Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G25: Liquid Crystals IIIRecordings Available
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Sponsoring Units: DSOFT Chair: Tim Atherton, Tufts Room: McCormick Place W-187A |
Tuesday, March 15, 2022 11:30AM - 11:42AM |
G25.00001: Dusty Plasma Liquid Crystal Analogues Eva Kostadinova, Emerson Gehr, Evie Guay, Lorin S Matthews, Truell W Hyde This study examines dusty plasma structures (mixtures of ions, electrons, neutrals, and micron-sized dust particles) using data from the Plasmakristall-4 (PK-4) facility on board the International Space Sstation. Dust grains immersed in plasma become negatively charged and can self-organize into strongly coupled fluids and crystalline structures. Microgravity dusty plasmas have been observed to form extended field-aligned filaments in the DC discharge of the PK-4 experiments, which have been compared to the filamentary state in electrorheological (ER) fluids. However, pair correlation functions and other structural analysis of the filamentary state of dusty plasma suggests that meaningful comparisons can be made between those structures and liquid crystals (LCs) with rod-shaped molecules. Specifically, we demonstrate that the coupling between dust particles within filamenrs is crystal-like, while the coupling across filaments is liquid-like. In addition to common orientation along a director axis (nematic behavior), the dust filaments also appear to align in large-scale nested structures, or shells (smectic behavior). In this talk we will demonstrate the liquid crystal properties of microgravity dusty plasma and propose ways to study universality of liquid crystal phase transitions and pattern formation using a dusty plasma analogue. |
Tuesday, March 15, 2022 11:42AM - 11:54AM |
G25.00002: Energetics of twisted spindle-shaped polymer nematics Helen S Ansell, Randall D Kamien During deswelling, initially spherical bipolar polymer microparticles with nematic ordering become elongated spindle shapes in which the polymer chains are observed to follow a twisted pattern. Using geometric arguments, we have previously shown that this twisting pattern follows curves that are well-described by a type of spiral called a loxodrome, for which the angle between the principal directions of the surface and tangent vector to the spiral is constant. In this talk, I will discuss an energetic model of this loxodrome twisting and show that the observed behavior can be captured by the Frank free energy along with an additional term that constrains the length of the integral curves in the system, a key constraint in the geometric model. I will demonstrate that the model predicts distinct regimes for the expected twisting behavior that depend on the relative values of the Frank elastic constants of the system. |
Tuesday, March 15, 2022 11:54AM - 12:06PM |
G25.00003: Inequalities for Elastic Constants in Nematic Liquid Crystals Cheng Long, Jonathan V Selinger In the early years of liquid crystal physics, Ericksen derived a set of inequalities for the elastic constants of nematic liquid crystals [1], which can be written as K11 – K24 > 0, K22 – K24 > 0, K33 > 0, and K24 > 0. However, recent experiments have shown that lyotropic chromonic liquid crystals violate the inequality on K22 – K24 [2]. Motivated by this discrepancy, we re-examine the inequalities, using a recent reformulation of elasticity theory [3]. This analysis shows that the Ericksen inequalities are stronger than necessary for thermodynamic stability. Rather, thermodynamic stability only requires the weaker set of inequalities K11 > 0, K22 > 0, and K33 > 0. In the regime between the strong and weak inequalities, the liquid crystal’s director field becomes nonuniform, but it is still thermodynamically stable. We compare these results with experiments on lyotropic chromonic liquid crystals. |
Tuesday, March 15, 2022 12:06PM - 12:18PM |
G25.00004: Smectic liquid crystal foam under confinement Changshuo Fu, Hongjie Lin, Mohamed Amine Gharbi |
Tuesday, March 15, 2022 12:18PM - 12:30PM |
G25.00005: Cholesteric Tactoids Eoghan Downey Chirality is a common propery of all kinds of materials from liquid crystals to polymers; of particular interest is how the chirality of a mesogenic unit imprints itself upon the higher order structures. For example, chiral polymers may form gyroid phases. Here we examine the problem of a determining the shape of droplet of chiral fluid: a cholesteric liquid crystal. We determine equilibrium structures for this system with a finite element approach as a function of the degree of chirality, measured by a cholesteric pitch. In doing so, we examine whether chirality can be transferred from a droplet to its shape. |
Tuesday, March 15, 2022 12:30PM - 12:42PM |
