Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L29: Liquid Crystals I: Fields and Interfaces |
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Sponsoring Units: DSOFT DPOLY Chair: Daniel Beller, University of California, Merced Room: 501 |
Wednesday, March 4, 2020 8:00AM - 8:12AM |
L29.00001: Coarse-grained theory for motion of solitons and skyrmions Cheng Long, Jonathan Selinger Recent experiments have found that applied electric fields can induce motion of skyrmions in chiral nematic liquid crystals [1]. To understand the magnitude and direction of the induced motion, we develop a coarse-grained approach to describe dynamics of skyrmions, similar to our group’s previous work on the dynamics of disclinations [2]. In this approach, we represent a localized excitation in terms of a few macroscopic degrees of freedom, including the position of the excitation and the orientation of the background director. We then derive the Rayleigh dissipation function, and hence the equations of motion, in terms of these macroscopic variables. We demonstrate this theoretical approach for 1D motion of a sine-Gordon soliton, and then extend it to 2D motion of a skyrmion. Our results show that skyrmions move in a direction perpendicular to the induced tilt of the background director. When the applied field is removed, skyrmions move in the opposite direction but not with equal magnitude, and hence the overall motion may be rectified. |
Wednesday, March 4, 2020 8:12AM - 8:24AM |
L29.00002: Energy dependence of resonant x-ray scattering for the twist bend nematic phase, a helix of liquid crystals Yu Cao, Jun Feng, Asritha Nallapaneni, Yuki Arakawa, Keqing Zhao, Feng Liu, Chenhui Zhu We examined the manifestation of resonance effects in twist bend nematic (NTB), smectic A (SmA) and crystalline phases via tender resonant X-ray scattering (TReXS) at sulfur K-edge. While SmA layering peak is partially resonant, scattering peak of NTB is purely resonant visible only near S K-edge. The difference is attributed to the periodic molecular orientation variation with weak electron density modulation. We further demonstrate that the energy-dependence of such scattering peak arising from pure orientation variation scales with the sum of f1(E)^2 and f2(E)^2, corresponding to the real and imaginary parts of dispersion correction respectively, while the resonant contribution in sulfur-containing SmA scales with f1(E), and is a first order perturbation to the relatively strong non-resonant peak from electron density modulation. |
Wednesday, March 4, 2020 8:24AM - 8:36AM |
L29.00003: Behavior of Superparamagnetic Microbeads in Smectic Films Emmanuel Bamidele, Xi Chen, Jackson Durm, Cheol Park, Joseph E MacLennan, Noel Anthony Clark We investigate the behaviour and magnetic response of superparamagnetic, silica-coated iron oxide microbeads embedded in freely suspended films of the smectic A liquid crystal 8CB. The microbeads, which are 5 mm in diameter, are mixed into the liquid crystal material at the edge of the film holder before the films of a few smectic layers are spread. Small aggregates and short chains of beads are observed in the film a few minutes after spreading, with longer chains forming along layer steps and at the heavily dislocated meniscus. The microbeads appear to affect the line tension of the layer steps, resulting in an increased tendency for thicker material to sweep in from the meniscus onto the initially thin film and causing islands to shrink much more rapidly than in films of neat 8CB. Moderate applied magnetic fields can be used to translate the beads on the film and to change the rate of island shrinkage. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L29.00004: Dynamics of magnetically induced defect walls in Nematic Lenses Zoltan Karaszi, Haumed Rahmani, Viktor Kenderesi, Peter Salamon, Agnes Buka, Antal Istvan Jakli We studied nematic liquid crystal (NLC) droplets with lens-shaped free surface placed on solid substrates under magnetic field applied parallel to the solid substrate. Both the solid substrate and the air pose homeotropic anchoring of the NLC director. At low magnetic-fields, the director field with a small radial in-plane component becomes distorted toward the field. At moderate fields it transforms into two regions with opposite tilt direction, separated by a wall. Polarizing optical microscopy (POM) with monochromatic light illumination reveals two sources of magnetic field driven fringe generation. At high fields, a Helfrich-type inversion defect wall arises normal to the magnetic field and moves outward. In addition to the experimental POM results, we present a simple theory and computer simulations that explain the magnetic field dependence of the director field and the time dependence of the outward motion of the inversion wall at constant magnetic fields. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L29.00005: Dynamic pattern generation of topological defect arrays in nematic liquid crystals MinSu Kim, Francesca Serra Some photonic devices use the periodic nature of repeating lattices to modulate light.[1] Here we create tunable patterns of topological defect arrays in nematic liquid crystals (LCs) that can be simply controlled by electrical waveforms.[2,3] The refractive index of LCs is spatially modulated owing to the presence of the topological defect arrays. The symmetry and regularity of patterns is controllable thanks to micro-pillars created by photolithography. Adjusting the frequency and amplitude of electrical waveforms changes the defect spacing. Where the defect spacing is reduced, the number density of defects increases, compatibly with the presence of the pillars. The patterns are in two dimensions and this opens up more possibilities for the arrangement of the defects. When a laser beam propagates through the spatially modulated refractive index LC layer, the direction of the wave front changes and the beam can be split and steered. We utilize these complex patterns as a diffraction grating, which can change both diffraction (polar) angle and azimuthal angles. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L29.00006: Elongation of Tactoids in the presence of an electric field Mohammadamin Safdari, Paul Van der Schoot, Roya Zandi Tactoids are spindle-shaped droplets of a nematic phase suspended in the co-existing isotropic phase. They are found in dispersions of a wide variety of elongated colloidal particles, including actin, fd virus, carbon nanotubes, vanadium peroxide, and chitin nanocrystals. Recent experiments on tactoids of chitin nanocrystals dispersed in water show that electrical fields can strongly elongate tactoids, but only if the tactoids have a sufficiently large volume. We explain this by extending the Oseen-Frank elastic model of Kaznacheev et al. [1] and Prinsen et al. [2] to partially bipolar tactoids and evaluate the level of elongation of the tactoids as a function of the surface tension, elastic constants, volume, and electric field strength. By invoking a free-energy-based relaxational dynamics, we also describe the time evolution after switching on the electric field, confirming that, counterintuitively, large tactoids take more time to elongate to their equilibrium value in an electric field than small ones do. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L29.00007: Observation of Ferroelectricity in a Thermotropic Nematic Liquid Crystal Using its Spectacular Electro-Optics Xi Chen, Eva Korblova, Renfan Shao, Leo Radzihovsky, Matthew Glaser, Joseph E MacLennan, David M. Walba, Noel Anthony Clark We have synthesized and studied the literature compound RM734 [1], a polar rod shaped molecule exhibiting a typical nematic phase and a second nematic phase assigned to be a "splay nematic" state [2]. We find that the second phase is rather a thermotropic ferroelectric nematic, an identification based on the first observation in a nematic of the defining characteristics of ferroelectricity: (1) the formation, in absence of applied electric field, of spontaneously polar domains of opposite sign of polarization separated by distinct domain boundaries; and (2) field-induced polarization reversal mediated by movement of these domain boundaries. A truly remarkable feature of this phase is the magnitude of its spontaneous polarization, saturating at 6 μC/cm2, the largest ever reported for an organic material or for any fluid, and larger than that of all but a few crystalline ferroelectrics. |
Wednesday, March 4, 2020 9:24AM - 9:36AM |
L29.00008: Ferromagnetic Fréedericksz transition in ferromagnetic nematic filaments Min Shuai, Xi Chen, Nathan Cobasko, Joseph E MacLennan, Matthew A. Glaser, Noel Anthony Clark Barium hexaferrite nanoplates at sufficiently high concentrations in 1-butanol spontaneously form a ferromagnetic nematic phase due to the interplay between Onsager excluded volume effects and magnetic dipole-dipole interactions [Nat Comm, 7: 10394, 2016]. We have discovered a rich variety of ferromagnetic structures, such as droplets, toroids, and filaments, which form in the isotropic background when the concentration of the nanoplates is within the isotropic-nematic phase coexistence region. Here, we demonstrate a Fréedericksz transition in such ferromagnetic filaments. In the absence of applied magnetic field, the local magnetization direction of the filaments is parallel to their long axes. When a magnetic field is applied parallel to the magnetization direction, the existing alignment of the nanoplates is reinforced and the shape of the filaments remain unchanged. However, when a sufficiently strong magnetic field is applied antiparallel to the magnetization direction, an undulation instability sets in and the filaments form wavy structures. A simple model was developed to describe the defomation of the filaments close to the transition points. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L29.00009: Electrostatically controlled surface boundary conditions in hybrid molecular-colloidal liquid crystals Haridas Mundoor, Bohdan Senyuk, Mahmoud Almansouri, Sungoh Park, Blaise Fleury, Henricus Wensink, Ivan I Smalyukh In fundamental studies of liquid crystal (LC) colloids, surface interactions define the boundary conditions for molecules on particle surfaces and ultimately determine the induced defects and interactions. We study the influence of surface charge and ions on surface anchoring properties, with a focus on defining the behavior of anisotropic colloids dispersed in a nematic LC. We demonstrate a systematic variation of boundary conditions through controlling the competing aligning effects of surface functionalization and electric field arising from surface charging and bulk counterions. The control of ionic content in the bulk and at surfaces allows for tuning the equilibrium orientations of charged colloidal particles with respect to the far-field director n. When dispersed in high concentration, the charged colloids form biaxial nematic, columnar, and crystalline assemblies with tunable symmetries and ordering enabled by the anisotropic elastic and electrostatic interactions. |
Wednesday, March 4, 2020 9:48AM - 10:00AM |
