Annihilation dynamics of umbilical defects in nematic liquid crystals under applied electric fields

Dierking, Ingo; Marshall, Owen; Wright, John; Bulleid, Neil J.

doi:10.1103/physreve.71.061709

Cited by 55 publications

(54 citation statements)

References 25 publications

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“…Furthermore, Svenšek andŽumer showed that the backflow mechanism is responsible for changing the director relaxation completely [117]. Subsequently, the numerical results were verified experimentally for semi-integer [60] and integer [118,119] …”

Section: Topological Defects In Flowmentioning

confidence: 74%

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Sengupta

Herminghaus

Bahr

2011

Molecular Crystals and Liquid Crystals

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Where the mind is without fear and the head is held high;Where knowledge is free;Where the world has not been broken up into fragments by narrow domestic walls;Where words come out from the depth of truth;Where tireless striving stretches its arms towards perfection;Where the clear stream of reason has not lost its way into the dreary desert sand of dead habit;Where the mind is led forward by thee into ever-widening thought and actionInto that heaven of freedom, my Father, let my country awake.Rabindranath Tagore (1861Tagore ( -1941 To my parents, my brother, and all the sacrifices they made for me AbstractThe doctoral thesis presented here is one of the first systematic attempts to unravel the wonderful world of liquid crystals within microfluidic confinements, typically channels with dimensions of tens of micrometers. The present work is based on experiments with a roomtemperature nematic liquid crystal, 5CB, and its colloidal dispersions within microfluidic devices of rectangular cross-section, fabricated using standard techniques of soft lithography. To begin with, a combination of physical and chemical methods was employed to create well defined boundary conditions for investigating the flow experiments. The walls of the microchannels were functionalized to induce different kinds of surface anchoring of the 5CB molecules: degenerate planar, uniform planar, and homeotropic surface anchoring. Channels possessing composite anchoring conditions (hybrid) were additionally fabricated, e. g. homeotropic and uniform planar anchoring within the same channel. On filling the microchannels with 5CB in the isotropic phase, different equilibrium configurations of the nematic director resulted, as the sample cooled down to nematic phase. For a given surface anchoring, the equilibrium director configuration varied also with the channel aspect ratio. The static director field within the channel registered the initial conditions for the flow experiments. The static and i ii dynamic experiments have been analyzed using a combination of polarization, and confocal fluorescence microscopy techniques, along with particle tracking method for measuring the flow speeds. Additionally, dual-focus fluorescence correlation spectroscopy is introduced as a generic velocimetry tool for liquid crystal flows.The flow of nematic liquid crystals is inherently complex due to the coupling between the flow and the nematic director. The presence of the four confining walls and the nature of surface anchoring on them complicate the flow-director interactions further. In microchannels possessing degenerate planar anchoring, four different flow-induced defect textures were identified with increasing Ericksen number: π-walls, disclination lines pinned to the channel walls, disclination lines with one pinned and one freely suspended end, and disclination loops freely flowing in a chaotic manner. However, such textures and sequence of defects were not observed for flows within channels with homogeneous anchoring.Using experiments and numerical modeling...

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Section: Topological Defects In Flowmentioning

confidence: 74%

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Sengupta

Herminghaus

Bahr

2011

Molecular Crystals and Liquid Crystals

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“…The latter scaling exponent relating to defect annihilation has been confirmed experimentally -also using liquid crystals [8,9]. The scaling law of defect formation, eq.…”

mentioning

confidence: 78%

“…Further confirmation for the defect annihilation scaling was achieved using electric field induced defects generated in a nematic liquid crystal. [9] These so called umbilical defects [26][27][28] do not end in a singular point, but are otherwise very similar to the Schlieren defects. They only differ in so far as the director continuously tilts into the direction of the substrate normal, being parallel to it at the defect core, which is small, but of finite size.…”

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confidence: 99%

Kibble–Zurek Scaling during Defect Formation in a Nematic Liquid Crystal

Fowler

Dierking

2017

ChemPhysChem

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Symmetry breaking phase transitions are often accompanied by the formation of topological defects, like in cosmological theories of the early universe, superfluids, liquid crystals or solid state systems. This scenario is described by the Kibble-Zurek mechanism, which predicts respective scaling laws for the defect density ρ. One such scaling law suggests a relation ~ Q -1/2 with  Q the change of rate of a control parameter. In contrast to the scaling of the defect density during annihilation with ρ~t -1 , which is governed by the attraction of defects of the same strength but opposite sign, the defect formation process, which depends on the rate of change of a physical quantity initiating the transition, is only very scarcely investigated. We here use nematic liquid crystals as a different system to demonstrate the validity of the predicted scaling relation for defect formation. It is found that the scaling exponent is independent of temperature and material employed, thus universal, as predicted.Topological defects[1] are often generated following symmetry breaking phase transitions in many aspects of physics including cosmology, superfluids and liquid crystals. Kibble proposed a universal mechanism by which cosmological defects were formed in the early universe as it expanded and cooled shortly after the Big Bang[2.3]. During a cosmological symmetry breaking phase transition, uncorrelated domains with randomly determined order are produced which subsequently coalesce. Defects are formed that exist between two domains with non-compatible order. The initial density of defects is dependent on the rate at which domains grow which is limited by the finite speed of information transmission (causality). It became known as the "Kibble-Zurek mechanism" after Zurek suggested that

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“…Introduction -The understanding of ordering processes in condensed matter has been the focus of considerable research over the last decades [1][2][3][4][5][6][7][8][9][10][11]. One of the main aspects of the ordering process is the dynamical behavior of defects present in the material.…”

mentioning

confidence: 99%

“…In particular, optical textures of liquid crystals [23][24][25] are known to be an excellent system for studying the dynamical behavior of defects. In fact, there are several studies on the annihilation dynamics of defects in liquid crystals employing thermotropics [7,[26][27][28], thermotropic polyester materials [29][30][31] and lyotropic [32]. Moreover, the experimental analysis of annihilation processes has also motivated many numerical simulations [4,6,10,33,34].…”

mentioning

confidence: 99%

Antipersistent behavior of defects in a lyotropic liquid crystal during annihilation

et al. 2013

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We report on the dynamical behavior of defects of strength s = ± 1/2 in a lyotropic liquid crystal during the annihilation process. By following their positions using time resolved polarizing microscopy technique, we present statistically significant evidence that the relative velocity between defect pairs is Gaussian distributed, anti-persistent and long-range correlated. We further show that simulations of the Lebwohl-Lasher model reproduce quite well our experimental findings.

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Annihilation dynamics of umbilical defects in nematic liquid crystals under applied electric fields

Cited by 55 publications

References 25 publications

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Kibble–Zurek Scaling during Defect Formation in a Nematic Liquid Crystal

Antipersistent behavior of defects in a lyotropic liquid crystal during annihilation

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