Download or read book Light Scattering Studies of Defects in Nematic twist bend Liquid Crystals and Layer Fluctuations in Free standing Smectic Membranes written by Shokir A. Pardaev and published by . This book was released on 2017 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This research described in this dissertation comprises three experimental topics and includes the development of an appropriate theoretical framework to understand the various observations in each. In the first part, we present results from angle-resolved second-harmonic light scattering measurements on three different classes of thermotropic nematic liquid crystals: polar and non-polar rodlike compounds, and a bent-core compound. We analyze the data in terms of the "flexoelectric" polarization induced by distortions of the nematic director field around topological defects known as inversion walls, which are analogous to Neel walls in magnetic spin systems and which often exhibit a closed loop morphology in nematic systems.The second part of this dissertation explores the possible existence of a helical polarization field in the nematic twist-bend (NTB) phase of dimeric liquid crystals, utilizing a similar nonlinear light scattering approach. The NTB phase is characterized by a heliconical winding of the local molecular long axis (director) with a remarkably short, nanoscale pitch. According to theoretical conjecture, a helical electric polarization field accompanies this director modulation, but, due to the short pitch, presents a significant challenge for experimental detection. Our study focuses on topological defects, classified as parabolic focal conics, in two achiral, NTB-forming liquid crystals. These defects generate distortions of the polarization field on sufficiently long (micron) lengths to enable a confirmation of the existence of polar structure. We analyze our results with a coarse-grained free energy density that combines a Landau-deGennes expansion of the polarization field, the elastic energy of a nematic, and a bilinear coupling between the two.The last part of the dissertation focuses on the layer dynamics of thin, free-standing membranes of a smectic-A liquid crystal, with a particular consideration of the surface (interfacial) parameters that control these dynamics. We utilize photon correlation spectroscopy to probe the contributions of distinct under- and overdamped processes to the membrane motion. According to hydrodynamic theory, the frequency and damping rate of underdamped layer motion should scale with scattering vector in a manner controlled by the relative magnitude of a surface elastic constant, which is associated with gradients in surface tension, as well as by the average surface tension. In addition, the damping in very thin films is predicted to be quite sensitive to the presence of an atmosphere surrounding the film. A distinct, overdamped mode, observable in sufficiently thick films, is also predicted to couple to the layer motion. We present results on these dynamical modes and their dispersion and demonstrate their consistency with the hydrodynamic theory subject to appropriate surface boundary conditions.

Download or read book Dynamic Light Scattering Studies of Layer Fluctuations in Freely Suspended Smectic Liquid Crystal Films written by Sunil K. Sharma and published by . This book was released on 2007 with total page 157 pages. Available in PDF, EPUB and Kindle. Book excerpt: Smectic liquid crystalline films or membranes have been widely studied by using the hydrodynamic theory and dynamical scattering methods, but most studies have been confined to either frequency domain light scattering measurements above ~1GHz, which probe sound waves in a limit where damping is a weak effect, or by low frequency photon correlation spectroscopy, performed in the time domain below 10 MHz (i.e., for times) and usually designed to study overdamped fluctuations of the smectic layer orientation when inertia is negligible. The interesting crossover regime, where inertia, damping, and elastic forces all contribute significantly in the hydrodynamic description, has posed a long standing challenge to scattering experiments. In this dissertation we present an extension of the optical photon correlation spectroscopy (PCS) technique down to ~100 picosecond resolution that fully closes the historical gap between frequency and time domain scattering techniques. We study smectic films from which we can determine, for optical wavevectors 10^4-10^5/cm, both the propagation frequency and damping rate for undulatory motion of thin free standing smectic films (a few tens of molecular layers), as well as the relaxation rates associated with overdamped layer dynamics of bulk films (thousands of layers thick), by measuring the time correlation function of the intensity of light scattered from thermal motion of the film surfaces. We find that the standard hydrodynamic model for incompressible, isothermal smectic films provides a good overall framework for quantitative analysis of our correlation data in both under and overdamped regimes for films of common cyanobiphenyl liquid crystals in the SmA phase. In particular, the main elastic properties - the film surface tension and compression elastic constant - are found to be reasonably consistent with values obtained from other, more invasive techniques. However, there are significant aspects in which the standard model appears incomplete. First, in relatively thin films where the dominant scattering comes from nearly pure undulation modes of the film, the effective layer sliding viscosity is more than an order of magnitude less than values reported in bulk samples, typically measured between substrates. On the other hand, in a very thick film, we obtain a result, from purely overdamped dynamics, which basically agrees with quoted bulk values of the layer viscosity. Second, in thinner films, we detect an additional, overdamped mode not expected in the light scattering derived from the standard hydrodynamic theory. We propose that this mode is associated with consequences of a higher degree of order on the film surfaces. We have also carried out preliminary surface light scattering experiments on layer motion in thin smectic-C and hexatic-B films. The dynamical parameters obtained from fits to these data show only modest variations through the SmA-SmC and SmA-HexB transitions. The additional, overdamped mode is revealed to be clearly hydrodynamic (relaxation rate ~ square of wavevector) over the range probed.

