Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C30: Liquid Crystals IIFocus
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Sponsoring Units: GSOFT Chair: Nasser Abukhdeir, University of Waterloo Room: BCEC 162B |
Monday, March 4, 2019 2:30PM - 2:42PM |
C30.00001: Nanocomposites of Ferroelectric Liquid Crystals and Magnetic Nanoparticles: Potential Application as a MRI Contrast Agent Luz Martinez-Miranda, Patricio N Romero-Hasler, Eduardo A Soto-Bustamante, Lynn K. Kurihara, Lamar O Mair, Irving N Weinberg Nanocomposites of ferroelectric liquid crystals in combination with FeCo magnetic nanoparticles are considered as a MRI contrast agent, by creating a magnetic field as a function of the body’s electric field. The rotational viscosity of the sample needs to be reduced in order to reduce the electric field strength required for liquid crystal rotation to the values observed in many live biological states. This is achieved by having the nanoparticles act as lubricants to the ferroelectric liquid crystal. The magnetic field depends on the concentration of nanoparticles, which cannot be toxic to the biological environment. The concentrations used form aggregates that tend to exist in stable spherulites, that are also reversible with temperature. We find a relation between the magnetic field and the number of spherulites per unit area that align as a function of the electric field in the presence of a very small sample such as our model system. Finally, we present a model of how the electric field rearranges the spherulites and produces the magnetic field. |
Monday, March 4, 2019 2:42PM - 2:54PM |
C30.00002: Can a minimum principle describe shear alignment of liquid crystals? Xingzhou Tang, Jonathan Selinger When a static perturbation is applied to a liquid crystal, the director configuration changes to minimize the free energy. When a shear flow is applied to a liquid crystal, one might ask: Does the director configuration change to minimize any effective potential? To address that question, we derive the Leslie-Ericksen equations for dissipative dynamics, and determine whether they can be expressed as relaxation toward a minimum. The answer may be yes or no, depending on the number of degrees of freedom. Using theory and simulations, we consider two specific examples, reverse tilt domains under simple shear flow and dowser configurations [1] under plane Poiseuille flow, and determine whether each example shows relaxation toward the minimum of an effective potential. |
Monday, March 4, 2019 2:54PM - 3:06PM |
C30.00003: Surface reconstruction of chiral liquid crystalline oligomers under the action of volatile organic compounds Petr Shibaev Significant changes in surface morphology as well as in chiral fingerprint structure are discovered in solid chiral oligomers and their compositions with low molar mass liquid crystals affected by volatile organic compounds. The changes are manifested by inversion of chiral pattern and surface periodic structure. Surprisingly, the changes are reversible with a relaxation time of several days at room temperature. This reversebility of the effect underscores the importance of chiral structure beneath the surface and not affected by the volatile compounds in restoration of initial morphology. The importance of underlying chirality is also confirmed by the absence of this effect in replicas of chiral surface created by means of common polymers (polystyreene, polyvinylpyridine). The discovered effect is also modelled by means of computer simulations and a simple theoretical model is presented. The discovered effect can find applications in gas detectors and optical devices. |
Monday, March 4, 2019 3:06PM - 3:18PM |
C30.00004: High order nonlinear electrophoresis in nematic liquid crystal Mojtaba Rajabi, Taras Turiv, O D Lavrentovich Electrophoresis is motion of particles relative to a surrounding fluid in a uniform electric field. In anisotropic fluids such as liquid crystals (LC) the particle’s velocity growth with the square of the electric field strength [1]. As a result, reversing polarity of the electric field does not change the direction of propulsion, which makes it possible to drive a sustained transport by an alternating-current (AC) field [2]. Here we report a higher-order nonlinear electrophoresis in which the velocity acquires a component proportional to the fourth power of the electric field strength. The effect is observed in a nematic LC with negative anisotropy of dielectric permittivity. The electric field realigns the director around the colloidal particle and thus modifies the geometry of spatial charge separation and the viscous drag; as a result, the velocity dependence on the field acquires higher-order nonlinearities. |
Monday, March 4, 2019 3:18PM - 3:30PM |
