Light Scattering Studies of Dynamics of Bent-core Liquid Crystals PDF Download
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Author: Strahinja Stojadinovic
Publisher:
ISBN:
Category : Light
Languages : en
Pages : 219
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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.
Author: Strahinja Stojadinovic
Publisher:
ISBN:
Category : Light
Languages : en
Pages : 219
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Book Description
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.
Author: M.S. Bancroft
Publisher:
ISBN:
Category :
Languages : en
Pages : 363
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Author: Shokir A. Pardaev
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ISBN:
Category : Liquid crystals
Languages : en
Pages : 0
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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.
Author: Elliot R. Bernstein
Publisher:
ISBN:
Category :
Languages : en
Pages : 13
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A total of 16 papers have appeared in the literature dealing with both the theoretical and experimental (light scattering) investigation of phase transitions and critical phenomena in simple molecular solids. New Brillouin scattering data have been obtained for benzil single crystals near the phase transition at 83.5K. A mean field approach to the dynamics of structural phase transitions in molecular crystals has been developed. Brillouin scattering study of the successive phase transitions in methmmonium iron chloride. In addition to the magnetic transition at lwo temperature, this system also possesses successive structural phase transitions. These have been studied by means of lattice dynamics and far infrared spectroscopy, birefringenence, x-ray and neutron diffraction, specific heat and ultrasonic sound velocity measurements, and ultrasonic attentuation measurements. Also pursued were high pressure Raman, Brillouin, and Rayleigh scattering studies of benzene in a diamond anvil cell.
Author: Pavan Kumar Challa
Publisher:
ISBN:
Category : Liquid crystals
Languages : en
Pages : 139
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This dissertation is mainly divided into three parts. First, the dynamic light scattering measurements on both calamitic and bent-core nematic liquid crystals, carried out in the new split-helix resistive magnet at the National High Magnetic Field Laboratory, Tallahassee is discussed. In a nematic liquid crystal the molecules tend to be aligned along a constant direction, labeled by a unit vector (or "director") n. However, there are fluctuations from this average configuration. These fluctuations are very large for long wavelengths and give rise to a strong scattering of light. The magnetic field reduces the fluctuations of liquid crystal director n. Scattered light was detected at each scattering angle ranging from 0° to 40°. The relaxation rate and inverse scattered intensity of director fluctuations exhibit a linear dependence on field-squared up to 25 Tesla. We also observe evidence of field dependence of certain nematic material parameters. In the second part of the dissertation, magneto-optical measurements on two liquid crystals that exhibit a wide temperature-range amorphous blue phase (BPIII) are discussed. Blue phase III is one of the phases that occur between chiral nematic and isotropic liquid phases. Samples were illuminated with light from blue laser; the incident polarization direction of the light was parallel to the magnetic field. The transmitted light was passed through another polarizer oriented at 90° with respect to the first polarizer and was detected by a photo-detector. Magnetic fields up to 25Tesla are found to suppress the onset of BPIII in both materials by almost 1 degree celcius. This effect appears to increase non-linearly with the field strength. The effect of high fields on established BPIII's is also discussed, in which we find significant hysteresis and very slow dynamics. Possible explanations of these results are discussed. In the third part of the dissertation, magneto-optic measurements on two odd-numbered dimer molecules that form the recently discovered twist-bend nematic (NTB) phase, which represents a new type of 3-dimensional anisotropic fluid with about 10 nm periodicity and accompanied optical stripes are discussed. In twist-bend nematic phase the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The pitch of the oblique helocoid is in the nanometer range. Light from a red laser was passed normally through the sample placed between crossed polarizers oriented at ±45° with respect to the vertical magnetic field. Optical birefringence was measured from the transmitted light. Magnetic field of B=25T shifts downward the N-NTB phase transitions by almost 1 Celsius. We also show that the optical stripes can be unwound by a temperature and material dependent magnetic induction in the range of B=5-25T. Finally, we propose a Helfrich-Hurault type mechanism for the optical stripe formation. Based on this model we calculate the magnetic field unwinding the optical scale stripes, and find agreement with our experimental results.
Author: David Wiant
Publisher:
ISBN:
Category :
Languages : en
Pages : 223
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Author: Sunil K. Sharma
Publisher:
ISBN:
Category : Fluctuations (Physics)
Languages : en
Pages : 157
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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.
Author: Toshiaki Harada
Publisher:
ISBN:
Category : Light
Languages : en
Pages : 290
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Author: Elliot Golias
Publisher:
ISBN:
Category :
Languages : en
Pages :
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We describe the construction of a light scattering apparatus that is first used to conduct optical reflectivity measurements in thin smectic-A liquid crystal films to measure the thickness of films. Furthermore, we continue by conducting dynamic light scattering studies of films to obtain surface tension measurements for 8CB.
Author: M. S. Sefton
Publisher:
ISBN:
Category :
Languages : en
Pages :
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