Download or read book Light Scattering Studies of Dynamics of Bent core Liquid Crystals written by Strahinja Stojadinovic and published by . This book was released on 2005 with total page 219 pages. Available in PDF, EPUB and Kindle. Book excerpt: Dynamical properties of the nematic and isotropic phase of several relatively new bent-core liquid crystals, as contrasted to conventional straight-core ("calamitic") liquid crystals, have been systematically studied by dynamic light scattering. Objective: Comparison of properties of bent-core nematics with straight-core nematics and search for a biaxial nematic phase due to the bent-core shape. Observations: Nematic phases are rather uncommon in bent-core compounds because nematic structure occurs only if the molecules can rotate relatively freely around their long axis, the condition apparently met in only a fraction of bent-core materials synthesized so far. The obtained results show that the elasticity to viscosity ratio for uniaxial bent-core nematics is typically two orders of magnitude lower than for straight-core nematics, due evidently to the large viscosity associated with optic axis distortions in the bent-core case. This result is independent of the normal mode of the director fluctuations probed. In one compound polarized light scattering data reveal fluctuations associated with the biaxial order parameter, occurring as a pretransitional effect on approach to the biaxial smectic-C phase. However, results on a homeotropically-aligned sample of another compound provide preliminary light scattering evidence for nematic biaxiality. Objective: To determine nature of isotropic to nematic phase transition and to measure dielectric fluctuations in the isotropic phase near the transition. Observations: The orientational order parameter fluctuations in the isotropic phase have been studied for the first time in bent-core compounds. Analogous to classic light scattering experiments performed on calamitic liquid crystals, fluctuations in nematic order in bent-core compounds exhibit a mean-field-like critical slowing down on approach to the isotropic-nematic transition from above. The fluctuations are intrinsically several orders of magnitude slower than for typical calamitics. In two related bent-core compounds, the transition is found to be weakly first-order, with the value of T NI -T* being ~3 times lower than the value typically obtained for calamitics. This reduction could be the effect of an enhanced density change at the I-N transition in the case of bent-core mesogens.

Download or read book Light Scattering Near Phase Transitions written by H.Z. Cummins and published by Elsevier. This book was released on 2012-12-02 with total page 685 pages. Available in PDF, EPUB and Kindle. Book excerpt: Since the development of the laser in the early 1960's, light scattering has played an increasingly crucial role in the investigation of many types of phase transitions and the published work in this field is now widely dispersed in a large number of books and journals. A comprehensive overview of contemporary theoretical and experimental research in this field is presented here. The reviews are written by authors who have actively contributed to the developments that have taken place in both Eastern and Western countries.

