Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M49: Liquid Crystals I: Chromonics, Discotics, Polymers and Functional Uses |
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Sponsoring Units: GSOFT Chair: Robin Selinger, Kent State University Room: 217D |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M49.00001: Double helix configuration of lyotropic chromonic liquid crystals in cylindrical capillaries with homeotropic anchoring Rui Chang, Karthik Nayani, Jinxin Fu, Elsa Reichmanis, Jung Ok Park, Mohan Srinivasarao We investigate the formation of double helix configuration of Sunset Yellow FCF, a nematic lyotropic chromonic liquid crystal (LCLC), confined in cylindrical capillaries with homeotropic boundary conditions. Like many other nematic liquid crystals, escape radial configuration is observed right after LCLC fills the capillary. Few minutes after filling the capillary, the escape radial configuration evolves to a new form with an axial twist mediated with domain walls separating opposite handedness. We attribute this behavior to the low twist elastic constant of chromonics. After 12 hours aging, a new ground state of LCLC, two line defects with a double helical configuration is observed. This ground state is the chosen configuration until evaporation of water forces a phase transition to the columnar phase. Additionally, the nematic-isotropic phase transition of LCLC in cylindrical capillaries has some striking and completely unique aspects. Unlike the typical tactoids with nematic and isotropic phases, we find a coexistence of double helix configuration and escape radial configuration mediated with point defects in biphasic temperature range. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M49.00002: Director distortion reduces molecular order of lyotropic chromonic liquid crystal on large scales Shuang Zhou, Sergij Shiyanovskii, Oleg Lavrentovich In thermotropic liquid crystals, energy required to change molecular order is usually orders of magnitude higher than the energy associated with elastic distortions. The scalar order parameter (S) is a constant beyond nanometers range from the defect core. However, in lyotropic chromonic liquid crystals (LCLCs), the elastic distortion reduces S on a scale up to tens of micrometers. Using PolScope, we study the optical retardance of areas tens of micrometers near half integer disclinations and find that the birefringence, a quantity proportional to S, is lower than that of uniform samples. Combining PolScope and TEM techniques, we also demonstrate that the ratio between splay and bend elastic constants $K_1/K_3$ increases to 1 as the distance to the defect core reduces to tens of nanometers. We explain our findings based on the fact that LCLCs are formed by reversible rod-shape aggregates of disc-shape molecules, unlike their thermotropic counterparts, where the basic building units are rigid molecules made of covalent bounds. In LCLCs the combining force to form the molecular stacks is only weak van der Waals force on the order of $k_B T$, thus the aggregation can be influenced by elastic distortion. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M49.00003: Observation of large nematic domains of discotic liquid crystals Abhijeet Shinde, Xuezhen Wang, Zhengdong Cheng Discotic liquid crystals are commonly found in nature in the form of clay, nacre. They are technologically important in applications such as conductive polymers, semiconductors and photovoltaics. Size and its distribution play an important role in their self-assemblies. Here we observed large nematic domains of discotic liquid crystals grown on a time scale of months. The development of such domains is observed to be faster for nanodisks that relatively smaller in size. The orientation of nanodisks is affected by gravity and inter-particle interactions which are yet to be fully understood. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M49.00004: Nucleation and Growth of Discotic Liquid Crystals Zhengdong Cheng, Xuezhen Wang, Lecheng Zhang, Abhijeet Shinde We investigate the nucleation and growth of liquid crystals of plate-shaped charged zirconium phosphate (ZrP) monolayers with various sizes, temperature and salt concentrations. The smaller the platelets size, or the higher the temperature, or the higher the salt concentration (from 0 to 0.6M), the faster the Isotropic-Nematic (I-N) separation took place. We established the I-N transition phase diagram of charged platelets in the temperature verse volume fraction plane, and discovered that N phase can be melted by increasing temperature, and coexistent samples are more sensitive to polydispersity at higher temperature and higher concentrations. We also found that salt concentration in the ZrP suspensions contributed to the formation of an apparently twisted phase. This work is supported by NSF (DMR-1006870) and NASA (NASA-NNX13AQ60G). X.Z. Wang acknowledges support from the Mary Kay O'Connor Process Safety Center (MKOPSC) at Texas A{\&}M University. