Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R41: Liquid Crystalline Behavior at the Supramolecular Scale in Biopolymer and Colloidal SystemsInvited
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Sponsoring Units: GSOFT DBIO Chair: Margaret Gardel, University of Chicago Medical Center Room: LACC 502A |
Thursday, March 8, 2018 8:00AM - 8:36AM |
R41.00001: Morphology of Carbon Nanotube Liquid Crystalline Phases: Insights into Tactoids and Columnar Phase Invited Speaker: Vida Jamali Solutions of carbon nanotubes (CNTs) in chlorosulfonic acid (CSA) form liquid crystalline phase at high concentrations. CNTs offer a high degree of polydispersity in length which makes them an ideal candidate for testing theories of liquid crystals for polydisperse rod-like systems. Moreover, high concentration CNT-CSA solutions can be processed into macroscopic materials. The properties of the CNT-based macroscopic materials, however, depends on the morphology of the liquid crystalline dope. So far, most of the literature on the liquid crystals of CNTs has been focused on qualitative aspects of the phase behavior. Here, we present our recent results on quantitative understanding of the morphology of the liquid crystal phase and how it is affected by the CNT properties and concentration. Polarized optical micrographs of the semi-dilute solutions of long CNTs shows the formation of the bi-continuous phases, while short CNTs form spindle-shaped nematic droplets, known as tactoids, coexisting with the isotropic phase. Such nematic droplets often partially wet the solid glass surface of the capillary. We developed the first method for characterizing the line tension in lyotropic liquid crystal systems by studying the shape of the partially wetting droplets and comparing that with our theoretical predictions. Additionally, we show that higher concentration solutions of CNTs form a hexagonal columnar phase as revealed by small angle x-ray scattering results. This nematic-columnar phase transition occurs at much lower concentrations than what has been predicted theoretically. We attribute this early transition to steric forces rising from electrostatic repulsions between the CNTs. |
Thursday, March 8, 2018 8:36AM - 9:12AM |
R41.00002: Self-organization in active, anisotropic biopolymer droplets Invited Speaker: Kimberly Weirich Biological assemblies self-organize from macromolecules into assemblies with distinctive spatial structure that have cellular function rooted in spatial asymmetry. The anisotropy of the assemblies is derived from the anisotropy of the components, such as biopolymer filaments. We present a model system of the cross-linked biopolymer filament, actin, to experimentally investigate how anisotropy and enzymatic activity drive self-organization and shape changes in biological materials. When actin filaments are short, cross-linker induces the formation of anisotropic liquid droplets, or tactoids. The cross-linker acts as a cohesion and controls the tactoid shape and dynamics. Polymers of myosin, which are molecular motors that bind to and translocate actin filaments, localize to actin tactoids. We find that myosin polymers self-organize within the tactoid, cluster, and drive tactoid shape changes. We present simple continuum model of an adhesive particle imposing alignment in an anisotropic droplet to describe the experimentally observed motor organization and tactoid deformation. Our results demonstrate how anisotropy in biological materials directs self-organization and shape changes. |
Thursday, March 8, 2018 9:12AM - 9:48AM |
R41.00003: Geometric percolation in chiral nematic liquid crystals of hard particles Invited Speaker: Paul Van der Schoot In geometric percolation, two particles are connected if their shortest distance is smaller than a cut-off distance known as the connectedness criterion. Clusters are then defined as mutually connected particles. The average number of particles in a cluster increases with particle loading and diverges upon approach of the percolation threshold. The average cluser becomes system-spanning at the percolation threshold, and the physical properties of the dispersion change drastically beyond it. |
Thursday, March 8, 2018 9:48AM - 10:24AM |
R41.00004: Why aren't tissues like biopolymer networks? Packing colloidal particles in a fibrous matrix. Invited Speaker: Paul Janmey The stiffness of tissues in which cells are embedded has effects on cell structure and function that can act independently of or override chemical stimuli. Rheologic measurements of liver, brain, and other soft tissues over a range of shear, compressive, and elongational strains show that the viscoelastic response of these tissues differs from that of synthetic hydrogels that have similar elastic moduli when measured in the linear range. The shear moduli of soft tissues generally decrease with increasing shear or elongational strain, but they strongly increase under uniaxial compression. In contrast, networks of crosslinked collagen or fibrin soften under compression, but strongly increase shear modulus when deformed in shear or extension. The compression softening of fibrous networks converts to compression stiffening characteristic of tissues as cells or cell-sized volume-conserving inert beads are incorporated into the fibrous networks at sufficiently high volume fractions. Compression stiffening in fibrous networks does not require the inclusions to become close packed or jammed, but the volume occupied by the cells or beads constrains the relaxation modes of the fibers to limit soft bending modes and enhance stiff stretching modes. |
Thursday, March 8, 2018 10:24AM - 11:00AM |
R41.00005: Instabilities, Phase Transitions, and Thermodynamics of Active Matter Invited Speaker: Daniel Needleman The self-organization of the microtubule cytoskeleton underlies diverse cell biological processes, ranging from chromosome segregation to neuronal morphogenesis. In order to gain insight into these biological processes, and the properties of active matter more generally, we are studying the large-scale dynamics, mechanics, and thermodynamics of collections of microtubules and molecular motors in cell extracts and reconstituted systems of purified components. I will present our recent work characterizing spontaneous contractions, ordering, instabilities, and heat production in these systems. |
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