Download or read book Light Scattering Studies of Orientational Order in Liquid Crystalline Tetrapodes and Lyotropic Chromonic Liquid Crystals written by Krishna Prasad Neupane and published by . This book was released on 2009 with total page 249 pages. Available in PDF, EPUB and Kindle. Book excerpt: In part because of its anticipated application for faster and lower power-consuming electro-optic devices, the biaxial phase of nematic liquid crystals, characterized by two optic axes, has long been sought after. We have investigated the existence of thermotropic nematic biaxiality in a relatively new system, liquid crystalline tetrapodes, through dynamic light scattering. Liquid crystalline tetrapodes are formed by the attachment of four mesogenic molecules to a single silicon or germanium atom through flexible siloxane chains. Our results, obtained in various scattering geometries and tested against available theory, strongly support the existence of a biaxial nematic phase. The temperature dependent slowing down of the biaxial order parameter fluctuations indicates that the uni - biaxial transition is weakly first order in a 4-ring tetrapode and second order in a 3-ring homolog. The temperature dependence of the relaxation rates of the biaxial order parameter mode and of the scattered intensity associated with biaxial optic axis fluctuations is explained by a Landau-deGennes model of the free energy. In particular, we have confirmed that the intensity exhibits the expected scaling with the uniaxial and biaxial order parameter magnitudes, for several distinct geometries, in the biaxial phase. Another relatively new class of nematics is the lyotropic chromonic liquid crystals (LCLCs) formed from concentrated mixtures of disc-like dye molecules in water. Because of their potential applications in dye based polarizing and compensating films and in biological sensing, and because of recent analogies drawn between LCLCs and the liquid crystalline phases of DNA solutions, LCLCs are interesting systems to study. Using dynamic light scattering on well aligned samples, we have explored particularly the temperature dependence of the elastic constants and orientational viscosities of nematic Disodium cromoglycate LCLCs. These parameters show a significant anisotropy. In particular, the bend and splay moduli K33 and K11 are an order of magnitude higher than the twist modulus K22, and the ratio K33/K11 shows an anomalous increase as temperature increases, which we attribute to the shortening of the aggregates. The bend viscosity is three orders of magnitude smaller than the splay and twist viscosities; all viscosity coefficients exhibit a strong temperature dependence.

Download or read book Light Scattering Studies of the Smectic A nematic Transition written by Henryk Birecki and published by . This book was released on 1976 with total page 202 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Dynamic Light Scattering Studies of Some Nematic Liquid Crystals written by M.S. Bancroft and published by . This book was released on 1989 with total page 363 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Light Scattering at the Nematic to Smectic A Transition in Binary Liquid Crystal Mixtures written by Lorraine Solomon and published by . This book was released on 1983 with total page 170 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Modern Topics In Liquid Crystals From Neutron Scattering Ferroelectricity written by Agnes Buka and published by World Scientific. This book was released on 1993-12-17 with total page 354 pages. Available in PDF, EPUB and Kindle. Book excerpt: This volume contains 19 review articles, written by leading experts in the field of neutron scattering, NMR, dielectric spectroscopy, ferroelectricity, liquid crystal polymers as well as related subjects. The articles cover a broad range of topics which are currently the center of focus and interest in this field. The book will be useful for experienced researchers as well as students and those who want to enter the field. Apart from the fact that such a publication covers a gap in the literature, there is also a personal actuality. This volume will be devoted to Professor L Bata, who started the liquid crystal research in Hungary some 25 years ago and who is still head of the department at KFKI today. He initiated a lot of new subjects in the field and supported many young scientists during these years. He is celebrating his 60th birthday this year.