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M49.00005: Reconstruction of the surface of freely suspended films of heptyloxybenzylidene heptylaniline Daniel Martinez Zambrano, Jeffrey Collett Surfaces of freely suspended thick films of 4-n-heptyloxybenzylidene-4-n-heptylaniline (7O.7) in the crystalline-B phase have been imaged using non-contact mode atomic force microscopy. Steps are observed on the surface of the film with a height of 3.0 $\pm$ 0.1 nm corresponding to the upright molecular length of 7O.7. In addition, we find that the step width varies with temperature between 56$^{\circ}$C and 59$^{\circ}$C. The steps are many times wider than the molecular length suggesting that the steps are not on the surface, but instead originate from edge dislocations in the interior. Using a strain model for liquid crystalline layers above an edge dislocation to estimate the depth of the dislocation, we estimate that the number of reconstructed surface layers decreases from 50 to 4 layers as the temperature increases from 56$^{\circ}$C to 59$^{\circ}$C. This trend tracks the behavior of the phase boundary in the thickness dependent phase diagram of freely suspended films of 7O.7, suggesting that the surface may be reconstructed into a Smectic-F phase. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M49.00006: Tension and Rupture Dynamics of Freely-Suspended Bent-Core Liquid Crystalline Fibers Oliver Kress, Seyyed Muhammad Salili, Tanya Ostapenko, Christopher Bailey, Alexey Eremin, Ralf Stannarius, Antal J\'akli Euler buckling, a physical mechanism which classically describes deformations in an elastic beam, has been expanded to describe the recoil of viscoelastic liquid crystalline filaments. Rupture of the freely suspended filaments resulted in a buckling instability that propagated through the filament. A characteristic wavelength and a time constant emerge as the filament recoils. Tensions of the suspended filaments were measured by induced mechanical deflection. The analysis of the results reveals a temperature dependent competition between surface and bulk effects that distinguishes these viscoelastic filaments from classical elastic beams. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M49.00007: Dynamics of Stable and Metastable Structures of Liquid Crystal and Lipid Systems at Interfaces Lawrence Honaker, Piotr Popov, Elizabeth Mann, Edgar Kooijman, Antal Jakli Due to the amphiphilic structure and character of liquid crystal molecules, they tend to align in a planar fashion at a boundary with water and homeotropically at a boundary with air. However, the introduction of heteromolecules with long aliphatic tails, such as phospholipids, into the system promotes homeotropic alignment, a conformational change which is easily visually observable. It can be expected that the presence of these lipids induces a uniformly homeotropic texture in the liquid crystalline system, but experimental observations show otherwise. Studies of the textures and features that arise in such systems are presented here with an emphasis on the study of the metastable hybrid textures that develop, their stability, the characteristics of their alignment, and factors that influence their presence. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M49.00008: Self-assembly of DNA origami particles in suspension of non-absorbing depleting polymers Mahsa Siavashpouri, Mark Zakhary, Christian Wachauf, Hendrik Dietz, Zvonimir Dogic The connection between the macroscopic properties of a liquid crystalline material and the microscopic features of the constituent molecules is the essential theme that permeates the field of liquid crystals. Previous studies have shown that monodisperse rod-like colloids such as filamentous bacteriophage self-assemble into 1D twisted ribbons in presence of attractive interactions mediated by non-absorbing polymers. The microscopic properties of the colloidal particles play an important role in determining the physical properties of these mesoscopic assemblages. Using structural DNA nanotechnology, we present the design and structure of DNA origami six-helix bundles with tunable microscopic properties, which can be used as a new building block for the self-assembly of rod-like colloidal particles. We demonstrate that formation of higher order structures from the assembly of colloidal rods is universal. By tuning the chirality, aspect ratio and flexibility of the DNA origami particles we can control the physical properties of the entire self-assembled structures. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M49.00009: Dynamic Theory for Polydomain Structures in Liquid-Crystal Elastomers Ayhan Duzgun, Jonathan Selinger Liquid-crystal elastomers are remarkable materials that combine the elastic properties of cross-linked polymer networks with the anisotropy of liquid crystals. Any distortion of the polymer network affects the nematic order of the liquid crystal, and, likewise, any change in the magnitude or direction of the nematic order influences the shape of the elastomer. When elastomers are prepared without any alignment, they develop disordered polydomain structures as they are cooled into the nematic phase. To model these polydomain structures, we develop a dynamic theory for the isotropic-nematic transition in elastomers. In this theory, the local nematic order is coupled to the strain tensor, which satisfies the constraint of elastic compatibility. If the system is rapidly cooled into the nematic phase, a polydomain state with a characteristic length scale can emerge. This polydomain state may eventually become uniform, or it may be locked in by quenched impurities. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M49.00010: Novel confinement of liquid crystals in Janus droplets Wei-Shao Wei, Joonwoo Jeong, Peter J. Collings, Tom C. Lubensky, A. G. Yodh In this work we create and investigate Janus droplets composed of liquid crystal (LC) and polymer. The Janus droplets are formed when homogeneous droplets of LC-polymer-solvent phase separate into LC and polymer regions during solvent evaporation through aqueous continuous phase. This scheme enables us to realize unique confinement geometries for LCs such as spherical caps and bowls, which are difficult to be achieved via other systems. The morphologies and surface anchoring conditions can be controlled by changing the size of droplets, the volume ratio between LC and polymer, and the type/concentration of surfactants in aqueous background phase. We explore a variety of defects in these novel confined geometries including dislocations and focal conic defects of smectic LCs. Nematic and cholesteric LCs are also explored. Models that balance the energetics of bulk elasticity and surface anchoring determine the director configurations of confined liquid crystals (LCs). [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M49.00011: Microlenses of smectic flowers Francesca Serra, Mohamed-Amine Gharbi, Iris B. Liu, Yimin Luo, Nathan D. Bade, Randall D. Kamien, Shu Yang, Kathleen J. Stebe The search for new and tunable optical components finds suitable candidates in liquid crystals, which have both reconfigurability and unique optical properties. Here we discuss smectic liquid crystals arranged in focal conic domains (FCDs), which can work as gradient-refractive index microlenses. We exploit this property to create an assembly of microlenses that resembles an insect compound eye. The system consists of a thin layer of smectics on a substrate patterned with microposts. The smectic film is pinned at the microposts, creating a curved interface that induces a hierarchical assembly of FCDs called the ''flower pattern'': each FCD resembles the petal of a flower around the micropost. The arrangement of FCDs, with the largest FCDs pinned at the top of the microposts and the smallest FCDs in the low-curvature regions far from the post, is mirrored into a hierarchy of focal lengths of the microlenses. This structure is reconfigurable by melting and cooling and it allows visualizing objects placed at different distances, hence it can be exploited for 3D image reconstruction. Similarly to the insect eyes, the flower pattern is sensitive to light polarization: the large FCDs, with the largest eccentricity, only work as microlenses for one direction of light polarization. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M49.00012: Phototropic liquid crystal materials containing naphthopyran dopants Mariacristina Rumi, Seth Cazzell, Tamas Kosa, Ludmila Sukhomlinova, Bahman Taheri, Timothy Bunning, Timothy White Dopant molecules dispersed in a liquid crystalline material usually affects the order of the system and the transition temperature between various phases. If the dopants undergo photoisomerization between conformers with different shapes, the interactions with the liquid crystal molecules can be different for the material in the dark and during exposure to light of appropriate wavelength. This can be used to achieve isothermal photoinduced phase transitions (phototropism). With proper selection of materials components, both order-to-disorder and disorder-to-order photoinduced transition have been demonstrated. Isothermal order-increasing transitions have been observed recently using naphthopyran derivatives as dopants. We are investigating the changes in order parameter and transition temperature of liquid crystal mixtures containing naphthopyrans and how they are related to exposure conditions and to the concentration and molecular structure of the dopants. We are also studying the nature of the photoinduced phase transitions, and comparing the behavior with that of azobenzene-doped mixtures, in which exposure to light leads to a decrease, instead of an increase, in the order of the system. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M49.00013: A Simple Method For The Determination of High-Accuracy Refractive Indices of Liquid Crystals via High-Resolution Birefringence Data Senay Ustunel, Erlin Kutlu, Mehmet Can Cetinkaya, Haluk Ozbek We proposed a simple procedure to determine extraordinary (n$_{\mathrm{e}})$ and ordinary refractive indices (n$_{\mathrm{o}})$ of LCs both in the N and Sm A phases based on the high-accuracy birefringence measurements. We show that, apart from the birefringence data, the procedure needs only a single value for the refractive index which is the value of that in the isotropic (I) phase just above the N-I transition temperature. By checking the consistency of our approximation model using the criteria found in the literature, we then conclude that our proposal is self-consistent. Additionally, we show that the temperature variation of refractive indices is well portrayed by the fit expression presented here for the first time contrary to the Haller extrapolation method. Furthermore, we then show that, without addressing density measurements, the proposed method allows one to obtain the temperature dependence of normalized molecular polarizabilities for extraordinary and ordinary rays, and the effective geometry parameter. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M49.00014: Liquid Crystal Based Sensor to Detect Beta-Sheet Formation of Peptides Monirosadat Sadati, Aslin Izmitli Apik, Nicholas L. Abbott, Juan J. de Pablo Protein aggregation into amyloid fibrils is involved in the progression of Alzheimer's, typeII diabetes and Huntington's diseases. Although larger aggregates remain important for clinical determination, small oligomers are of great interest due to their potentially toxic nature. It is therefore crucial to develop methods that probe the aggregation process at early stages and in the vicinity of biological membranes. Here, we present a simple method that relies on liquid crystalline materials and a Langmuir monolayer at the aqueous-liquid crystal (LC) interface. The approach is based on the LC's specific response to $\beta $-sheet structures, which abound in amyloid fibrils. When the system is observed under polarized light, the fibrils formed by amyloidogenic peptides give rise to the formation of elongated and branched structures in the LCs. Moreover, the PolScope measurements prove that the LCs are predominantly aligned along the fibrils when exposed to a $\beta $-sheet forming peptide. In contrast, non-amyloidogenic peptides form ellipsoidal domains of irregularly tilted LCs. This method is capable of reporting aggregation at lipid-aqueous interfaces at nanomolar concentrations of the peptide, and much earlier than commonly used fluorescence-based techniques. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M49.00015: Liquid Crystalline Compositions as Gas Sensors Petr Shibaev, John Murray, Anthony Tantillo, Madison Wenzlick, Jordan Howard-Jennings Droplets and films of nematic and cholesteric liquid crystalline mixtures were studied as promising detectors of volatile organic compounds (VOCs) in the air. Under increasing concentration of VOC in the air the detection may rely on each of the following effects sequentially observed one after the other due to the diffusion of VOC inside liquid crystalline matrix: i. slight changes in orientation and order parameter of liquid crystal, ii. formation of bubbles on the top of the liquid crystalline droplet due to the mass transfer between the areas with different order parameter, iii. complete isotropisation of the liquid crystal. All three stages can be easily monitored by optical microscopy and photo camera. Detection limits corresponding to the first stage are typically lower by a factor of 3-6 than detection limits corresponding to the beginning of mass transfer and isotropisation. The prototype of a compact sensor sensitive to the presence of organic solvents in the air is described in detail. The detection limits of the sensor is significantly lower than VOC exposure standards. The qualitative model is presented to account for the observed changes related to the diffusion, changes of order parameter and isotropisation. [Preview Abstract] |
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