Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T50: Liquid Crystals II: Chiral and Nanocomposites |
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Sponsoring Units: GSOFT Chair: Chenhui Zhu, Lawrence Berkeley National Laboratory Room: 218 |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T50.00001: ABSTRACT MOVED TO V1.00117 |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T50.00002: Chiral defects of achiral nematic liquid crystals in capillaries with homeotropic anchoring Louis Kang, Joonwoo Jeong, Zoey S. Davidson, Peter J. Collings, Tom C. Lubensky, A. G. Yodh Nematic liquid crystals (LCs) with strong elastic anisotropy can give rise to interesting nontrivial structures in confined geometries. We placed achiral LCs with a small twist elastic modulus in cylindrical capillaries with homeotropic anchoring. The LCs adopt degenerate twisted- and escaped-radial (TER) configurations in which they bend, or escape, towards one end of the capillary and twist along the radius with a handedness. The experimental system using Sunset Yellow FCF, a lyotropic chromonic LC, was investigated by polarized optical microscopy and showed excellent agreement with numerical calculations. Moreover, defects between TER domains of opposite escaping direction and/or twist handedness exhibit a rich phenomenology. Those spanning domains of opposite escaping direction and same twist handedness are strongly energetically disfavored. These findings enrich our understanding about how chiral structures can arise from achiral building blocks and how topological defects interplay with chirality. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T50.00003: Chiral liquid crystal droplets near the isotropic phase Jose Martinez-Gonzalez, Mohammad Rahimi, Ye Zhou, Juan de Pablo Liquid crystalline blue phases (BPs) are found between the helical and the isotropic phase of highly chiral compounds. A common feature of BPs is that the local director field forms double twist cylinders leading to defects that form regular, periodic patterns. As a result of their physical properties, which include selective light reflection, high viscosity and a small elastic shear modulus, BPs could find intriguing potential applications in photonic materials, micro-lasers, electrically switchable colour displays and light filters, to name a few. In the bulk, BPs are only formed in a narrow range of temperature. In this work, we use theory and simulations to examine the phase behavior of chiral liquid crystal droplets with weak planar anchoring in the vicinity of the isotropic phase, with special emphasis on the effects of confining blue phase I and blue phase II. We identify several new morphologies, whose complexity increases with the chirality of the medium, and we find that confinement of blue phases in micro-droplets increases their range of stability. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T50.00004: Probing helical structures in liquid crystals with resonant soft x-ray scattering at carbon edge Chenhui Zhu, Cheng Wang, Anthony Young, Ilja Gunkel, Alexander Hexemer, Feng Liu, Dong Chen, David Walba, Noel Clark, Wim Bras We report the first in-situ measurement of the helical pitch in nanofilament B4 phase, using resonant soft x-ray scattering at carbon resonant edge. A strong scattering peak was observed corresponding to $\sim$100 nm periodicity in layer orientation variation. The scattering is anisotropic due to the nano-filament helical structure and bond orientation sensitivity enabled by the linearly-polarized soft x-rays. In-situ measurements of the helical pitch as a function of temperature provide unique information on the B4 structure and the nature of the B2-B4 phase transition. This approach can be extended to other helical structures in liquid crystals and beyond. [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T50.00005: Biosensing using smectic and cholesteric liquid crystals Piotr Popov, Elizabeth Mann, Antal Jakli Liquid-crystal-based biosensors utilize liquid crystal alignment's high sensitivity to the presence of lipids and proteins self-assembled at the liquid crystal/aqueous solution interface. The optical response of the bulk liquid crystal to the interface offers inexpensive, easy optical detection of such biologically relevant molecules. Present technique uses nematic liquid crystal phase state that typically has a planar-to-homeotropic response only. Here we show that smectic and cholesteric phase states of liquid crystals can be used as new sensing modes that can provide additional information or improve the characteristics of a potential biosensor device. Smectic-A phase extends the detection range both toward the lower and higher concentration. Cholesteric phase (nematic with a chiral dopant) may be sensitive to the chirality of biological surface-active molecules such as phospholipids. Additionally, the ``finger-print'' texture of a cholesteric phase may show the differences between biomolecule homologues, thus providing a promising way of distinguishing between subtle differences