Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B17: Liquid Crystals II: Topology and Defects |
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Sponsoring Units: GSOFT Chair: Linda Hirst, University of California - Merced Room: 276 |
Monday, March 13, 2017 11:15AM - 11:27AM |
B17.00001: Orientation of topological defects in 2D nematic liquid crystals Xingzhou Tang, Jonathan Selinger Topological defects are an essential part of the structure and dynamics of all liquid crystals, and they are particularly important in experiments and simulations on active liquid crystals. In a recent paper [1], Vromans and Giomi pointed out that topological defects are not point-like objects but actually have orientational properties, which strongly affect the motion and energetics of the defects. That paper developed a mathematical formalism which describes the orientational properties as vectors. Here, we agree with the basic concept of defect orientation, but we disagree with the mathematical formalism of vectors, because it depends on an arbitrary choice of coordinate system. Instead, we represent the defect orientation by a tensor, with a rank that depends on the topological charge: rank 1 for a charge of +1/2, rank 3 for a charge of -1/2. Using this tensor formalism, we calculate the orientation-dependent interaction between defects, and we present numerical simulations of defect motion. [1] A. J. Vromans and L. Giomi, Soft Matter 12, 6490 (2016). [Preview Abstract] |
Monday, March 13, 2017 11:27AM - 11:39AM |
B17.00002: The static and dynamic behaviors of the topological defects in a nematic liquid crystal reveal its material characteristics Rui Zhang, Takuya Yanagimachi, Nitin Kumar, Margaret Gardel, Paul Nealey, Juan de Pablo Topological defects in nematic liquid crystals (LCs) play a key role in phase transitions, domain growth, and morphology evolution. Their ability to absorb impurities offers promise for design of self-assembled, hierarchical materials. Past work has primarily studied defects in thermotropic LCs. In this work, we focus on lyotropic chromonic LCs and biopolymer LCs, and investigate how the static and dynamic properties of topological defects depend on the LC's material characteristics. Specifically, we rely on a Landau-de Gennes free energy model that accounts for variable material constants and back-flow effects, and adopt a hybrid lattice Boltzmann simulation method. We first show that the fine structure of half-charge defects is a function of the ratio of splay and bend constants. This morphological information is in turn used to infer the elasticity of an in vitro, actin-based LC suspension. We then examine the annihilation process of a defect pair of opposite topological charge. We find that the ratio of the two defect velocities is an outcome of the interplay between the LC's elastic moduli, its viscosities, and the organization of the defects. Our calculations predict a strong post-annihilation transverse flow that is further confirmed by our experiments with non-equilibrium LCs. An analysis of the asymptotic behavior of the elastic moduli allows us to elucidate the material at phase transitions. Our modelling provides a general, unified framework within which a wide class of LC materials can be understood. [Preview Abstract] |
Monday, March 13, 2017 11:39AM - 11:51AM |
B17.00003: Topological defects and shapes of triatic liquid crystal vesicles Francesco Serafin, Oksana Manyuhina, Mark Bowick Is shape the manifestation of function, or does shape determine function? Since the time of Aristotle, the study of shape has proven to be a fruitful way to understand the behavior of physical systems, from atomic to biological systems scales. Two dimensional soft membranes are a perfect setting to understand the emergence of shape. An interesting possibility is to control and design new self-assemblable supramolecular shapes by coating the surface of soft closed vesicles with liquid crystals (LC) of various symmetries. The microscopic geometry of the liquid crystal molecules, in particular the structure of topological defects, when combined with the topology of the vesicle's surface, ultimately determines the vesicle's shape. Recent work has shown that the minimal energy shapes of smectic and nematic vesicles are faceted polyhedra. A very soft smectic vesicle develops sharp creases and forms a faceted tetrahedron. When the coating LC has the symmetries of the square, the vesicle forms a cube. In this work we extend these results to a 3-fold symmetric LC, proving that the vesicle's ground state is an octahedron. This gives a systematic way of predicting vesicle's shapes as we change the liquid crystal's symmetry. [Preview Abstract] |
Monday, March 13, 2017 11:51AM - 12:03PM |
