Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M09: Topological Defects in 3D Active and Equilibrium NematicsInvited Live
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Sponsoring Units: DSOFT Chair: Daniel Beller, University of California Merced; Miha Ravnik, Univ of Ljubijana |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M09.00001: Controlling topological defects within achiral and chiral nematics using surface geometry and anchoring Invited Speaker: Lisa Tran System boundary conditions are significant in determining the arrangement and dynamics of topological defects in passive nematics. In this talk, I will present two systems that demonstrate the use of boundary geometry to control defects. The first system comprises of an array of holes suspended within a planar nematic cell [1]. Each hole necessitates, topologically, the presence of an accompanying defect. Within an array, the defects within each hole can interact to create intricate defect structures, such as a network of disclination lines that span the system. These structures are determined by the saddle deformations of each hole, the elastic constants of the liquid crystal, and the anchoring of the cell. The second system confines chiral nematics into spherical shells. Adjusting the shell geometry as well as the type and strength of anchoring at the shell surfaces determines the kind and assembly of chiral nematic defects [2]. Outside of equilibrium configurations, I will present recent results where dynamically changing the surface anchoring can undulate the chiral nematic to produce defects, in a manner reminiscent of the classic Helfrich-Hurault instability [3]. Topological defects in passive nematics can be finely tuned through the command of surface boundary conditions using techniques and principles that could also translate for active nematic systems. |
Wednesday, March 17, 2021 12:06PM - 12:42PM Live |
M09.00002: Flows in 3D active nematic systems Invited Speaker: Aparna Baskaran In the well studied 2D active nematic systems there is a well known correlation between the statistical properties of the topological structures and the flow structures in the system, given by the defect densities and vortex statistics. These properties are controlled by the intrinsic active length scale that is proportional to the nematic elasticity and inverse of the activity. In 3D systems, the system size becomes important and the observed phenomenology of the flow is determined by the system size that enters due to viscosity and the active length scale that is set by the nematic properties. In this talk, I will illustrate this fact in the context of the linearized hydrodynamic equations and numerical solutions of the full hydrodynamic theory in channel geometries. |
Wednesday, March 17, 2021 12:42PM - 1:18PM Live |
M09.00003: Geometry and mechanics of disclination lines in 3D nematic liquid crystals Invited Speaker: Jonathan Selinger In 3D nematic liquid crystals, disclination lines have a range of geometric structures. Locally, they may resemble +1/2 or -1/2 defects in 2D nematic phases, or they may have 3D twist. Here, we analyze the structure in terms of the director deformation modes around the disclination, as well as the nematic order tensor inside the disclination core. Based on this analysis, we construct a vector to represent the orientation of the disclination, as well as tensors to represent higher-order structure. We apply this method to simulations of a 3D disclination arch, and determine how the structure changes along the contour length. We then use this geometric analysis to investigate three types of forces acting on a disclination: Peach-Koehler forces due to external stress, interaction forces between disclination lines, and active forces. These results apply to the motion of disclination lines in both conventional and active liquid crystals. |
Wednesday, March 17, 2021 1:18PM - 1:54PM Live |
M09.00004: Dynamics of Active Defect Loops Invited Speaker: Gareth Alexander We describe the flows and morphological dynamics of topological defect lines and loops in three-dimensional active nematics and show, using theory and numerical modeling, that they are governed by the local profile of the orientational order surrounding the defects. Analyzing a continuous span of defect loop profiles, ranging from radial and tangential twist to wedge ±1/2 profiles, we show that the distinct geometries can drive material flow perpendicular or along the local defect loop segment, whose variation around a closed loop can lead to net loop motion, elongation, or compression of shape, or buckling of the loops. We demonstrate a correlation between local curvature and the local orientational profile of the defect loop, indicating dynamic coupling between geometry and topology. To address the general formation of defect loops in three dimensions, we show their creation via bend instability from different initial elastic distortions. |
Wednesday, March 17, 2021 1:54PM - 2:30PM Live |
M09.00005: Topological structure and dynamics of three-dimensional active nematics Invited Speaker: Guillaume Duclos Topological structures are effective descriptors of the nonequilibrium dynamics of diverse many-body systems. For example, motile, point-like topological defects capture the salient features of two-dimensional active liquid crystals composed of energy-consuming anisotropic units. We dispersed force-generating microtubule bundles in a passive colloidal liquid crystal to form a three-dimensional active nematic. Light-sheet microscopy revealed the temporal evolution of the millimeter-scale structure of these active nematics with single-bundle resolution. The primary topological excitations are extended, charge-neutral disclination loops that undergo complex dynamics and recombination events. Our work suggests a framework for analyzing the nonequilibrium dynamics of bulk anisotropic systems as diverse as driven complex fluids, active metamaterials, biological tissues, and collections of robots or organisms. |
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