of hydrocarbon chain or head-group size and structure. [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T50.00006: Toplogical defects in cholesteric liquid crystal shells Alexandre Darmon, Michael Benzaquen, Olivier Dauchot, Teresa Lopez-Leon Confining rod-like molecules, such as the nematogens of liquid crystals, on a curved surface inevitably yields topological defects. Remarkably, in the specific case of a nematic sphere, Poincar\'e stated that the sum of the topological charges of the defects is equal to +2. Recent studies have shown that, in the case of nematic shells, three kinds of configuration are possible. Each of these configurations correspond to different arrangements of defects that satisfy Poincar\'e's theorem. But much richer scenarii appear when playing with the shell thickness, since bulk effects start competing with surface effects. In particular, we show that inducing chirality in the liquid crystal can have a dramatic effect in the defect structure of the shell, where the ratio $c$ between the cholesteric pitch and the shell thickness becomes the control parameter. We also study into details the equilibrium configuration obtained for $c>>1$, which corresponds to the structure with a single disclination line of charge +2, also know as the Frank-Pryce structure. Following the idea brought by the simulations of Sec et al. on cholesteric droplets, we show experimentally that this structure is not a pure +2 line but is actually found to be two non-singular +1 lines that are winded around each other. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T50.00007: Geometry and topology of the cholesteric pitch axis: Which way is everything twisting? Daniel Beller, Thomas Machon, Simon Copar, Daniel Sussman, Gareth Alexander, Randall Kamien, Ricardo Mosna In a cholesteric with distortions and topological defects, it is not obvious how to even define the pitch axis and locate its singularities. We propose a construction of the cholesteric pitch axis for an arbitrary director field as an eigenvalue problem. With this tool we are able to compare the defects of the cholesteric phase with seemingly analogous defects in the biaxial nematic and smectic phases. Our results show the limitations of these analogies and indicate in what ways the cholesteric's topology is intermediate between, and distinct from, the topologies of the biaxial nematic and smectic phases. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T50.00008: Spontaneous Ferromagnetic Ordering of Nanoplatelets in Isotropic Solvent Min Shuai, Arthur Klittnick, Michael Tuchband, Matthew Glaser, Joseph Maclennan, Noel Clark, Rolfe Petschek, Alenka Mertelj, Darja Lisjak, Martin Copic Room-temperature ferromagnetic fluids were first experimentally demonstrated by Mertelj, \textit{et al} (\textit{Nature}, \textbf{504}: 237--241, 2013), by suspending surfactant wrapped coated barium hexaferrite (BHXF) nanoplates in the liquid crystal 5CB. We have studied the liquid crystal phase behavior of BHXF magnetic platelets suspended in isotropic solvent (1-butanol) at high volume fraction, where simulations predict an N-I transition for monodisperse hard plates. In these suspensions, the anisotropic particles can be aligned by magnetic fields as weak as 2 gauss, leading to a state with substantial birefringence and dichroism. When the volume fraction of the magnetic platelets is higher than 28{\%}, we observe a phase co-existence, with an isotropic state at the top of a capillary and a birefringence phase at the bottom. In the lower phase, domains are found to have different magneto-optical response from each other and the response is dependent on the \textit{sign} of the magnetic field, showing broken time-reversal symmetry and ferromagnetism. Spike structures are observed at the interface between the isotropic and ferromagnetic states. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T50.00009: Effects of ferroelectric nanoparticles on ion-transport in a liquid crystal Alfred Garvey, Rajratan Basu A small quantity of BaTiO$_{3}$ ferroelectric nanoparticles (FNPs) of 50 nm diameter was doped in a nematic liquid crystal (LC), and the free ion concentration was found to be significantly reduced in the LC$+$FNP hybrid compared to that of the pure LC. The strong electric fields, due to the permanent dipole moment of the FNPs, trapped some mobile ions, reducing the free ion concentration in the LC media. The reduction of free ions was found to have coherent impacts on the LC's conductivity, rotational viscosity, and electric field-induced nematic switching. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T50.00010: Smectic Liquid Crystals in the Immediate Vicinity of the Superstructure of a Nanoparticle in a Nanocomposite Luz J. Martinez-Miranda We have studied how the nanoparticle (or the superstructure the nanoparticle forms) aligns the liquid crystal in smectic liquid crystal nanocomposites to determine how charges are transferred between the liquid crystal and the nanoparticle or nanoparticle superstructure [1,2,3]. This superstructure depends on the density of aliphatic functionalizations at the surface of the nanoparticle [4]. We have found evidence that the asymmetry of the liquid crystal distorts those superstructures that belong to the cubic structure. In addition, we have observed some differences in how the smectic liquid crystal behaves depending on the nanoparticle used and how the liquid crystal is aligned by the substrate. Knowing how the smectic liquid crystals align in the immediate vicinity of this superstructure with respect to the bulk alignment gives us an idea of how charges are transferred. \\[4pt] [1] Taylor, J. W. Ph. D. Thesis. University of Maryland, College Park, USA, 2013.\\[0pt] [2] Martinez-Miranda, L. J. et al, \textit{Applied Physics Letters }2010, 97, 223301.\\[0pt] [3] Branch, J. et al , \textit{Journal of Applied Physics }2014, 115, 164303.\\[0pt] [4] See, for example, Draper, M. et al, \textit{Advanced Functional Materials }2011, 21, 1260-1278. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T50.00011: Nano Liquid Crystal Droplet Impact on Solid Surfaces Rui Zhang, Juan de Pablo Liquid droplet impaction on solid surfaces is an important problem with a wide range of applications in everyday life. Liquid crystals (LCs) are anisotropic liquids whose internal structure gives rise to rich optical and morphological phenomena. In this work we study the liquid crystal droplet impaction on solid surfaces by molecular dynamics simulations. We employ a widely used Gay-Berne model to describe the elongated liquid crystal molecules and their interactions. Our work shows that, in contrast to isotropic liquids, drop deformation is symmetric unless an instability kicks in, in which case a nano scale liquid crystal droplet exhibits distinct anisotropic spreading modes that do not occur in simple liquids. The drop prefers spreading along the low viscosity direction, but inertia can in some cases overcome that bias. The effects of the director field of the droplet, preferred anchoring direction and the anchoring strength of the wall are investigated. Large scale (0.1 micron) simulations are performed to connect our nano scale results to the experiments. Our studies indicate that LCs could provide an interesting alternative for development of next-generation printing inks. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T50.00012: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T50.00013: Nanoparticle interfacial assembly in liquid crystal droplets Mohammad Rahimi, Tyler Roberts, Julio Armas-Perez, Xiaoguang Wang, Emre Bukusoglu, Nicholas L. Abbott, Juan J. de Pablo Controlled assembly of nanoparticles at liquid crystal interfaces could lead to easily manufacturable building blocks for assembly of materials with tunable mechanical, optical, and electronic properties. Past work has examined nanoparticle assembly at planar liquid crystal interfaces. In this work we show that nanoparticle assembly on curved interfaces is drastically different, and arises for conditions under which assembly is too weak to occur on planar interfaces. We also demonstrate that LC-mediated nanoparticle interactions are strong, are remarkably sensitive to surface anchoring, and lead to hexagonal arrangements that do not arise in bulk systems. All these elements form the basis for a highly tunable, predictable, and versatile platform for hierarchical materials assembly. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T50.00014: Translational and Rotational Diffusion of Nanoparticle Aggregates of Irregular Shape in 2D Fluid Membranes Kyle Meienberg, John Papaioannou, Cheol Park, Matt Glaser, Joe Maclennan, Noel Clark, Tatiana Kuriabova, Thomas Powers We observe directly the diffusion and aggregation of nanoparticles (buckyballs) embedded in thin, freely suspended smectic A liquid crystal films of 8CB using reflected light microscopy Individual buckyballs, initially homogeneously dispersed in the film, are too small to see but after some hours form nanoscale clusters. These, in turn, aggregate to form extended, micron-scale objects which diffuse in the film, enabling the measurement of 2D rotational and translational mobilities of inclusions with a wide variety of different shapes. The experimental mobilities are compared with predictions of the extended Saffman-Delbr\"{u}ck (SD) model used successfully to describe the diffusion of micron-sized objects in thin fluid membranes in a variety of experimental systems. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T50.00015: Towards 2D nanocomposites Hyun-Sook Jang, Changqian Yu, Robert Hayes, Steve Granick Polymer vesicles (``polymersomes'') are an intriguing class of soft materials, commonly used to encapsulate small molecules or particles. Here we reveal they can also effectively incorporate nanoparticles inside their polymer membrane, leading to novel ``2D nanocomposites.'' The embedded nanoparticles alter the capacity of the polymersomes to bend and to stretch upon external stimuli. [Preview Abstract] |
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