C30.00005: Comparison of material parameters among flexible dimer, trimer, tetramer of nematic mesogens Zeinab Parsouzi A.Sh, Greta Babakhanova, Rony Saha, Mojtaba Rajabi, Taras Turiv, Chris Welch, Georg H Mehl, James Gleeson, Antal Istvan Jakli, O D Lavrentovich, Samuel Sprunt We compare the temperature dependencies of the birefringence, elastic constants and orientational viscosities for four homologous nematic liquid crystal oligomers: dimer (DTC5C9), its associated trimer and tetramer. These materials all exhibit the twist-bend nematic (NTB) phase. The magnitudes of the splay (K11), twist (K22) and bend (K33) elastic constants, as well as the corresponding orientational viscosities are determined by dynamic light scattering measurements. Interestingly K33 in the dimer and tetramer shows the monotonic decrease with temperature below the nematic-isotropic transition followed by a pretransitional increase close to the nematic to NTB transition. The trimer, in contrast, shows essentially no pretransitional change in K33 . This behavior can be attributed to odd-even effect associated with the number of monomer unites in oligomers. The orientational viscosities associated with splay, twist and bend fluctuations in the N phase of dimer, trimer and tetramer are comparable to those of nematics formed by rod-like molecules. All three show strong temperature dependence, increasing sharply near the N – NTB transition. |
Monday, March 4, 2019 3:30PM - 3:42PM |
C30.00006: Enhanced Dissipation Behavior of Main-Chain LCE Networks Cristina Martin Linares, Alejandro Martin Linares, Nicholas Traugutt, Thao Nguyen, Christopher Yakacki Liquid Crystal Elastomers (LCEs) are composed of mesogens bound to an elastomeric network of polymer chains. The orientation mesogens relative to the polymer network leads to reversible actuation at the nematic-isotropic transition (Tni), soft-elasticity, and enhanced dissipation. Instead of a peak at Tg, the tan-delta curve for LCEs is elevated over Tg-Tni, indicating enhanced dissipation. At large strains, the material exhibits rate-dependent soft-elasticity with hysteresis[1]. We hypothesize that the enhanced dissipation behavior arises from the motion of the mesogens relative to the polymer network and measure the effect of mesogen ordering and network orientation. We developed uniaxial tension tests using 3D-DIC to measure the rate-dependent load-unload response and hysteresis for 3 main chain LCE networks with different microstructures, including polydomain and monodomains. The modulus and hysteresis increased significantly with strain rate for all networks. The modulus for a monodomain stretched parallel to the director was an order of magnitude higher than that of a polydomain. The monodomain stretched perpendicular to the director had the greatest dissipation. |
Monday, March 4, 2019 3:42PM - 3:54PM |
C30.00007: Defect nucleation in quenched freely suspended smectic liquid crystal films Adam Green, Cheol Park, Joseph E MacLennan, Matthew Glaser, Noel Anthony Clark Freely suspended films of smectic liquid crystals, which can be as thin as two molecular layers, are a natural choice for studying ordering and phase transitions in 2D. One 2D system of ongoing interest is that of the XY-model, which describes the U(1) symmetry group of rotations, broken by either a complex number or 2D vector. The XY-model gives rise to topological defects, or vortices, with interesting implications for phase transitions in two dimensions eg. Kosterlitz-Thouless transitions. Here, we describe our recent experiments focusing on the nucleation of topological defects in the quasi-2D system of freely-suspended smectic films. By rapidly quenching a film (inducing a rapid phase transition from an ordered to a disordered phase), and studying the evolution of the dynamics with a high-speed video camera, we are able to visualize defect nucleation in smectic C (tilted) films, and we will discuss the role that islands (pancake-like, circular regions with additional smectic layers bounded by an edge dislocation) play in mediating the creation of these defects. |
Monday, March 4, 2019 3:54PM - 4:06PM |
C30.00008: Shaping nanoparticle fingerprints at the interface of cholesteric emulsions Lisa Tran, Hye-Na Kim, Ningwei Li, Shu Yang, Kathleen Stebe, Randall D Kamien, Martin Haase In this work, we balance the interfacial energy of nanoparticles against the elastic energy of cholesteric liquid crystals to dynamically shape nanoparticle assemblies at a fluid interface. By adjusting the concentration of surfactant that plays the dual role of tuning the nanoparticle hydrophobicity and altering the anchoring of liquid crystals, we pattern nanoparticles at the interface of cholesteric liquid crystal emulsions. Interfacial assembly is tempered by elastic patterns that arise from the geometric frustration of confined cholesterics. Patterns are tunable by varying both surfactant and chiral dopant concentrations. Adjusting the particle hydrophobicity more finely by regulating the surfactant concentration and solution pH further modifies the rigidity of assemblies, giving rise to surprising assembly dynamics dictated by the underlying elasticity of the cholesteric. Because assembly occurs at the interface with the desired structures exposed to the surrounding water solution, particles can be readily cross-linked and manipulated. This study serves as a foundation for better understanding inter-nanoparticle interactions at interfaces by tempering their assembly with elasticity. |
