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Center for the Science and Technology of Advanced Materials and Interfaces

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  • Item
    Defect Unbinding in Active Nematic Tori
    (Georgia Institute of Technology, 2018-05-15) Fernandez-Nieves, Alberto ; Community for Research on Active Surfaces and Interfaces ; Center for the Science and Technology of Advanced Materials and Interfaces
    We will discuss our recent results with active nematics on toroidal surfaces and show how, despite the intrinsic activity and out-of-equilibrium character of our system, we still observe remnants of the expected curvature-induced defect unbinding predicted for nematics in their ground state. In our experiments, however, the number of defects is far larger than what one would expect for conventional nematics. In addition, these defects move throughout the toroidal surface and explore "phase space", bringing about interesting analogies with what we could call the high-temperature limit of a nematic liquid crystal. We unravel the role of activity by comparing our results to numerical simulations. Overall, our results illustrate the interplay between order, topological constraints, local geometry and activity.
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    Observation of Optical Weyl Points and Other Topics in Topological Photonics
    (Georgia Institute of Technology, 2018-05-15) Rechtsman, Mikael C. ; Community for Research on Active Surfaces and Interfaces ; Center for the Science and Technology of Advanced Materials and Interfaces
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    Disorder-Order Transitions in p-Conjugated Polymers
    (Georgia Institute of Technology, 2016-12-02) Köhler, Anna ; Center for Organic Photonics and Electronics ; Center for the Science and Technology of Advanced Materials and Interfaces ; Universität Bayreuth
    The aggregation of p-conjugated materials significantly impacts on the photophysics, and thus on the performance of optoelectronic devices. Nevertheless, we know comparatively little about the laws governing aggregate formation of p-conjugated materials from solution. In this talk, I shall compare, discuss and summarize how aggregates form for three different types of compounds, that is, homopolymers, donor-acceptor type polymers and low molecular weight compounds. To understand how aggregates form, we employ temperature dependent optical spectroscopy, which is a simple yet powerful tool for such investigations. I shall discuss how optical spectra can be analysed to identify distinct conformational states and to obtain quantitative information on changes in the inter-chain coupling, the conjugation length and the oscillator strength upon aggregate formation. We find aggregate formation to proceed alike in all these compounds by a coil-to-globule like first order phase transition. Notably, the chain expands before it collapses into a highly ordered dense state. I will address the role of side chains and the impact of changes in environmental polarization.
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    Mechanics of Active Networks – Lessons from Fire Ant Aggregations
    (Georgia Institute of Technology, 2018-04-19) Sridhar, Shankar Lalitha ; Vernerey, Franck ; Fernandez-Nieves, Alberto ; Shen, Tong ; Soft Matter Incubator ; Center for the Science and Technology of Advanced Materials and Interfaces ; University of Colorado Boulder
    Biological assemblies in nature are seen as active matter due to their ability to perform intelligent collective motion based on neighbor interactions and sometimes without any centralized control or leadership. Fire ants are a great example in this context and display a rich class of material behaviors, including elasticity, viscous flow, and self-healing. Although classical theories in mechanics have enabled us to mechanically characterize this system, there is still a gap in our understanding on how individual ant behavior affects the emerging response of the aggregation. I will discuss an alternative approach from a statistical perspective where the population distribution of ants evolves due to mechanical deformation, and individual ant’s leg detachment and attachment events. Numerical simulations of the aggregation’s response in diverse situations, such as jamming (density) and shear thinning (reduced viscosity) will be presented and compared to experimental measurements.
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    Topological Edge Floppy Modes in Disordered Fiber Networks
    (Georgia Institute of Technology, 2018-05-14) Mao, Xiaoming ; Community for Research on Active Surfaces and Interfaces ; Center for the Science and Technology of Advanced Materials and Interfaces
    Disordered fiber networks are ubiquitous in natural and manmade materials. The dilute nature of these networks permits floppy modes which only bend the fibers without changing their length, and these floppy modes govern mechanical response of the material. In this talk, we show that the geometry of the fiber network dictates the nature of these floppy modes. In particular, an ideal network in which all fibers are straight hosts bulk floppy modes, whereas perturbing the network geometry induces floppy modes exponentially localize on the edge of the network. Various activities present in fiber networks, such as active driving of motors in the cytoskeleton and actuators in manmade fiber networks, could lead to such edge floppy modes. We show that the localization of these edge floppy modes is protected by the topology of the phonon structure of the fiber networks, analogous to topological edge floppy modes in Maxwell lattices.