B17.00004: Near-field interaction of colloid near wavy walls Yimin Luo, Francesca Serra, Denise Wong, Edward Steager, Kathleen Stebe Anisotropic media can be used to manipulate colloids, in tandem with carefully designed boundary conditions. For example, in bulk nematic liquid crystal, a wall with homeotropic anchoring repels a colloid with the same anchoring; yet by changing the surface topography from planar to concave, one can turn repulsion into attraction. We explore the behaviors of micro-particles with associated topological defects (hedgehogs or Saturn rings) near wavy walls. The walls locally excite disturbance, which decays into bulk. The range of influence is related to the curvature. The distortion can be used to position particles, either directly on the structure or at a distance away, based on the ``splay-matching'' rules. When distortion becomes stronger through the deepening of the well, the splay field created by the wall can prompt transformation from a Saturn ring to a hedgehog. We combine wells of different wavelength and depth to direct colloid movement. We apply a magnetic field to reset the initial position of ferromagnetic colloids and subsequently release them to probe the elastic energy landscape. Our platform enables manipulation, particle selection, and a detailed study of defect structure under the influence of curvature. [Preview Abstract] |
Monday, March 13, 2017 12:03PM - 12:15PM |
B17.00005: The role of disclinations on the organization and conductivity in liquid crystal nanocomposites Luz J Martinez-Miranda, P. Romero-Hasler, A. Meneses-Franco, E A Soto-Bustamante The structure of TiO2 nanoparticles in a liquid crystal nanocomposite was found to be an oblique structure due to the alignment of the TiO2 with respect to the liquid crystals. This order is anisotropic due to the ordering of the liquid crystals. The particles are highly localized in the nanocomposite, which has consequences in the electrical percolation. We want to obtain an understanding of how the nanoparticles organize in this highly localized fashion. The nanoparticles and the liquid crystals phase separate, with the nanoparticles accumulating in the defects exhibited by the liquid crystal even after being sonicated initially. The liquid crystal is polymerized by the process of electropolymerization that takes place in the isotropic phase of the monomers. The nanoparticles are free to move away from the defects where they phase separate since the defects disappear in the isotropic. We believe the polymerization imposes a limitation in the movement of the nanoparticles. The combination of the accumulation in the disclinations, the polymerization in the isotropic and the formation of the liquid crystal unit side chains can affect the conductivity of the nanocomposite. [Preview Abstract] |
Monday, March 13, 2017 12:15PM - 12:27PM |
B17.00006: Molecular engineering of colloidal liquid crystals using DNA origami Mahsa Siavashpouri, Christian Wachauf, Mark Zakhary, Florian Praetorius, Hendrik Dietz, Zvonimir Dogic Understanding the microscopic origin of cholesteric phase remains a foundational, yet unresolved problem in the field of liquid crystals. Lack of experimental model system that allows for the systematic control of the microscopic chiral structure makes it difficult to investigate this problem for several years. Here, using DNA origami technology, we systematically vary the chirality of the colloidal particles with molecular precision and establish a quantitative relationship between the microscopic structure of particles and the macroscopic cholesteric pitch. Our study presents a new methodology for predicting bulk behavior of diverse phases based on the microscopic architectures of the constituent molecules. [Preview Abstract] |
Monday, March 13, 2017 12:27PM - 12:39PM |
B17.00007: Ordered Nematic Colloidal Dispersions of Anisotropic Nanoparticles. Ivan Smalyukh, Qingkun Liu, Haridas Mundoor A promising approach of designing mesostructured composite materials with novel physical behavior is based on combining unique optical and electronic properties of solid nanoparticles with long-range ordering and facile switching of soft matter. We practically realize molecular-colloidal organic-inorganic composites formed via multi-scale self-assembly in dispersions of anisotropic inorganic nanoparticles in liquid crystals. Using charged anisotropic rod-like and platelet-shaped nanoparticles, we demonstrate self-assembly of triclinic colloidal crystals and biaxial colloidal fluids with low-symmetry ordering. We show that these unexpected forms of self-assembly emerge from the competing elastic and electrostatic interactions in the nematic host. We characterize the facile response of these colloidal dispersions to external fields and discuss their potential technological uses. [Preview Abstract] |
Monday, March 13, 2017 12:39PM - 12:51PM |
B17.00008: Tuning the synergistic functionality of semiconducting and magnetic nanoparticle co-assemblies Mark Bartolo, Randy Espinoza, Jussi Amaral, Sayantani Ghosh A much sought after goal of nano-synthesis is the ability to create artificial materials from self-assembled nano-constituents that would exhibit multifunctionality and \textit{in situ} responsiveness to external stimuli. We are investigating the ensemble behavior of Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ magnetic nanoparticles (MNPs) and CdeSe/ZnS quantum dots (QDs) when dispersed in electro-optically active liquid crystalline (LC) matrix. Prior research has demonstrated an enhancement in the QD emission with a small, applied magnetic field, a result of synergistic interactions between the nanoparticles. Here, we have expanded the phase space by varying the sizes and relative proportions of QDs and MNPs in the assemblies. Our goal is to identify the limits of sensitivity of the enhanced brightening effect as a function of concentration of MNPs.. Using scanning confocal photoluminescence microscopy, we observe that 20 nm MNPs co-assembled with 6 nm QDs exhibit the brightening effect at very dilute limit of 1:100 for MNP:QD, while decreasing MNP sizes require higher proportion of the latter. This could be a potential platform for developing high sensitivity, small footprint, and low power magnetic field detectors. [Preview Abstract] |