Monday, March 4, 2019 4:06PM - 4:18PM |
C30.00009: Twist fluctuations and rotational diffusion of the director near hedgehog defects in nematic droplets Alexis De la Cotte, Olaf Stenull, Peter Collings, Tom C Lubensky, A. G. Yodh When strong homeotropic boundary conditions are set at the interface of emulsion droplets containing a nematic liquid crystal (NLC), the spherical confinement combined with the system bulk elasticity is known to produce a point-like defect called a radial hedgehog with topological charge +1. We measure the director fluctuations about this state and compare them to the predictions of a theoretically calculated normal mode analysis. Three distortion modes are evident experimentally using two different optical microscopy techniques. Using polarizing microscopy, we observe the motion of the two extinction axis and identify two angular diffusions: a fast one, following the angle the axes make in respect to each other, and a slow one, tracking the angle they make in respect to the polarizers’ orientations. In bright field, we are also able to measure the translational motion of the hedgehog. Analysis of these distortion modes indicates the possibility of an instability toward a twisted hedgehog configuration in droplets of the NLC 5CB. |
Monday, March 4, 2019 4:18PM - 4:30PM |
C30.00010: Artificial spin (and Potts) ice of skyrmions in liquid crystals Ayhan Duzgun, Cristiano Nisoli Artificial spin ices [1] are engineered arrays of mutually interacting, frustrated, single domain magnetic nano-islands that can be characterized at the degree of freedom level and designed in different arrangements for different emergent behaviors [2]. Beyond magnets, similar ideas have been exported to trapped colloids [3] and vortices in pinned superconductors [4]. In this work, we present a proposal to realize them in a new platform: Liquid crystals. Liquid Crystals possess various advantages as a spin ice platform. Skyrmions in liquid crystals [5], can be confined in traps made of light or field gradients where their preferential position in the trap can mimic a binary Ising spin or a Potts variable. By solving partial differential equations for chiral nematic liquid crystals in the presence of constraints, we demonstrate that skyrmions can reproduce the ice-state of square ice and honeycomb ice. We then extend these simulations to Potts systems. |
Monday, March 4, 2019 4:30PM - 4:42PM |
C30.00011: Two-Dimensional Hexagonal Boron Nitride Nanosheet as the Planar-Alignment Agent in a Liquid Crystal-Based Electro-Optic Device Rajratan Basu, Lukas Atwood The planar-alignment agent in an electro-optic liquid crystal (LC) device plays an essential role for the LC’s electro-optical characteristics. Rubbed polyimide (PI) layers are conventionally used as the planar-alignment agent in traditional liquid crystal displays (LCDs). Here we experimentally demonstrate that the 2D hexagonal boron nitride (h-BN) nanosheet can serve as the planar-alignment agent in an LC cell. Two h-BN-covered indium tin oxide (ITO) glass slides are placed together to fabricate an LC cell. A nematic LC inside this h-BN-based cell exhibits uniform planar-alignment. This planar-alignment at the molecular scale is achieved due to the coherent overlay of the benzene rings of the LC molecules on the hexagonal BN lattice. This h-BN-based LC cell shows the typical electro-optical effect when an electric field is applied via ITO electrodes. The dielectric measurement across this h-BN-based electro-optic cell shows a standard Fréedericksz transition of the LC, confirming that the 2D h-BN, as the planar-alignment agent, supplies adequate anchoring energy. Finally, we show the h-BN-based LC cell exhibits more optical transparency than a regular PI alignment layer-based LC cell. |
Monday, March 4, 2019 4:42PM - 4:54PM |
C30.00012: Peculiar Nonlinear Elasticity of Liquid Crystal Elastomers Revealed by Biaxial Stretching Asaka Takebe, Haruki Tokumoto, Kenji Urayama Liquid crystal elastomers (LCE) are a combination of liquid crystal and elastomer. The coupling of liquid crystal alignment and rubber elasticity results in a unique mechanical property which is often called soft elasticity. The stretching in the direction normal to the initial director requires very small mechanical work due to the director rotation along the stretching axis. This is clearly observed as a finite plateau region with very low stress in the stress-strain curve. Most of the earlier characterizations of finite deformation of LCEs have been conducted using uniaxial stretching. In general, uniaxial stretching, however, provides a limited basis for comprehensive understanding of nonlinear elasticity of elastomers, because it is only a particular one among all physically accessible deformations. Present work investigates the biaxial stress-strain behavior of polydomain nematic elastomers with varying the strains independently in the two directions. We have found the peculiarity in nonlinear elasticity of LCEs which has not been characterized by uniaxial stretching measurements. |
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