Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S57: Self-assembly in Liquid Crystals and other Complex Solvents IFocus
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Sponsoring Units: GSOFT DBIO DPOLY Chair: Cecilia Leal, University of Illinois at Urbana-Champaign Room: LACC 518 |
Thursday, March 8, 2018 11:15AM - 11:51AM |
S57.00001: Topological defects in liquid crystals as templates for molecular self-assembly Invited Speaker: Nicholas Abbott Topological defects in liquid crystals (LCs) have been widely used to organize colloidal dispersions and template polymerizations, leading to a range of elastomers and gels with complex mechanical and optical properties. However, little is understood about molecular-level self-assembly processes within the defects. This presentation will describe how nanoscopic environments defined by LC topological defects can selectively trigger processes of molecular self-assembly. By using fluorescence microscopy, cryogenic transmission electron microscopy and super-resolution optical microscopy, key signatures of molecular self-assembly of amphiphilic molecules in topological defects are observed - including cooperativity, reversibility, and controlled growth of molecular assemblies. By using polymerizable amphiphiles, we also demonstrate preservation of molecular assemblies templated by defects, including nanoscopic “o-rings” synthesized from “Saturn-ring” disclinations. Our results reveal that topological defects in LCs are a versatile class of three-dimensional, dynamic and reconfigurable templates that can direct processes of molecular self-assembly in a manner that is strongly analogous to other classes of macromolecular templates (e.g., polymer—surfactant complexes). Opportunities for the design of exquisitely responsive soft materials will be discussed using bacterial endotoxin as an example. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S57.00002: Hydrogels dehydrate embedded liposomes Sarith Bandara, Tom Molley, Kristopher Kilian, Cecilia Leal Amphiphiles self-assemble into different structures depending on packing and environment. The effect of hydrogels on amphiphile self-assembly—particularly liposome self-assembly—has not been extensively studied. This is relevant for both the design of new scaffold materials for cell growth and the understanding of how drug carriers interact with the extracellular matrix. In this presentation, we will show recent Small-Angle X-ray Scattering (SAXS) data demonstrating that when liposomes are incorporated into polyethylene glycol (PEG) hydrogels, they suffer significant restructuring. Specifically, in addition to a characteristic correlation length of 14 nm for the crosslinked hydrogel, Bragg peaks are detected, indicating that liposomes have transformed into a tight stack of lipid bilayers with interlamellar separation of 5.7 nm. These results demonstrate that hydrogels have a strong influence on liposome self-assembly. We hypothesize that the hydrophilic PEG hydrogel osmotically absorbs a significant volume of water, inducing a transformation of liposomes into multilamellar bodies. When liposomes are protected with a PEG-moiety corona, the influence of the surrounding hydrogel appears to be damped. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S57.00003: Quantifying Block Copolymer Bindings to Lipid Bilayers with Different Composition and Curvature Wenjia Zhang, Frank Bates, Timothy Lodge We have studied the effects of lipid bilayer curvature and composition on the interaction with poly(ethylene oxide)-b-poly(propylene oxide) diblock and triblock copolymers, using unilamellar phospholipid vesicles ranging from 90 to 220 nm in mean diameter. The composition of the bilayer membranes was manipulated by varying the lipid headgroup chemistry and degree of unsaturation in binary lipid mixtures, as well as with the concentration of cholesterol. The polymer-lipid bilayer association was investigated by probing polymer diffusion via pulsed-field-gradient NMR (PFG-NMR). In the presence of lipid vesicles, a fraction of polymer was bound to lipid bilayers, resulting in slower diffusion than that of free polymer. The coexistence of free and bound polymer gave rise to biexponential echo decay curves, which enabled the quantification of polymer binding fraction. Larger molecular weight and higher hydrophobicity of the polymer led to stronger interactions with lipid bilayers. A higher binding fraction was observed when lipid bilayer curvature increased. The polymer-lipid bilayer association was also strongly correlated with the membrane composition. This work demonstrated that PFG-NMR is a powerful tool to quantify polymer binding on the molecular level. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S57.00004: Supramolecular Self-Assembly of Filamentary Structures from Bent-Core Liquid Crystals RAYSHAN VISVANATHAN, Maria Kolber, Alexandra Duncan, Michael Tuchband, Keri Graber, Eva Korblova, Joseph MacLennan, Matthew Glaser, Noel Clark We have used small angle x-ray scattering (SAXS) and freeze fracture transmission electron microscopy (FFTEM) to characterize a novel filamentary phase of bow-shaped liquid crystals (LCs) with nanophase segregating properties. Bow-shaped LCs with a rigid core and a flexible trisilane-terminated alkyl tail self-organizes into filamentary structures upon cooling from conventional orthogonal bent-core smectic phases. The formation of filamentary structure is driven by nanophase segregation and influenced by the length of alkyl linker between the core and trisilane unit. Nanophase segregation occurs because the core of the molecule is immiscible with its trisilane tail. We performed systematic SAXS studies to investigate the formation of nanofilaments in a homologous series of bow-shaped molecules consisting of n-carbon alkyl linker (5 < n < 11). We find that the homologues with n > 9 (longer tail) have a strong tendency to self-assemble into a fully bilayered nanofilament structure, while for n < 8 (shorter tail) the LCs tend to retain their partial bilayered structure also seen in the SmA phase. We propose a nanoscale model to explain the structural changes and the filament formation. