions condense to values lying within a narrow range of angles; finally these condense to a single value while at the same time ensuring a little or no concomitant shrinkage in the layer spacing. The azimuthal distribution of the in-layer directors is probed using IR and polarized Raman spectroscopic techniques. The latter allows us to obtain the ODF and the various order parameters for the uniaxial and the biaxial phases. Based on the results of these measurements, we conclude that the "cone-shaped" (or volcano-shaped) de Vries type of distribution can most preferably describe SmA* where "a first-order phase transition from SmA* to SmC*" and a low layer shrinkage can both be easily explained.

9 citations


Cited by
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Journal ArticleDOI
TL;DR: In this article, a specific practical method devoted to the design of functional multicomponent ferroelectric liquid crystalline (FLC) mixtures based on chiral components exhibiting the Ferroelectric and antiferroelectric polar order is presented.

37 citations

Journal ArticleDOI
TL;DR: In this paper, an assessment of the data processing and analysis methods used to obtain the second and fourth-rank orientational order parameters of liquid crystals from X-ray scattering experiments has been carried out, using experimental data from four extensively studied alkyl-cyanobiphenyls and calculated data generated from two general types of theoretical orientational distribution function.
Abstract: An assessment of the data processing and analysis methods used to obtain the second- and fourth-rank orientational order parameters of liquid crystals from X-ray scattering experiments has been carried out, using experimental data from four extensively studied alkyl-cyanobiphenyls and calculated data generated from two general types of theoretical orientational distribution function. The application of a background subtraction and two different baseline correction methods to the scattering profiles is assessed, along with three different methods to analyse the processed data. The choice of baseline correction method is shown to have a significant effect: an offset to zero overestimates the order parameters from the experimental and calculated data sets, particularly for lower order parameters arising from broad distributions, whereas an offset to a value estimated from regions of low scattering intensity provides experimental values close to those reported from other experimental techniques. By contrast, the three different analysis methods are shown generally to result in relatively small absolute differences between the order parameters. We outline a straightforward general approach to experimental X-ray scattering data processing and analysis for uniaxial phases that results in order parameters that match well with those reported using other experimental techniques.

34 citations

Journal ArticleDOI
TL;DR: One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers, providing an alternative non‐chiral way for the transition from anticlinic to synclinic tilt.
Abstract: A series of bent-shaped 4-cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short-pitch heliconical lamellar liquid-crystalline phase with incommensurate 3-layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric-(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field-induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non-tilted and ferreoelectric switching lamellar phase, providing an alternative non-chiral way for the transition from anticlinic to synclinic tilt.

22 citations

Journal ArticleDOI
TL;DR: The behaviour of dielectric relaxation process has been investigated in four ferroelectric liquid crystal (FLC) materials having different spontaneous polarisation (Ps) values in this article.
Abstract: The behaviour of dielectric relaxation process has been investigated in four ferroelectric liquid crystal (FLC) materials having different spontaneous polarisation (Ps) values. Ps effect on the permittivity in four different FLCs has been carried out in highly anchored sample cells around ~8 μm thick. It has been found that the main contribution to the dielectric permittivity in chiral Smectic C (SmC*) phase is due to Goldstone mode (GM) and partially unwound helical mode (p-UHM). In higher PS value FLC materials, the p-UHM process is found to dominate the dielectric properties. It has also been observed that p-UHM process is highly dependent on the probing ac voltage and temperature, whereas GM is found to be weakly dependent of probing voltage and temperature in SmC* phase of all the studied FLC materials. The influential contribution of p-UHM has exhibited the dielectric properties in its intrinsic frequency range making the materials suitable for futuristic display and photonics devices.

18 citations

Journal ArticleDOI
TL;DR: Results show that inclusion of ferroelectric nanoparticles in the discotic matrix consolidates the stability of the columnar matrix of the Col_{h} phase by virtue of their ferroic nature, which makes such systems highly appropriate for one-dimensional conductors.
Abstract: Liquid crystal nanocomposites have been a hot topic of research due to optimization of physical properties with such blending. There are several reports on enhancement of physical properties of nematic liquid crystals due to the blending of the nanomaterials. L. M. Lopatina and J. V. Selinger [Phys. Rev. Lett. 102, 197802 (2009)]10.1103/PhysRevLett.102.197802 have even proposed a theory based on experimental results for the enhancement of the properties of the nematic mesophase in the presence of ferroelectric nanoparticles. However, discotic liquid crystal nanocomposites are less studied. In the present experimental work, we have studied the effect of ferroelectric (BaTiO_{3}) nanoparticles on a room temperature discotic liquid crystalline material, namely 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone. We investigated the physical properties of low concentration ferroelectric nanoparticle dispersed discotic columnar structure, using calorimetric, optical, x-ray diffraction, and dielectric spectroscopy tools. Results show that inclusion of ferroelectric nanoparticles in the discotic matrix consolidates the stability of the columnar matrix of the Col_{h} phase by virtue of their ferroic nature. An enhancement in charge carrier conductivity by several orders of magnitude at ambient conditions has been observed which makes such systems highly appropriate for one-dimensional conductors. Low concentration of BaTiO_{3} nanoparticles substantially enhanced permittivity of the system also. A molecular relaxation mode has been observed in the middle frequency range of the dielectric spectra. Enhancement of these important parameters could be possible due to the ferroelectric nature of the dispersed nanoparticles.

16 citations