Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y15: Liquid Crystals III |
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Sponsoring Units: DFD Chair: Luz Martinez-Miranda, University of Maryland Room: 316 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y15.00001: Magnetic-field induced isotropic to nematic liquid crystal phase transition J.T. Gleeson, T.B. Ostapenko, D. B. Wiant, S.N. Sprunt, A. Jakli We report on measurements of magnetic field induced nematic order in the bent-core liquid crystal 4-chlororesorcinol bis[4- (4-n-dodecyloxybenzoyloxy) benzoate]. Using the 31 Tesla solenoid at the National High Magnetic Field Laboratory, we have observed, at temperatures less than one degree above the clearing point, a first-order transition to the nematic phase. The critical magnetic field at which this occurs increases with temperature. We discuss these results within the context of both Maier-Saupe and Landau-deGennes mean field models for the nematic-isotropic transition. The implications of possible tetrahedratic order are also discussed. To our knowledge, this is the first observation of such a magnetic field-induced transition in a thermotropic liquid crystal; the reasons for which this behavior is now attainable are discussed. \\ \\ This work was supported by the NSF (DMR-0606160) and Kent State University. Work performed at NHMFL supported by NSF cooperative agreement DMR- 0084173, the State of Florida and the DOE. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y15.00002: High Magnetic Field-Induced Birefringence in Lyotropic Chromonic Liquid Crystals T. Ostapenko, Yu. Nastishin, J.T. Gleeson, S.N. Sprunt, O.D. Lavrentovich, P.J. Collings We studied the effect of magnetic-field induced birefringence of a 14{\%} solution of disodium cromoglycate (DSCG) in water at temperatures above the nematic-isotropic coexistence region. According to Landau-deGennes mean field theory, we expect to find a linear relationship between the inverse of the induced birefringence, $\Delta $n, and the quantity (T-T*), where T* is the stability limit of the isotropic phase. Using the 31 T resistive magnet at the National High Magnetic Field Laboratory, we observed that, as we increase the temperature above the coexistence region, we deviate from this linear dependence. Our data shows that $\Delta $n goes to zero, whereas Landau-deGennes predicts that $\Delta $n should decrease asymptotically. This may be due to the lack of isodesmic aggregate formation at a finite temperature above the coexistence region.\\ \\ Supported by NSF (DMR-0710544 and DMR-0606160). Work performed at NHMFL, supported by NSF cooperative agreements DMR-0084173, the State of Florida and the DOE. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y15.00003: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y15.00004: Chromonic liquid crystalline properties of dyes Xuxia Yao, Jung Park, Mohan Srinivasarao As a new class of lyotropic liquid crystals, chromonic liquid crystals (CLCs) can self-assemble into an ordered complex fluid, potentially useful for organic solar cells. Different from common amphiphilic lyotropic mesophases, CLCs have no optimum aggregation size, which implies the order parameter increases with concentration. We used capillary flow and magnetic field to induce alignment in chromonic dyes and studied the aggregation behavior by Vis-spectroscopy, the phase behavior by POM and DSC, and the order distribution by Raman Scattering. We also investigated how the molecular structures influence the structures of mesogens and the morphology in the dried film which will further influence the charge mobility in the solar cells. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y15.00005: Liquid Crystalline Phase Transition of Colloidal Platelets with Identical Thickness Dazhi Sun, Zhengdong Cheng, Hung-Jue Sue The disorder -- order transition in discotic colloids has been an active research area since the observation of the nematic phase in clay suspensions by I. Langmuir in 1938. In the past decade, synthetic platelets have been used extensively to investigate the discotic liquid crystal phase transitions. Here, we report the phase behavior of a new model platelet system -- alpha-zirconium phosphate (ZrP). After exfoliation, the monolayer ZrP platelets possess uniform thickness, but have a high polydispersity in diameter. We observed an isotropic -- nematic transition in our system upon increasing the platelet volume fraction, followed by the formation of the discotic smectic phase, an elusive phase that has been rarely seen in discotic liquid crystals. The discotic smectic phase (domain) is characterized by X-ray diffraction, high-resolution transmission electron microscopy, and optical microscopy. The equation of state (EOS) of our system is also discussed. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y15.00006: Study of the Isotropic-Nematic and the Nematic-Smectic-A Phase Transitions in Carbon Nanotubes and Liquid Crystal Composites Krishna Sigdel, Germano Iannacchione A high-resolution ac-calorimetric study of the isotropic to nematic (\emph{I-N}) and the nematic to smectic-A (\emph{N-SmA}) phase transitions of carbon nanotubes (CNTs) and liquid crystal octyl-cyanobiphenyl (8CB) composites (8CB+CNTs) as a function of CNTs concentration is reported. Scans were performed on heating and cooling for all samples (0.5-6 wt\% of CNTs) over a wide temperature range well above and below the two transitions in pure 8CB. Both the \emph{I-N} and the \emph{N-SmA} transitions evolve in character and have their transition temperatures shift lower as the wt\% of CNTs increases. For intermediate wt\% values, new transitions features are observed, which suggest new phase ordering of the CNTs within the liquid crystal host. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y15.00007: Recovery and stabilization of a reversed phase sequence in a ternary liquid crystal mixture Ronald Pindak, Shun Wang, LiDong Pan, B.K. McCoy, Suntao Wang, H.T. Nguyen, Cheng-Cher Huang The nOHFBBB1M7 (n =10) compound, 10OHF, shows a reversed SmC*$_{FI2}$ - SmC* phase sequence, unique among all known antiferroelectric liquid crystals. This reversed phase sequence is stabilized upon doping with 9OTBBB1M7(C9) or 11OTBBB1M7(C11). In contrast, doping with homologous compounds (n = 9, 11, or 12) eliminates the SmC*$_{FI2}$ phase. One 10OHF/11OHF mixture without the SmC*$_{FI2}$ phase was selected for further studies. Adding C9 into this mixture revives the reversed phase sequence. Unexpectedly, even though 11OHF destabilizes the SmC*$_{FI2}$ phase in binary mixtures with 10OHF, it significantly increases the SmC*$_{FI2}$ temperature range in 10OHF/11OHF/C9 ternary mixtures. The extended temperature range is important for device applications. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y15.00008: Room-Temperature Liquid Crystal Blue Phases Stefanie Taushanoff, Khoa Van Le, Robert Twieg, Antal Jakli The ``blue phases'' of a highly chiral liquid crystal are defect-studded structures of double-twist cylinders that are laced together. The three phases, BPI*, BPII* and BPIII* differ only in the packing of the double-twist cylinders. Until recently, blue phases were of limited practical use because they appeared for only a very narrow temperature range. Mixtures that show BPI* and BPII* phases for wide temperature ranges at or around room temperature are now available [1]. Relatively wide temperature BPIII (the blue fog) phase so far was available only at very high temperatures [2]. Here we present mixtures with room-temperature wide range BPIII* phase and compare the ability of chiral dopants to form the different blue phases in a base nematic mixture. PDLC films cast with blue-phase material are also examined.\\[3pt] [1] H. Coles and M. Pivnenko, Nature 2005 436-18 997-1000 \\[0pt] [2] C. V. Yelamaggad, I. S. Shashikala, G. Liao, D.S. Shankar Rao, S. K. Prasad , Q. Li A. Jakli, Chem. Mater Comm, 2006, 18, 6100-6102 [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y15.00009: Hybrid shells of nematic liquid crystal Alberto Fernandez-Nieves, Teresa Lopez-Leon We investigate the consequences of changing the boundary conditions for the nematic director at the outer surface of a spherical shell from planar to homeotropic. We find there are different routes to the final equilibrium configuration, depending on the initial shell structure. For bipolar shells, which are shells having two pairs of s=+1 boojums on either surface, a disclination ring forms, shrinks and disappears in a process that is highly reminiscent of that seen in bipolar drops. By contrast, shells with four s=+1/2 defects develop open disclination lines in the inner surface; these lines form between the original s=+1/2 defects and force their approach and coalescence. These results highlight the fascinating range of behaviors that are driven by the interplay between topological constraints and the nematic order of liquid crystals. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y15.00010: Optical waveguiding in bent core liquid crystal filaments Antal Jakli, Jake Fontana, Chris Bailey, Wolfgang Weissflog, Istvan Janossy, Peter Palffy-Muhoray We demonstrate optical waveguiding in recently discovered free-standing bent core liquid crystal filaments. The bent core liquid crystal molecules self-assemble into a novel cylindrical geometry that is ``solid-like'' in the radial direction of the filament and liquid in the axial direction of the filament. Waveguiding properties of filaments of millimeter lengths were characterized. The transmitted power density through the filament was independent of temperature from 180oC to near room temperature. Initial defect of newly pulled filaments were found to self anneal, thus leaving perfectly defect free fibers, where light scattering was found to be insignificant. The absorbance was found to be strongly wavelength dependent in the visible regime and very small in the infrared range. A self-assembled optical waveguide with self-annealing fluid properties may have promising applications in optical communications. