Flow-driven transition and associated velocity profiles in a nematic liquid-crystal cell

Jewell, S. A.; Cornford, Stephen; Yang, F.; Cann, P. S.; Sambles, J. R.

doi:10.1103/physreve.80.041706

Cited by 34 publications

(42 citation statements)

References 13 publications

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“…We confirm the transition reported by Denniston et al 18 and Jewell et al 19 , and find that it is driven by a dynamical instability, rather than free energy considerations. We also observe a very strong departure from the Poiseuille relation that does not relate to any obvious morphological transition.…”

Section: Introductionsupporting

confidence: 81%

“…Obviously, in the absence of flow, the director will uniformly orient itself in the z direction. Following the terminology of Jewell et al 19 , we call this configuration the 'vertical' state, V.…”

Section: Homeotropic Anchoringmentioning

confidence: 99%

“…Again following Jewell et al 19 we term this new state the horizontal state, H. The change V → H represents a topological change and cannot be effected by continuous deformations. As noted by Denniston et al 18 , the transition occurs by the formation of topological defects, which unwind to produce the new state.…”

Section: Homeotropic Anchoringmentioning

confidence: 99%

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The effect of anchoring on the nematic flow in channels

2015

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Understanding the flow of liquid crystals in microfluidic environments plays an important role in many fields, including device design and microbiology. We perform hybrid lattice-Boltzmann simulations of a nematic liquid crystal flowing under an applied pressure gradient in two-dimensional channels with various anchoring boundary conditions at the substrate walls. We investigate the relation between flow rate and pressure gradient and the corresponding profile of the nematic director, and find significant departures from the linear Poiseuille relation. We also identify a morphological transition in the director profile and explain this in terms of an instability in the dynamical equations. We examine the qualitative and quantitative effects of changing the type and strength of the anchoring. Understanding such effects may provide a useful means of quantifying the anchoring of a substrate by measuring its flow properties.

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Section: Introductionsupporting

confidence: 81%

Section: Homeotropic Anchoringmentioning

confidence: 99%

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The effect of anchoring on the nematic flow in channels

2015

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“…Due to lack of adequate fabrication techniques previously, the fundamental experimental studies by Pieranski et al were conducted within confinements with only one characteristic length, i. e. the channel depth, typically few hundred micrometers. A set of promising experiments investigating the effect of confinement and surface properties were initiated by Sambles and co-workers in Exeter, UK [42][43][44] using wide channels (width depth ≈ 30 µm).…”

Section: Motivationmentioning

confidence: 99%

“…It is only very recently that experiments on backflow dynamics in microfluidic setups are being carried out: reporting on flow induced transitions [42,43], effects of molecular pretilt at confining surfaces [44], nematic textures in untreated channels (see (1) on the publication list), and electrically or mechanically modulated microflows [41,232].…”

Section: Nematic Flow In a Homeotropic Microchannelmentioning

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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Introduction

Sengupta¹

2013

Topological Microfluidics

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Flow-driven transition and associated velocity profiles in a nematic liquid-crystal cell

Cited by 34 publications

References 13 publications

The effect of anchoring on the nematic flow in channels

The effect of anchoring on the nematic flow in channels

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

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