G25.00006: Manipulating Liquid Crystal Tactoids with Applied Fields Cole E Wennerholm, Tim Atherton, Eoghan Downey, Chaitanya S Joshi In contrast to isotropic fluids, liquid crystals can sustain elastic deformations and order-dependent surface tension. Hence, equilibrium shapes of liquid crystal droplets (known as a tactoid) are much richer than the spherical shapes adopted by isotropic fluids. Here, we consider the problem of a nematic liquid crystal droplet deformed by an imposed electric field. Finite element simulations are presented to determine the droplet shape as a function of the electric field, anchoring conditions and the elastic properties of the material. Our simulation method allows us to features like cusps that are challenging with other methods. Prospects for simulation of tactoids involving other kinds of liquid crystal phase will also be discussed. |
Tuesday, March 15, 2022 12:42PM - 12:54PM |
G25.00007: Hydrodynamics in quasi 2D liquid crystal films - lattice structures, coalescence & coarsening - Christoph Klopp Hydrodynamic phenomena in thin films play a crucial role in biological systems, in nature and modern technology. Various experimental and theoretical studies explored the motion of objects in two dimensional films [1,2], the merging behavior of objects in different systems [3,4] and the structural change of emulsions during long-term observations [5]. |
Tuesday, March 15, 2022 12:54PM - 1:06PM |
G25.00008: Flow-induced order–order transitions in amyloid fibril nematic and cholesteric tactoids Hamed Almohammadi, Massimo Bagnani, Raffaele Mezzenga Liquid crystalline droplets, also known as tactoids, forming by nucleation and growth within the phase diagram region where isotropic and nematic phases coexist, challenge our understanding of liquid crystals under confinement due to anisotropic surface boundaries at vanishingly small interfacial tension, resulting in complex, non-spherical shapes. Little is known about their dynamical properties, since they are mostly studied under quiescent, quasi-equilibrium conditions. Here we show that different classes of amyloid based nematic and cholesteric tactoids undergo order–order transitions by flow-induced deformations of their shape. Tactoids align under extensional flow, undergoing extreme deformation into highly elongated prolate shapes, with the cholesteric pitch decreasing as an inverse power-law of the tactoids aspect ratio. Free energy functional theory and experimental measurements are combined to rationalize the critical elongation above which the director-field configuration of tactoids transforms from bipolar and uniaxial cholesteric to homogenous and to debate on the thermodynamic nature of these transitions. |
Tuesday, March 15, 2022 1:06PM - 1:18PM |
G25.00009: Chiral domains in a flowing achiral nematic liquid crystal Qing Zhang, Rui Zhang, Shuang Zhou, Irmgard Bischofberger An achiral nematic liquid in a microfluidic cell could be expected to form achiral structures in a pressure-driven flow. Surprisingly, we find the spontaneous emergence of chiral structures when an achiral lyotropic chromonic liquid crystal in the nematic phase relaxes from a high velocity flow to a steady-state lower velocity flow. The chirality results from a periodic double-twist deformation of the liquid crystal and leads to striking stripe patterns vertical to the flow direction. We show that the double-twist deformation is triggered at regions of biaxial-splay deformation which becomes unstable. The transition from this biaxial-splay deformation to the double-twist deformation can be rationalized by the low Frank–Oseen elastic energy stored in double-twist deformations compared to that stored in biaxial-splay deformations. We show that the size of the chiral structures can be tuned by the local flow velocity and is set by the competition between the elastic torque and the viscous torque acting on the structures. |
Tuesday, March 15, 2022 1:18PM - 1:30PM |
G25.00010: Templating polymer networks with chiral liquid crystals Zhaofei Zheng, Francesca Serra Surface anchoring and geometry of the boundary strongly influence the configuration of liquid crystals (LCs). These properties not only affect the LC molecules’ alignment near the surface but also include the topological property in the bulk. Instead of manipulating only the external surface, a useful internal ‘surface’ can be introduced by using polymerizable monomers as a host LC. |
Tuesday, March 15, 2022 1:30PM - 1:42PM |
