Cooperative liquid-crystal alignment generated by overlaid topography

Yi, Yu Hyeon; Maclennan, Joseph E.; Clark, Noel A.

doi:10.1103/physreve.83.051708

Cited by 13 publications

(14 citation statements)

References 32 publications

(27 reference statements)

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“…The investigations hitherto cover a wide range of homeotropic confinements: LC cells [2,[11][12][13], capillaries [14][15][16][17], droplets [18,19], micro-cavities [20,21], and porous networks [22,23]. In spite of the practical relevance of confinements with rectangular cross-sections, especially microchannels, the resulting equilibrium of the nematic director has not been experimentally analyzed yet.…”

Section: Microchannels With Homeotropic Surface Anchoringmentioning

confidence: 99%

Nematic Liquid Crystals Confined Within a Microfluidic Device: Static Case

Sengupta

2013

Topological Microfluidics

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Section: Microchannels With Homeotropic Surface Anchoringmentioning

confidence: 99%

Nematic Liquid Crystals Confined Within a Microfluidic Device: Static Case

Sengupta

2013

Topological Microfluidics

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“…The investigations hitherto cover a wide range of homeotropic confinements: LC cells [178,[184][185][186], capillaries [99,[187][188][189], droplets [36,190], micro-cavities [125,191], and porous networks [192,193]. In spite of the practical relevance of confinements with rectangular cross-sections, especially microchannels, the resulting equilibrium of the nematic director has not been experimentally analyzed yet.…”

Section: Microchannels With Homeotropic Surface Anchoringmentioning

confidence: 99%

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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“…Email: Youngwoo.yi@colorado.edu patterns has been studied widely for thermotropic LCs [10][11][12]. Nanoimprinting can generate polymer films with topographic patterns in a simple and practical way and such films have been extensively used to demonstrate alignment of a variety of LCs in novel ways [13][14][15][16].…”

Section: Introductionmentioning

confidence: 99%

Orientation of chromonic liquid crystals by topographic linear channels: multi-stable alignment and tactoid structure

Clark

2013

Liquid Crystals

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We study the alignment behaviour of the chromonic liquid crystal phases of sunset yellow (SSY)/water, disodium cromoglycate (DSCG)/water and their mixtures when confined in cells by polymer films topographically imprinted with linear channels of 250 nm width, depth and spacing. A variety of novel alignment effects are generated by contact with such a patterned surface, as follows. Nematic DSCG and nematic SSY at low concentration and their nematic mixtures orient with the long axes of stacked chromonic aggregates on average parallel to the channels, that is, with the molecular planes normal to the channel axis. These oriented nematics exhibit isotropic/nematic tactoids aligned with their major axes along the channels. Two geometries of the tactoids, elliptic cylinder and rectangular cuboid with hemi cylindrical ends are observed. Additionally, a transition of DSCG tactoids from a three-dimensional (3D) director configuration to a two-dimensional one is observed when the tactoids on one surface of a cell touch the other surface of the cell. In SSY solutions of sufficiently high concentration, multi-stable alignment is found, including preferential in-plane orientation of the director parallel to, perpendicular to and 45 • rotated from the channels.

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Cooperative liquid-crystal alignment generated by overlaid topography

Cited by 13 publications

References 32 publications

Nematic Liquid Crystals Confined Within a Microfluidic Device: Static Case

Nematic Liquid Crystals Confined Within a Microfluidic Device: Static Case

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Orientation of chromonic liquid crystals by topographic linear channels: multi-stable alignment and tactoid structure

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