Download or read book Light Scattering in Liquids and Macromolecular Solutions written by V. Degiorgio and published by Springer. This book was released on 1980-12 with total page 336 pages. Available in PDF, EPUB and Kindle. Book excerpt: This volume contains most of the papers presented at the "Workshop on Quasielastic Light Scattering Studies of Fluids and Macromolecular Solutions" held at CISE, Segrate (Milano), Italy, from 11 to 13 May, 1979. Quasielastic light scattering (also called self-beating spectroscopy or intensity correlation spectroscopy) is the technique, introduced by Benedek and coworkers and by Cummins and coworkers about 15 years ago, by which dynamical information about a scattering medium is obtained through the measurement of the power spectrum (or the intensity correlation function) of the laser light scattered from the medium. The technique received in the early seventies a considerable impulse from the development of real-time fast digital correlators. The aim of the Workshop was to bring together a selected number of researchers in order to discuss recent developments in quasielastic light scattering and related optical methods and to report about new applications of the technique in physics, chemistry and biology. The first two days of the meeting were devoted to the oral presentations of papers. In the third day an informal session was held which included a very lively and thorough discussion of the highlights of the Workshop. Most part of the technical problems were debated during the informal session and during the subsequent visits to the light scattering laboratory in CISE.

Download or read book Dynamic Light Scattering of Surface Fluctuations of a Nematic Liquid Crystal written by Lih-Bin Shih and published by . This book was released on 1981 with total page 420 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Inelastic Light Scattering in Crystals written by Mikhail Mikhaĭlovich Sushchinskiĭ and published by Nova Publishers. This book was released on 1989 with total page 260 pages. Available in PDF, EPUB and Kindle. Book excerpt: Translation from the 1987 Russia edition. These proceedings address issues in solid state optics and physics: Raman scattering in crystals and dispersive media, Rayleigh and inelastic scattering with phase transitions, the features of ferroelectrics in connection with the general concept of soft mod

Download or read book Dynamic Light Scattering from Twist Fluctuations in Nematic Media Containing Polymers written by Pei-Yuan Liu and published by . This book was released on 2000 with total page 444 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Liquid and Solid State Crystals written by Józef Żmija and published by . This book was released on 1993 with total page 616 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book LIGHT SCATTERING BY LIQUID CRYSTALS written by R. S. Stein and published by . This book was released on 1968 with total page 46 pages. Available in PDF, EPUB and Kindle. Book excerpt: The scattering of light by liquid crystals is primarily a result of orientation fluctuations and is formally similar to the scattering by solid crystalline polymers. Some experimental results for the photographic scattering of mesophases of cholesteryl myristate are presented. The patterns for the cholesteric state are similar to those for spherulitic polymers and are analyzed using the theory for scattering from media exhibiting non-random orientation correlations. As a result of the light scattering studies it may be shown that the cholesteric state is characterized by disc-like correlations, while the smectic state and solid cholesteryl myristate both show random orientation correlations. (Author).

Download or read book Acoustooptical Phenomena in Liquid Crystals written by O. A. Kapustina and published by Routledge. This book was released on 1984 with total page 176 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Opticals Effects in Liquid Crystals written by I. Jánossy and published by Springer. This book was released on 2011-12-25 with total page 234 pages. Available in PDF, EPUB and Kindle. Book excerpt: In 1988 physicists and chemists commemorated the centenary of the discovery of the first liquid crystals. Fora long period after this discovery, although many significant results were found, liquid crystal research remained a marginal topic of condensed matter physics. The situation changed in the sixties. At that time the remarkable electro-optical properties of liquid crystals were recognized and found soon widespread application in numeric displays. From a more fundamental point of view, the interest in disordered systems. increased in general at the same time. Liquid crystals represented an important dass of such systems. Among others, phase transitions, hydrodynamics and topological defects occurring in them attracted considerable attention. The connection between the liquid-crystalline state and the structure of biological membranes stimulated a Iot of works also. In the present volume we discuss a relatively new and rapidly developing branch of the fi. eld, namely nonlinear optical effects in liquid crystals. Optical studies have always played a signifi. cant role in liquid crystal science. Research of optical nonlinearities in liquid crystals began at the end of the sixties. Since then it became a powerful tool in the investigation of symmetry properties, interfacial phenomena or dynamic behaviour. Furthermore, several new aspects of nonlinear processes were demonstrated and studied extensively in liquid crystals. The subject covered in this book is therefore of importance both for liquid crystal research and for nonlinear optics itself. The term "nonlinear optics" is used here in a broad sense.