L29.00010: Cellulose Nanocrystal well alignment with addition magnetic nanoplates in weak gradient magnetic field mingfeng chen, Zhengdong Cheng, Dali Huang Rod like Cellulose nanocrystal (CNCs) are emerging nanomaterials that form chiral nematic liquid crystals and the controlling of orientation of formed liquid crystal phased has attracted broad attention. To enhance the response of CNC rods in the magnetic field, barium hexaferrite (BF) nanoplates, with positive magnetic susceptibility and mean nanoplatelet magnetization is mo= 2*10-18 A m2, were introduced. Here, we investigated the effect of addition BF nanoplatelets on the phase transition and orientation of CNCs suspension in a weak external gradient magnetic field (Several hundred Gauss) . We observed the nematic phase prefer to move to the lower magnetic field region and a floating nematic phase is formed when the direction of magnetic field is contrast with the gravity field. We also demonstrate the gradient magnetic field can accelerate the phase separation. Based on these, we establish a method to control the phase separation speed and the position of nematic phase, since the movement of nematic phase depends on the competition of magnetic and gravity field. Moreover, the nematic phase were nearly perfect alignment along the magnetic field, which indicate with the BF magnetic nanoplates, the CNCs can be aligned by very weak magnetic field. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L29.00011: Application of monolayer WSe2 as the planar-alignment agent in an electro-optic liquid crystal device Rajratan Basu, Lukas J Atwood Two-dimensional (2D) tungsten diselenide (WSe2) nanosheets were transferred onto indium tin oxide (ITO) coated glass slides. Two such 2D WSe2-covered ITO glass slides were placed together to fabricate a liquid crystal (LC) cell. A nematic LC inside this WSe2-based cell exhibits uniform planar-alignment. This planar-alignment at the molecular scale is achieved due to the coherent overlay of the LC molecules along the selenium (Se) array on the WSe2 lattice. This WSe2-based LC device shows the typical electro-optical effect when an electric field is applied via ITO electrodes. This electro-optical effect also reveals a standard Fréedericksz transition of the LC, confirming that the 2D WSe2, as the planar-alignment agent, supplies adequate planar anchoring energy—which can be overcome by the Fréedericksz threshold voltage. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L29.00012: Topographical variations in thin liquid crystal films Tim Atherton, Andrew Ferris, Charles Rosenblatt A thin film of isotropic fluid with a free interface is always perfectly flat far away from boundaries and inclusions. In contrast, liquid crystals can, in principle, sustain a surface topography because elastic distortions in the bulk can be relaxed by deformation(s) of the surface. In practice, surface tension tends to dominate over elastic forces and so boundary deformations are usually neglected. (However, these have been observed in smectics and in nematics, as De Gennes famously predicted, where defects are confined to the boundary.) Here we present a different scenario: Topography at the free interface is induced by surface patterning of the substrate in a thin, hybrid-aligned nematic liquid crystal. The proposed mechanism requires finite anchoring at the free interface leading to modulation of the nematic orientation; deformation of the surface then allows the nematic to better satisfy the anchoring condition. We present analytical and numerical theory for several surface patterns, which are compared with our experimental actualizations. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L29.00013: Microscopic look at mesogen interactions with two-dimensional nanosheets Paul Brown, Sean A Fischer, Jakub Kolacz, Christopher Spillmann, Daniel Gunlycke Recent interest surrounding the combination of liquid crystals (LC) and two-dimensional (2D) nanosheets has shown when 2D nanosheets are utilized liquid crystal-optical refractors obtain faster response times, smaller device dimensions, and the replacement of mechanical components that suffer from deterioration. Additionally, 2D materials can operate as an electrode and alignment agent simultaneously. In this talk, we use density functional theory to focus on the microscopic origins of recent observations where liquid crystals are combined with two-dimensional nanosheets. We highlight the origins of critical interactions between 5CB and graphene, boron nitride, and phosphorene. We demonstrate qualitatively different band structures among the substrates, and, in particular, boron nitride shows an alignment dependence for 5CB. The effect associated with the 5CB orientations on boron nitride will be discussed in more detail, where the possibility of edge modes form. Finally, we discuss the influence of point defects in the crystal lattice of each substrate. |
Wednesday, March 4, 2020 10:36AM - 10:48AM |
L29.00014: Nucleation and Growth of BPII Liquid Crystals on Chemically Patterned Surfaces: A Surface Anchoring Assisted BPII-Coherence Length Xiao Li, Kangho Park, jose Martínez-González, Orlando Guzman, Juan De Pablo, Paul F Nealey In condensed matter, the correlation length is an essential parameter for describing the distance over which a material maintains its structural properties. In liquid crystals, such a characteristic length is of the order of 10 um while in solid crystals it can be extent up to macroscopic dimensions. Here, we report how to measure the correlation length of blue-phases (BPs), which are chiral liquid crystals with long-range 3D-crystalline structures and submicron-sized lattice-parameters. We design patterned substrates made of a binary array of regions with different anchoring, which facilitate a uniform nucleation and growth of BP-crystals with (100)-lattice orientation and a simple cubic symmetry. Our results indicate that this simple cubic blue-phase (BPII), forms first on the patterned surface, thereby setting the growth of domains in directions that can be parallel or perpendicular to the patterned regions. These results are used to understand the emergence of a surface anchoring assisted BPII-coherence length in terms of time and pattern characteristics. We found that BPII single crystals can be achieved on patterned regions whose lateral dimensions are equal or larger than 10 µm, which is consistent with our measurements of the BPII-coherence length. |
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