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    Unusual Director Configurations and Diffusion Driven by Liquid Crystal Elastic Anisotropy
    (Georgia Institute of Technology, 2018-04-18) Yodh, Arjun G. ; Soft Matter Incubator ; Center for the Science and Technology of Advanced Materials and Interfaces
    I will describe experiments that probe effects of twist elastic anisotropy in lyotropic chromonic liquid crystals (LCLCs) on the director configurations in cylinders/spheres and on particle diffusion. Time permitting, I will also describe measurements of LCLC “coffee rings” and of twist fluctuations in suspended spherical LC droplets.
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    How Do We Create and Process Materials for Flexible, Transparent Electronic Circuitry?
    (Georgia Institute of Technology, 2018-03-29) Marks, Tobin ; Center for Organic Photonics and Electronics ; Center for the Science and Technology of Advanced Materials and Interfaces ; Northwestern University (Evanston, Ill.)
    This lecture focuses on the challenging design, realization, understanding, and implementation of new materials families for unconventional electronics. Fabrication methodologies to achieve these goals include high-throughput, large-area, high-resolution printing techniques. Materials design topics will include: 1. Rationally designed high-mobility p- and n-type organic semiconductors for printed organic CMOS, 2. Self-assembled high-k nanodielectrics enabling ultra-large capacitance, low leakage, high breakdown fields, minimal trapped interfacial charge, and device radiation hardness, 3. Polycrystalline and amorphous oxide semiconductors for printable transparent and mechanically flexible electronics, 4. Combining these materials sets to fabricate a thin-film transistor-based circuitries, 5. The relevance of these advances to unconventional photovoltaics.
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    Workshop on Topological Protection in Messy Matter - Welcome and Overview
    (Georgia Institute of Technology, 2018-05-14) Rocklin, D. Zeb ; Community for Research on Active Surfaces and Interfaces ; Center for the Science and Technology of Advanced Materials and Interfaces
    The Workshop on Topological Protection in Messy Matter is sponsored by Georgia Tech's Community for Research on Active Surfaces and Interfaces (CRĀSI). The workshop will bring together distinguished researchers from diverse intellectual communities to present and moreover to develop new ways of exploiting topology in "messy" systems, including but not limited to disordered, fluid, amorphous, active, structured and quasicrystalline systems.
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    Active Fluids as Topological Metamaterials: Structure Without H Periodic Order
    (Georgia Institute of Technology, 2018-05-15) Souslov, Anton ; Community for Research on Active Surfaces and Interfaces ; Center for the Science and Technology of Advanced Materials and Interfaces
    Active liquids are composed of self-driven microbots that endow the liquid with a unique set of mechanical characteristics. We present two designs for topological states using active fluids: one using periodic confinement and another using a bulk fluid without periodic order. In a periodic lattice, geometry of confinement controls the structure of topological waves. Without periodic order, topological edge waves can arise in a fluid of self-spinning particles undergoing spontaneous active rotation. This can occur because a fluid undergoing rotation experiences a Coriolis force that breaks Galilean invariance and opens a gap at low frequency. Alternatively, such edge waves can arise due to a Lorentz force in a magnetized plasma. We explore the interplay of topological states and an anomalous response coefficient called odd (or Hall) viscosity. For large odd viscosity, this transverse response can be measured via the profile shape of topologically robust edge waves.
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    Why is Structural Hierarchy So Prevalent in Biological Materials?
    (Georgia Institute of Technology, 2018-04-19) Michel, Jonathan ; Yunker, Peter J. ; Soft Matter Incubator ; Center for the Science and Technology of Advanced Materials and Interfaces
    Structural hierarchy, in which materials possess distinct features on multiple length scales, is ubiquitous in nature. Many biological materials, such as bone, cellulose, and muscle, have as many as ten hierarchical levels. While structural hierarchy confers many mechanical advantages, including improved toughness and economy of material, it also presents a problem as each hierarchical level substantially increases the amount of information necessary for proper assembly. This seems to conflict with the broad prevalence of naturally occurring hierarchical structures. At the present, there is no general framework for understanding the interplay between structures on disparate length scales; such a framework is a critical tool for accounting for the robustness of hierarchical materials to defects. Here, we use simulations and experiments to validate a generalized model for the tensile stiffness of hierarchical, stretching-stabilized networks with a nested, dilute hexagonal lattice structure, and demonstrate that the stiffness of such networks becomes less sensitive to errors in assembly with additional levels of hierarchy. Following seminal work by Maxwell and others on criteria for stiff frames, we extend the concept of connectivity in network mechanics, and find a similar dependence of material stiffness upon each hierarchical level. More broadly, this work helps account for the success of hierarchical, filamentous materials in biology and materials design, and offers a heuristic for ensuring that desired material properties are achieved within the required tolerance.