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y15.00011: Dielectric properties of bent-core nematic materials Peter Salamon, Nandor Eber, Samuel Sprunt, James Gleeson, Antal Jakli We report of dielectric spectroscopy measurements on bent-core nematic liquid crystals. The components of the relative dielectric permittivity and the dielectric loss have been measured as functions of frequency and temperature in the case of various bent-core mesogens in their nematic and isotropic phases. The results show that these liquid crystals have extraordinary dielectric behaviors if we compare them to the traditional calamitic materials, such as -- they show low frequency ( $<$ 10 kHz ) relaxations. Distortion elastic constants measurements reveal that K$_{1}$ / K$_{3} \quad \sim$ 1 in contrast to typical calamitics. The reasons for these anomalous behaviors will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y15.00012: Second Harmonic Generation in a Bent-core Nematic Liquid Crystal. Seung Ho Hong, Antal Jakli, James Gleeson, Samuel Sprunt, Brettt Ellman We studied second harmonic generation (SHG) as a function of optical polarization in a magnetically-aligned bent-core nematic liquid crystal (BCN). At the isotropic to nematic transition we detect the onset of a weak SH signal, which stays approximately constant through the nematic phase. Our results for polarization selectivity and for cells of different thickness indicate that the signal from the BCN cannot be explained by quadrupoles, defects in director orientation, fluctuations or cell-surface polarization. We discuss models for a noncentrosymmetric component of the BCN structure that can explain our data. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y15.00013: Viscoelastic parameters and flexoelectric effect in a bent-core nematic liquid crystal studied by dynamic light scattering Madhabi Majumdar, K. Neupane, James T. Gleeson, Antal Jakli, Samuel Sprunt Recent measurements of the flexure-induced electric polarization in certain bent-core nematics (BCNs) have demonstrated a giant flexoelectric effect [1]. We present a study of nematic elasticities and viscosities in one of these compounds, together with an attempt to characterize the flexoelectricity by its effect on director fluctuations. Our results combined with a reanalysis of earlier data indicate that the flexoelectricity is a phenomenon distinct from the ordinary director modes; additionally we observe unusual, very slow fluctuations in polarized scattering which suggest the BCN has a heterogeneous, ``glassy'' character. Dilution in a miscible calamitic indicates a dramatic development of the slow dynamics between 30 wt {\%} and 60 wt {\%} BCN. We suggest a model to account for both our present results and the giant flexoelectricity discovered in [1]. Reference: [1] J. Harden \textit{et al}., \textit{Phys. Rev. Lett., 97,} 157802 (2006). Acknowledgement: NSF DMR-0606160. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y15.00014: Large Flow-Birefringence of Nematogenic Bent-Core Liquid Crystals Christopher Bailey, Katalin Fodor-Csorba, Rafael Verduzco, James Gleeson, Samuel Sprunt, Antal Jakli We have found that bent-core liquid crystalline materials show exceptionally large flow birefringence in their isotropic liquid phase. The flow birefringence is over two orders of magnitude larger than usual for low molecular weight liquid crystals. Comparing the flow birefringence per unit viscosity, the observed values are an order of magnitude larger than low molecular weight and side-chain polymeric calamitic liquid crystals. This large flow birefringence is attributed to the nanostructure of these materials that contain temporary smectic clusters of a few smectic layers, which exist even in their isotropic phase. These smectic clusters appear to shear align resulting in the observed flow birefringence behavior. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y15.00015: Activated Kinetics of Nematic and Smectic Phase Transitions in an Aligned Matrix of Nano-colloidal Liquid Crystalline Gel Dipti Sharma This study investigates an interesting thermal behavior of an aligned aerosil nano-colloidal system in the aligned matrix of octyl-cyanobiphenyl liquid crystal. This system was prepared by solvent dispersion method (SDM) where different densities of aerosil nanoparticles were added into octyl-cyanobiphenyl liquid crystal. Then samples were cycled into magnetic field during SmA-I transition to get an aligned matrix of nanocolloids. Heating scans were performed at various heating rates from 20 to 1 K min$^{-1}$ using DSC. Aligned samples follow Arrhenius behavior and showed a decrease in transitions temperature for SmA--N and N--I transitions when compared with the unaligned samples. The activation energy of the aligned system increases and its respective enthalpy decreases for the lowest density of aerosils 0.05 g cm$^{-3}$, then for the further increase of aerosil density, the activation energy decreases and its respective enthalpy increases. The second order transition SmA--N shows a higher activated kinetics than the weak first order N--I transition. This can be explained in terms of molecular interaction in between aerosil nanoparticles and aligned liquid crystal molecules, and developed strain in the matrix of the aligned system. [Preview Abstract] |
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