G25.00011: Self-assembled nanoparticle capsules and their aggregation in anisotropic liquid crystal solvent Devika Gireesan Sudha, Jocelyn Ochoa, Linda S Hirst The mutual attraction between colloidal particles in an anisotropic fluid, such as the nematic liquid crystal phase can lead to the formation of hierarchical aggregate morphologies distinct from those that tend to form in isotropic fluids. Previously it was prohibitive to study this aggregation process for a large number of colloids due to the difficulty of achieving a well dispersed initial colloid distribution. I will be talking about a self-assembling colloidal system, recently developed by our group, to investigate this process. Hollow, micron-scale colloidal capsules are formed in-situ in the nematic phase and subsequently aggregate to produce fractal structures and dense gels, the structure of which is determined by capsule concentration and temperature quench depth through the isotropic to nematic phase transition point. This self-assembling colloidal system provides a unique method to study particle aggregation in liquid crystal over large length scales. We use confocal fluorescence microscopy over a wide range of length scales to measure aggregate structure as a function of temperature quench depth, observe ageing and explore the driving mechanisms in this unique system. Our analysis reveals that aggregate dynamics depend on a combination of Frank elasticity relaxation, spontaneous defect line annihilation and internal fracturing. |
Tuesday, March 15, 2022 1:42PM - 1:54PM |
G25.00012: Topological dereliction in liquid crystal-mediated nanoparticle assembly on spherical droplets Viviana Palacio-Betancur, Julio C Armas-Perez, Juan P Hernandez-Ortiz, Juan De Pablo A liquid crystal droplet with planar anchoring offers precise control over the assembly of nanoparticles. The two antipodal defects are sites with high elastic energy, and adsorbed particles alleviate nematic distortions. As the number of particles increases, the multitude of possible arrangements opens the portfolio of metastable arrays. We study the interplay between LC-mediated forces and entropic frustration of nanoparticle assembly on a bipolar droplet via computational methods. The LC is described in the framework of the Landau-de Gennes formalism, and a hybrid relaxation is carried out: a Metropolis sampling algorithm explores the order tensor Q and particles location, while a Ginzburg-Landau relaxation establishes the equilibrium configuration inside the droplet. Strongly anchored homeotropic particles reduce the global free energy by ∽103 kT when adsorbed at the defects. Through this algorithm, we find a bifurcation on the free energy surface as a function of the number of particles N. At N>5, an onset of entropic frustration aids the particle assembly resulting in kinetically trapped states. These results explain previous experimental observations in micron sized droplets. Futhermore, this algorithm can be generalized to other systems prone to free energy entrapment. |
Tuesday, March 15, 2022 1:54PM - 2:06PM |
G25.00013: The simulation of three-dimensional solitary waves driven by an alternating current electric field in nematic liquid crystal Xingzhou Tang, Ali Mozaffari, Noe d Atzin, Nicholas Abbott, Juan De Pablo Recent experiments reveal that in nematic liquid crystal, bullet-like three-dimensional solitary waves have been observed when electrically driven [1]. Being powered by an alternating current (AC) electric field, the bullet is a nonsingular bow-like perturbation of the director from the uniform state or form a butterfly-like structure. |
Tuesday, March 15, 2022 2:06PM - 2:18PM |
G25.00014: Oscillatory Shear Response of Freely Suspended Films of Lyotropic Chromonic Liquid Crystal Angel Martinez, Benjamin Kauffman, Claire Olney, Mara Niesyt, Rebecca Sipen, Pamela McKnight, Xiang Cheng, Nathan C Keim Lyotropic chromonic liquid crystals (LCLCs) have received a wide interest in recent years because their fundamental viscoelastic properties are not well understood and they have potential in new bio-sensing and optics applications. They are composed of plank-like molecules that self-assemble into cylindrical aggregates when in water. Their nematic phase has a huge viscoelastic anisotropy, exhibiting fascinating behavior like spontaneous chiral symmetry breaking and colloidal sub-diffusivity. Here, we study the structural response of freely suspended, thin LCLC films to oscillatory shear using nematic DSCG at 10 to 16 wt. % concentration. Oscillating shear is generated via forcing of a suspended magnetic needle. Unlike previous shearing experiments, the fluid is weakly confined at the scale of LC defects and we image in the plane of the quasi-2D shear flow. At small strain amplitude the flow is largely reversible, while at large amplitude, defect structures store elastic energy and are advected away from the needle, reminiscent of elastic turbulence. Understanding these materials under large strain and weak confinement is a step toward bulk processing, enabling applications such as paintable polarizers and deformable optics. |
Tuesday, March 15, 2022 2:18PM - 2:30PM |
G25.00015: Amphibious Transport of Fluids and Solids by Soft Magnetic Carpets Ahmet F Demiroers, Joost de Graaf, Arnold J Mathijssen |
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