Monday, March 13, 2017 12:51PM - 1:03PM |
B17.00009: Dielectric Anistropy, Elastic Constants, and Threshold Voltage Measurements of Gold-nanoparticle Colloids in Nematic 5CB Angelo Visco, Jon Foust, Joseph Belobradich, Rizwan Mahmood, Donald Zapien We have explored electro-optical and thermal properties of gold nanoparticles (GNPs) colloids in 4-cyano- 4'-pentylbiphenyl (5CB) liquid crystals (LCs). GNP's do not readily disperse in a LC host and, so, have been treated with either, 1-Hexane-thiol, 1-Dodecane-thiol, or 1-Octadecyl-thiol. This treatment suppresses the aggregation of GNPs within the 5CB host to a threshold of approximately 0.7{\%} GNP by weight. Our measurements on dodecanethiol and hexanethiol treated GNPs showed an unusual, steep trough in the dielectric anisotropy and elastic constants at a critical concentration of 0.0862 wt. {\%} GNPs in 5CB. Due to the order parameter, we have observed a peak in the transition temperature at the same critical concentration. Above the critical concentration the transition temperatures, dielectric anisotropy, and elastic constants level off to within experimental uncertainty. Measurements of dodecanethiol treated GNPs in 5CB reveal distinctions in the rate of change in dielectric anisotropy as compared to hexanethiol treated GNPs in 5CB. This effect is possibly due to the increased carbon concentration in dodeccanethiol compared to hexanethiol. Attempts to mix the Smectic A (SmA), 8CB liquid crystal using our hexanethiol and dodecanethiol GNPs were unsuccessful for particle sizes of 100nm and 28nm. We suspect that this is due to an insufficient length of the carbon-chain and U.V. spectroscopy measurements may prove useful in characterizing the resulting aggregation. We hope the system will be helpful in modifying the properties of mesophases that may ultimately results in developing new electro-optical devices. [Preview Abstract] |
Monday, March 13, 2017 1:03PM - 1:15PM |
B17.00010: Liquid Crystal Mediated Nano-assembled Gold Micro-shells Makiko Quint, Som Sarang, David Quint, Kerwyn Huang, Ajay Gopinathan, Linda Hirst, Sayantani Ghosh We have created 3D nano-assenbled micro-shell by using thermotropic liquid crystal (LC), 4-Cyano-4'-pentylbiphenyl (5CB), doped with mesogen-functionalized gold nanoparticles (AuNPs). The assembly process is driven by the isotropic-nematic phase transition dynamics. We uniformly disperse the functionalized AuNPs into isotropic liquid crystal matrix and the mixture is cooled from the isotropic to the nematic phase. During the phase transition, the separation of LC-AuNP rich isotropic and ordered 5CB rich domains cause the functionalized AuNPs to move into the shrinking isotropic regions. The mesogenic ligands are locally crystalized during this process, which leads to the formation of a spherical shell with a densely packed wall of AuNPs. These micro-shells are capable of encapsulating fluorescence dye without visible leakages for several months. Additionally, they demonstrate strong localized surface plasmon resonance, which leads to localized heating on optical excitation. This photothermal effect disrupts the structure, releasing contents within seconds. Our results exhibiting the capture and optically regulated release of encapsulated substances is a novel platform that combines drug-delivery and photothermal therapy in one versatile and multifunctional unit. [Preview Abstract] |
Monday, March 13, 2017 1:15PM - 1:27PM |
B17.00011: Blue-Phases on Nano-Patterned Surfaces Jose Martinez-Gonzalez, Xiao Li, Monirosadat Sadati, Ye Zhou, Rui Zhang, Paul Nealey, Juan de Pablo Blue-phases are states of matter with liquid properties and cubic crystalline structures of topological defects. Usually, a sample of this complex fluid consists of randomly oriented polycrystalline domains; this deteriorates the electro-optical properties of the material and places limits to its potential applications. In this work a theoretical analysis of channel-confined blue-phases was used to identify the preferred local molecular orientation at different crystallographic planes. This led to different template designs for surface patterning that were used to direct the lattice orientation of blue-phases with respect to the channel surface. Our theoretical predictions were confirmed trough experiments with nano-patterns accurately produced on chemical surfaces. This provides a new methodology to produce monocrystalline blue-phase domains as large as the patterned area with a desired lattice orientation. Our findings open the possibility to fully exploit the crystalline features of blue phases, which are useful for the design of a new generation of biological sensors, photonics materials and liquid crystal displays. [Preview Abstract] |