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S57.00005: Colloids in Nematics: Attractive and Repulsive Loci Near Corrugated Boundaries Yimin Luo, Francesca Serra, Daniel Beller, Kathleen Stebe We study colloids with well-defined anchoring near a corrugated boundary with hills and dales of radii of curvature greater than the colloid radius. By tailoring wall curvature, we define sites of attraction and equilibrium loci for colloids that vary from near contact to several particle radii from contact. Colloids with hedgehogs, boojums and Saturn rings interact distinctly with these boundaries; Saturn rings transform to dipolar configurations driven by wall interactions in some cases. We also define sites of repulsion to propel colloids away from these boundaries, and find unstable loci from which colloids depart along multiple paths. Small perturbations of colloid position allow selection among these paths to propel colloids toward or away from the wall using the nematic energy landscape. These abilities to transform defect configuration, to define docking sites and to direct particles toward or away from boundaries provide new tools to steer colloid motion, defect configuration and to guide structure formation. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S57.00006: Self-Assembly of Colloidal Particles with Optical Binding Dominique Davenport, Dustin Kleckner The properties of a material can be dramatically altered by patterning it at the microscopic scale, resulting in a composite known as a metamaterial. In principle, such a material could be created through self-assembly, however, it is difficult in practice to control the interactions between constituent particles. One potential method for doing so is optical binding, which is a highly tunable inter-particle force mediated by an intense optical field. The optical binding force has been shown to drive self-assembly of colloidal particles; we attempt to expand upon the tunable aspect of this force. What types of structures can be formed with optical binding? How can these structures be manipulated? We will discuss initial experiments designed to answer these questions and create a flexible platform for studies in self-assembly. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S57.00007: Sub-microsecond switching times using dynamically tunable plasmonic pixels Sebastian Etcheverry, Leonardo Araujo, Isabel Carvalho, Walter Margulis, Jake Fontana Liquid crystal based devices can arbitrarily control the amplitude, phase and polarization of light, enabling disruptive technologies such as flat screen televisions and smart phones. Yet, the Achilles heel of these devices are their slow, millisecond switching speeds, constraining potential applications. Here we develop the concept of a dynamic plasmonic pixel as a novel paradigm for liquid crystal devices using the electric field controlled alignment of gold nanorods. Experiments were performed using an electro-optic fluid fiber device, which enabled convenient interaction of light, electric fields and the nanorod suspension. We studied the evolution of the electric-field induced alignment of gold nanorods and demonstrate microsecond switching times, 3 orders of magnitude faster than a traditional Freederickcz-based liquid crystal alignment mechanism. We find that the dynamics of the alignment agrees well with the Einstein-Smoluchowski relationship. Furthermore, by digitally switching the nanorods between orthogonally aligned states, we show switching frequencies greater than MHz can be achieved. The development of these dynamically tunable plasmonic pixels may lead to ultrafast optical switches, filters, displays and spatial light modulators. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S57.00008: Relaxation dynamics of bola-shaped smectic island in a thin, spherical bubble of smectic C liquid crystal in microgravity Cheol Park, Eric Minor, Joseph MacLennan, Matthew Glaser, Noel Clark, Pavel Dolganov, Nikita Shuravin, Vladimir Dolganov We describe the relaxation dynamics of an elongated SmC* island embedded in a dense, two-dimensional emulsion of circular islands on a spherical liquid crystal bubble. Four-lobe extensional shear flow driven by air jets creates a bola-shaped inclusion comprising two roughly circular reservoirs connected by a long, extremely thin strip in the background island emulsion. After shear flow ceases, the bola relaxes slowly to become a single, circular domain. The connecting strip between the two islands gradually becomes shorter, eventually forming a broad bridge connecting the islands, which then proceed to merge, first transforming into an elliptical inclusion and then a single circular island. The dependence of the thin stripe length on time allows us to estimate the island mobility μ assuming the force acting on the islands are due to the line tension of the strip. The measured viscosity, particularly during the coalescence phase, is larger than expected from simple estimates of the dynamics on a uniform background film, an effect we suggest is probably related to additional drag from the background island emulsion. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S57.00009: Structure-Property Relationships in the Twist-Bend Liquid Crystal Phase of Odd Members of the CBnCB Homologous Series Michael Tuchband, Min Shuai, Daniel Paterson, Lee Foley, Jessica Riano, Alyssa Scarbrough, Victoria Normal, Cheng Wang, David Walba, John Storey, Chenhui Zhu, Corrie Imrie, Noel Clark We synthesize a series of homologues of CB7CB with different alkyl number n in the linker to investigate the effects of linker length on the properties of the twist-bend phase. We identify the TB phase in the odd members of the CBnCB series for n = 5 – 17 and investigate them primarily with resonant soft x-ray scattering (RSoXS) and non-resonant wide-angle x-ray scattering techniques. Increasing the length of the linker increases the molecular flexibility and decouple the mesogenic arms of the molecules, tending to stabilize the temperature range of the nematic phase while that of the TB phase diminishes. It also dramatically increases the helix pitch and contributes to enhanced structural fluctuations of the helix. We fit the measured helix pitch as a function of temperature for each compound to extract its ground state pitch at low temperature, when fluctuations are frozen out. We find that the ground state helix pitch scales nearly linearly with linker number. Our study not only demonstrates the nanoscale effects of lengthening the chemical linker, but also highlights the surprising importance of fluctuations in the behavior of the twist-bend phase. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S57.00010: Magnetically-Controlled Dynamics of Skyrmions in Chiral Ferromagnetic Liquid Crystals Jung-Shen Tai, Andrew Hess, Ivan Smalyukh Topological solitons, such as skyrmions, arise in field theories ranging from high energy physics to condensed matter and cosmology. Skyrmions can also be realized in the magnetization field of colloids with long-range ferromagnetic ordering, where the ability of optical imaging in the bulk reveals their structure and topology in detail. In this work, using a combination of experiments and numerical modeling, we demonstrate the magnetic control of equilibrium configurations and rich dynamics of skyrmions, revealing how ferromagnetic liquid crystal colloids can serve as a soft matter model system for directly visualizing effects previously predicted to occur in solid-state ferromagnets. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S57.00011: Active turbulence in a gas of self-assembled spinners Gašper Kokot, Alexey Snezhko Strongly interacting colloids driven out-of-equilibrium by an external periodic forcing often develop nontrivial collective dynamics and self-assembled patterns. Ferromagnetic micro-particles, suspended at a liquid interface and energized by a uniaxial in-plane alternating magnetic field spontaneously form arrays of self-assembled spinners rotating in either direction. We show that the spinners, emerging as a result of spontaneous symmetry breaking of clock/counterclockwise rotation of self-assembled particle chains, generate vigorous vortical flows at the interface. An ensemble of spinners exhibits chaotic dynamics due to self-generated advection flows. The same-chirality spinners (clockwise or counterclockwise) tend to aggregate and form dynamic clusters. Self-induced interface currents promote active diffusion that could be tuned by the parameters of the external excitation field. Furthermore, erratic motion of spinners at the interface generates chaotic fluid flow reminiscent of two-dimensional turbulence. The work provides insight into fundamental aspects of collective transport in active spinner materials. |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S57.00012: Controlled Real-Time Transition Between Bipolar and Radial Configuration of Liquid Crystal Droplets Jake Shechter, Jennifer Ross Active matter is the study of driven many-body systems that span length scales from flocking birds to molecular motors. A previously described self-propelled particle system was made from liquid crystal (LC) droplets in water with high surfactant concentration to move particles via asymmetric surface instabilities. Using a similar system, we increase the concentration of SDS surfactant to undergo a transition from the so-called bipolar configuration to a radial configuration and cause the droplets to begin swimming at sufficiently high concentration. Using optical tweezers, we can hold and observe a single LC droplet as the surfactant concentration is increased, allowing us to watch the dynamics of LC phase transitions directly. Further, we can use the same system to explore the dynamics of confined LCs in the presence of novel, custom surfactancts used to trigger phase transitions. |
Thursday, March 8, 2018 2:03PM - 2:15PM |
S57.00013: In situ characterization of sorption and diffusion in ionic liquids Alexandra Bayles, Connor Valentine, Matthew Helgeson, Todd Squires Ionic liquids possess unique physicochemical properties that make them advantageous for a broad range of applications including separation processes, homogeneous catalysis, and electrochemical devices. In these applications, the dynamics of molecular solute diffusion near interfaces plays a critical role, but is notoriously challenging to measure and model. To gain better mechanistic insight into transport in ILs, we developed a set of novel methods for characterizing solute transport across IL-fluid interfaces by measuring the spatiotemporal evolution of concentration fields using microfluidic Fabry-Perot interferometry. Here, we characterize the gradient-driven diffusion of water in methylimidazolium halide ILs. We find that the collective diffusivity is dominated by the molecular diffusivity of water, suggesting that the IL acts as an immobile matrix over time scales relevant for gradient diffusion. The results can be modeled by an activated diffusion process of water “hopping” between ion pairs, and the magnitude of the electrostatic activation barrier is consistent with the electronegativity of the anion. We anticipate that these results will help elucidate the influence of mesophase structure and concentrated ion effects on molecular transport in ILs and dense electrolytes. |
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