Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W50: Liquid Crystals III: Nematic and Twist Bend Phases |
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Sponsoring Units: GSOFT Chair: Luz Miranda-Martinez, University of Maryland-College Park Room: 218 |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W50.00001: Direct mapping of local director field of nematic liquid crystals at the nano-scale Yu Xia, Francesca Serra, Shu Yang, Randall Kamien The director field in liquid crystals (LCs) has been characterized mainly via polarized optical microscopy, fluorescence confocal microscopy, and Raman spectroscopy, all of which are limited by optical wavelengths -- from hundreds of nanometers to several micrometers. Since LC orientation cannot be resolved directly by these methods, theory is needed to interpret the local director field of LC alignment. In this work, we introduce a new approach to directly visualize the local director field of a nematic LC (NLC) at the nano-scale using scanning electron microscopy (SEM). A new type of NLC monomer bearing crosslinkable groups was designed and synthesized. It can be well-oriented at particle surfaces and patterned polymer substrates, including micron-sized silica colloids, porous membranes, micropillar arrays, and 1D channels. After carefully crosslinking, the molecular orientation of NLCs around the particles or within the patterns could be directly visualized by SEM, showing oriented nanofibers representing LC director from the fractured samples. Here, we could precisely resolve not only the local director field by this approach, but the defect structures of NLCs, including hedgehogs and line defects. The direct mapping of LC directors at the nanoscale using this method will improve our understanding of NLC local director field, and thus their manipulation and applications. More importantly, a theoretical interpretation will no longer be a necessity to resolve a new material system in this field. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W50.00002: Topological Defects in Liquid Crystals: Studying the Correlation between Defects and Curvature Charles Melton Topological defects have recently been the subject of many fascinating studies in soft condensed matter physics. In particular, linking the evolution of topological defects to curvature changes has been a focus, leading possible applications in the areas such as cosmetics, pharmaceuticals, and electronics. In this study, defects in nematic liquid crystal droplets are investigated via laboratory and theoretical techniques. Nematic liquid crystal defects are reproduced via Monte Carlo simulations using a modified 2D XY-Model Hamiltonian. The simulation is performed on a curved surface to replicate a nematic droplet and examine possible defect configurations. To complement this theoretical work, we have trapped nematic droplets inside a dual-beam optical trap. This system allows controllable non-contact droplet deformation on a microscope based platform. Future work will focus on using the trap to stretch nematic droplets, correlating the changing topological defects with theoretical predictions. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W50.00003: Ground States of Nematic Tori with radial boundary conditions Karthik Nayani, Perry Ellis, Jung Ok Park, Mohan Srinivasarao, Alberto Fernandez-Nieves We report ground states of radially anchored nematic tori. For strong anchoring conditions we observe the often reported escape radial configuration. However, as the aspect ratio of the torus is lowered the escape configuration evolves to a new structure where the additional splay on the liquid crystal imposed by the geometry of the torus is released as bend. At lower anchoring condition the ground state corresponds to a two $+$1/2 line defect wrapping the torus. However, as the aspect ratio is lowered we again observed a new ground state where a single line wraps the torus twice. We perform Jones calculus simulations to confirm that our director ansatz reflects the experimental findings. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W50.00004: Using chemically patterns with different anchoring behavior to control the orientation of nematic liquid crystal Xiao Li, Julio Armas Perez, Jose Adrian Martinez-Gonzalez, Helou Xie, Juan de Pablo, Paul Nealey We present experimental and theoretical study of nematic liquid crystal (5CB) confined to a thin cell between homeotropic anchoring top surface and chemically patterned planar/homeotropic anchoring bottom substrates. The chemically patterned substrate with different dimensions and $\sim$ 4 nm depth topography induce the 5CB to align as the pattern direction as non-degenerate behavior, until the width of the straight line pattern is too wide to confine the 5CB to one direction and back to degenerate behavior. By changing the width of the straight line pattern, a brightness change of the intensity is shown by their corresponding crossed polarizer images. This change is mainly due to a discontinuity of the average angle between the molecules and the surface in function of line width, which is in excellent agreement with the Landan-de Gennes theory when the balance between the elastic deformation in the bulk and orientation of molecules close to the surface is simulated for different pattern dimensions. An elastic free energy transition is also observed from the numerical analysis when the strong planar anchoring for presented experiments is changed to weak. This 3D confinement by chemically patterns and small depth topography offers a new way to generate any geometry pattern controllable non-degenerate orientation, achieving switchable optical properties. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W50.00005: Planar Anchoring of Achiral Nematic Liquid Crystals in Capillaries --- with a Twist Zoey S. Davidson, Joonwoo Jeong, Louis Kang, Peter J. Collings, Tom C. Lubensky, A. G. Yodh In the common three-term Frank free energy of a nematic liquid crystal, the ground state configuration will have no deformations and all nematic directors will be parallel. However, certain confining geometries can impose significant deformations on the ground state, even if a zero-deformation configuration can be drawn that satisfies all boundary conditions. By solving the Euler-Lagrange problem of the Frank free energy equation, including the saddle-splay term, with cylindrical confinement and degenerate planar anchoring, we find conditions for a highly deformed ground state configuration that has a double twist like structure. We explore these effects experimentally with both thermotropic and lyotropic liquid crystal materials, finding good agreement with the theoretically predicted configuration. We also observe a rich phenomenology of defect structures in the liquid crystal samples. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W50.00006: Dynamics of isothermal phase transition of liquid crystal with zero anchoring Jinxin Fu, Karthik Nayani, Jung Ok Park, Mohan Srinivasarao Liquid crystal (LC) is an ideal system to mimic the cosmological symmetry breaking in the laboratory. The formation of LC string defects in film and bubble has been shown to be analogous to the formation of cosmic strings previously. Here we study the dynamics of LC isothermal transition from isotropic to nematic phase in a three-dimensionally isotropic environment, which enables us to observe the simultaneous symmetry breaking of matter without any external heat transfer or anchoring boundary condition. The isothermal phase transition is realized by the photochemical conversion of the trans-form to the cis-form of an Azobenzene compound that is added into liquid crystal E7. And a medium composed of carbopol and SDS surfactant provides the zero anchoring. The dynamics of the nucleation of LC and defects are studied under microscope with high-speed camera. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W50.00007: Optical Switching of Nematic Liquid Crystal Film based on Localized Surface Plasmon Resonance Makiko Quint, Silverio Delgado, Zachary Nuno, Linda Hirst, Sayantani Ghosh We have demonstrated an all-optical technique to reversibly switch the spatial orientation of nematic liquid crystal molecules from homeotropic to planar in a few micron thick films. Our method leverages the highly localized electric fields that are generated in the near-field of a densely packed gold nanoparticle layer when the samples are excited by light resonant with the localized surface plasmon absorption. We present simulations and control measurements for off-resonance excitation, where the switching behavior is not observed. Using polarized microscopy and transmission measurements, we observe this switching over a temperature range starting several degrees below and up to the isotropic transition, and at on-resonance excitation power less than 10 $\mu$W. In addition, we controllably vary the in-plane directionality of the liquid crystal molecules in the planar state by altering the linear polarization of the incident excitation. [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W50.00008: Substrate induced gliding for a nematic liquid crystal layer Ensela Mema, Linda Cummings, Lou Kondic The interaction between nematic liquid crystals (NLC) and polymer substrates is of current industrial interest, due to a desire to manufacture a new generation of flexible Liquid Crystal Displays (LCDs) for use in portable electronic devices. Polymer substrates present challenges because they can interact with the NLC, exhibiting a phenomenon known as gliding: the preferred orientation of the NLC molecules at the interface changes over timescales of minutes to hours. We present two models for gliding, inspired by the physics and chemistry of the interaction between the NLC and polymer substrate. These models, though simple, lead to non-trivial results, including loss of bistability, a finding that may have implications for display devices. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W50.00009: A novel twisted nematic alignment and its effects on the electro-optical dynamics of nanoscale liquid crystalline films Brittany Rauzan, Lay Min Lee, Ralph Nuzzo Vibrational spectroscopic studies of a surface induced, twisted alignment of the nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB) and its temperature-dependent electro-optical (EO) dynamics were studied near the crystalline-nematic and nematic-isotropic transition temperatures, and at a median temperature in the nematic phase. A 50 nm thick film of 5CB was confined in nanocavities defined by the dimensions of a gold interdigitated electrode array patterned on a unidirectionally polished ZnSe substrate. The film was assembled between two polished substrates bearing extended nanometer-scaled grooves that are oriented orthogonally to one another. The results show that with this anchoring scheme, the molecular director of the LC film undergoes a ninety-degree twist. Step-scan time resolved spectroscopy (TRS) measurements were made to determine the rate constants for the temperature-dependent EO dynamics of both the electric field-induced orientation and thermal relaxation processes of the LC film. The work rationalizes the impacts of organizational anisotropy and illustrates how it can be exploited as a design principle to effectively influence the electric field-induced dynamics of LC systems. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W50.00010: Rheology of a Twist-bend Nematic Liquid Crystal Seyyed Muhammad Salili, Chanjoong Kim, Samuel Sprunt, James Gleeson, Owain Parri, Antal Jakli First detailed flow shear alignment studies and rheological measurements in the twist-bend nematic ($N_{tb} )$ liquid crystalline phase of odd numbered flexible dimer molecules is presented. It is found that the $N_{tb} $ phase is strongly shear-thinning. At shear stresses below $1Pa$ the apparent viscosity of the $N_{tb} $ phase is $1000$ times larger than in the nematic phase. At stresses above $10Pa$ the $N_{tb} $ viscosity drops by two orders of magnitude and the material exhibits Newtonian fluid behavior. The results are consistent with the behavior of a system with pseudo-layer structure with layer spacing determined by the heliconical pitch. From the measurements of dynamic modulus we estimate the compression modulus of the pseudo-layers to be $B\sim 2kPa$; this value is discussed within the context of a simple theoretical model based upon a coarse-grained elastic free energy. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W50.00011: Second Harmonic Light Scattering Study of a Twist-Bend Nematic Liquid Crystal Shokir Pardaev, James Gleeson, Antal Jakli, Samuel Sprunt The twist-bend nematic phase exhibited by certain liquid crystalline dimers has been the subject of intensive recent investigation. In this report we present the results of angle-resolved second harmonic (SH) light scattering measurements from a twist-bend (TB) nematic liquid crystal for various combinations of the fundamental and second harmonic polarizations. These measurements reveal a polarization-dependent pretransitional temperature dependence of the SH signal, as well as an evolution of the SH scattering pattern below the transition (in the TB phase). We will discuss our results in terms of other recent experiments, as well as the current theoretical understanding of the nematic to TB transition and the nature of the TB phase. We thank O. Parri at Merck Chemicals Ltd., Southampton, UK for providing the studied material for us. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W50.00012: Coarse-grained model and light scattering of the twist-bend nematic phase Shaikh Shamid, David Allender, Jonathan Selinger We develop a coarse-grained version of the continuum theory for the twist-bend (TB) nematic phase of liquid crystals. In this theoretical approach, we begin with an ideal, undistorted TB phase, which has a heliconical modulation of the director field. We then calculate the elastic free energy cost of a position-dependent local rotation of the director away from the ideal state. We diagonalize this free energy density to find the eigenmodes of the system. Of these eigenmodes, the soft mode can be regarded as a smectic-like distortion of periodic planes in the TB phase; this mode has effective elastic constants for layer compression and curvature. By comparison, the hard mode involves director variations away from the optimum cone angle. This calculation leads to a prediction for light scattering from the TB nematic phase. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W50.00013: Fullerene (C$_{60}$) nano-colloids in nematic liquid crystal Angelo Visco, Kevin Sobczak, Rizwan Mahmood We report high resolution homodyne light scattering studies to probe director fluctuations in bend/splay mode in bulk nematic liquid crystal and as a function of fullerene (C$_{60}$) nanoparticles concentration. The preliminary analysis shows that the relaxation time of these fluctuations is fairly constant with in the experimental uncertainty despite the constraints imposed on the director fluctuations due to the insertion of nano colloids. The relaxation time extracted from the data found to be in nano seconds range and the diffusion constant (D) found to be, D $=$ 4.29 x 10$^{6}$ cm/sec. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W50.00014: Computational and theoretical analysis of chiral rafts in colloidal membranes Raunak Sakhardande, Michael Hagan, Aparna Baskaran, Bulbul Chakraborty In contrast to bulk liquids or crystals clusters of finite size are rare and their assembly usually requires sophisticated engineering. Recent experiments conducted on monolayer membranes composed of two species of chiral rodlike molecules leads to the spontaneous formation of thermodynamically stable, rafts with a well-defined finite size. To understand the fundamental forces driving this self-limited assembly, we combine Monte Carlo simulations and a mean field theory to explore the phase diagram of a monolayer of bidisperse rodlike molecules as a function of interparticle interactions and chirality.~ The simulations demonstrate that differences in chirality between the two rod species can stabilize finite-sized rafts.~ We present a phase diagram which predicts parameter ranges over which finite-sized rafts are stable. [Preview Abstract] |
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