Monday, March 13, 2017 1:27PM - 1:39PM |
B17.00012: Stabilizing Monodomain in Blue Phase Liquid Crystal by Chemically Patterned Surfaces. Xiao Li, Jose A. Martinez-Gonzalez, Monirosadat Sadati, Ye Zhou, Juan J. de Pablo, Paul F. Nealey Topological defects in blue-phases liquid crystals (LCs) can be self-assembled into three-dimensional cubic crystalline structures, representing unique ordered states of matter among the other LCs. Stabilization of blue phases by trapping nanoparticles or polymerization right on the disclination lines and new mesogens synthesis are now serving as the major strategies for developing fast response optoelectronics. Polycrystalline, platelet and multi-domain topological defects of blue-phase LCs are difficult to overcame with current methods. We developed a chemically patterned surface that allows blue-phase LCs to be directed self-assembled upon the alternative planar/homeotropic (P/H) stripe pattern surface and into the uniform monodomain structure. The resulting blue-phases are single crystal, are oriented and guided with the underlying substrate and can be created over arbitrarily large areas. The single crystal blue-phase domain is determined by the period and the ration of P/H of the lithographically defined surface pattern rather than by the inherent limitations of the blue-phase material. Our results illustrate how directed self-assembly strategies by 2D chemically patterned surface allow for 3D disclination lines organization in existing manufacturing. [Preview Abstract] |
Monday, March 13, 2017 1:39PM - 1:51PM |
B17.00013: Resonant soft X-ray scattering study of twist bend nematic, cholesteric and blue phases. Miroslaw Slamonczyk, Ewa Grecka, Natasa Vaupotic, Damian Pociecha, JIm Gleesom, Antal Jakli, Sam Sprunt, Cheng Wang, Alexander Hexemer, Chenhui Zhu We have demonstrated that, when operated at carbon K-edge, the linearly polarized soft X-rays can enable bond orientation sensitivity, which can be utilized to probe the otherwise forbidden peak from the helices of twist bend nematic [2] and helical nanofilament phase [1]. Here we show that the same principle can be used to probe blue phase and chiral nematic phase. Furthermore, we discuss the relationship between the incoming linearly polarized X-rays, and the anisotropy in the scattering pattern. [1] C. Zhu, et al. \textit{Nano. Lett.} \textbf{15}, 3420 (2015). [2]. C. Zhu, et al. \textit{Phys. Rev. Lett.,} \textbf{116}, 147803 (2016). [Preview Abstract] |
Monday, March 13, 2017 1:51PM - 2:03PM |
B17.00014: Discussion on optical response of liquid-crystal BPIII driven by an inclined electric field. Hui-Yu Chen, Yen-Wen Wang Three blue phases exist between the chiral nematic and the liquid phase. Compared with the electro-optical properties of BPI and BPII, BPIII is a fast response photonic device with no residual birefringence, and less hysteresis effect when an in-plane electric field is applied. However, the in-the-plane field is not uniform and then the electro-optical properties is more complicate than that we can image. This is a key point for further application of BP. In this paper, a grating-like vertical electric field is used to induce the two different optical phenomena of BPIII. As the electric field is turned on, the light transmittance rapidly increases to a stable value (\textless 0.5 ms, Kerr effect). If the applied voltage is a dc, the transmittance will remind in this stable value. However, when the applied voltage is ac, the transmittance will oscillate with the frequency. The change in transmittance will be obvious in a low frequency. From our observation, we have known that the oscillation of the transmittance is not caused by the ion effect. It is induced by reorientation of the induced optical axis (flexoeletric effect). Thus, we can control the applied frequency and the amplitude to modulate the contribution of Kerr effect and flexoelectric effect. [Preview Abstract] |
Monday, March 13, 2017 2:03PM - 2:15PM |
B17.00015: Compatibility conditions for director fields in flat and curved 2D geometries Efi Efrati, Idan Niv Many biological and man made structures form geometrically frustrated assemblies where their constituents possess two or more mutually contradicting geometric tendencies. For example, banana shaped (bent-core) liquid crystals favor equally spaced director field lines that are also uniformly curved. Mathematically this leads to a locally preferred director field of vanishing splay and constant bend, correspondingly. One can easily show that these two tendencies cannot be simultaneously achieved in the plane. In this work we present a method for determining whether two prescribed bend and splay fields are compatible with a planar director field, and provide a direct formula for its reconstruction depending only on these fields and their derivatives when they are compatible. We generalize these methods for curved geometry, and explore how the geometric frustration is resolved in the specific case of bent core liquid crystals in the plane. [Preview Abstract] |
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