Electrorheological effect in the nematic phase of 4-<i>n</i>-pentyl-4′-cyanobiphenyl

Negita, Keishi

doi:10.1063/1.472564

Cited by 74 publications

(55 citation statements)

References 26 publications

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“…The situation however changes as the temperature is reduced: The degree of order becomes larger again, and the effective viscosity is now determined by the relative orientation between the director and the flow. Some interesting observations on the rheology of nematic 5CB under the application of an electric field have been reported by K. Negita [211]. Investigations for a more detailed understanding of the flow behaviour at the nematic-to-isotropic transition are underway.…”

Section: Nematic Flow Due To Pressure Gradientmentioning

confidence: 78%

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: Nematic Flow Due To Pressure Gradientmentioning

confidence: 78%

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Sengupta

Herminghaus

Bahr

2011

Molecular Crystals and Liquid Crystals

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“…Because of the richness of its bifurcation phenomena, EHC has received considerable attention over the past 2 decades [2]. In the field of rheology, the electrorheological (ER) effect in liquid crystals-namely the reversible change of apparent viscosity due to the application of an electric field-is of particular interest in terms of its mechanical applications [3][4][5][6][7][8][9][10][11][12][13][14]. The viscosity of rodlike nematics under a shear flow varies depending on the relative orientation of the director n (average direction of the long axis of molecules), the flow direction and the velocity gradient of the flow, which is known as the anisotropy of viscosity [15].…”

Section: Introductionmentioning

confidence: 99%

“…Normally, positive ER effect, i.e., the increase of viscosity * nagaya@oita-u.ac.jp † nara@elec.okayama-u.ac.jp ‡ yhna@hnu.kr § orihara@eng.hokudai.ac.jp by the electric field, is hoped for and η 1 is larger than η 2 for the most rodlike nematics. Accordingly, liquid crystals of positive dielectric anisotropy have predominantly been examined [3][4][5][6][7][8][9][10][11][12] because the increase of viscosity from η 2 to η 1 seems to be obtained easily when the electric field is applied on the liquid crystal perpendicularly to the flow direction. Although the magnitude of the ER effect for lowmolecular-weight nematics with positive dielectric anisotropy is not enough for general mechanical applications except for micromachines, liquid crystal polymer dissolved in a nematic solvent [10] and LC polymer solution [4] show potential for exhibiting a substantial ER effect.…”

Section: Introductionmentioning

confidence: 99%

Apparent viscosity ofp-methoxybenzylidene-p′-n-butylaniline in the presence of electrohydrodynamic convection

et al. 2013

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The apparent shear viscosity of p-methoxybenzylidene-p -n-butylaniline in the presence of electrohydrodynamic convection (EHC) is investigated experimentally. In the absence of an electric field, directors are almost aligned along the flow direction such that the viscosity is close to the minimum of the Miesowicz viscosities. Since EHC disturbs the flow-aligned director configuration, the viscosity increases as the applied voltage is increased in the low-voltage regime. In the high-voltage regime, however, further increasing the voltage leads to a decrease in viscosity. Microscope observations using a rheometer reveal that the decrease in viscosity occurs in the dynamic scattering mode 2 (DSM2) state, whose spatial director distribution is anisotropic due to the shear flow. By adopting the Ericksen-Leslie theory for the shear flow under the electric field, we find that the viscosity decrease can be attributed to the negative contribution of the electric stress caused by the anisotropic director distribution of the DSM2 state.

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“…Further, there have been reports of the effect on the steady shear viscosity of temperature (23) . The mechanism behind the field-induced viscosity increase in liquid crystals is thought to be that the individual liquid crystal molecules possess a dipole moment, and this tends to align in the electric field direction (as illustrated schematically in Fig.…”

Section: Introductionmentioning

confidence: 99%

“…This viscosity change can also be expressed through the Miesowicz viscosities (24) . Theoretical models of the electro-rheological response of liquid crystals have been developed by Negita (23) (a modified Leslie-Ericksen approach), and by Reyes et al (25) (a hydrodynamic model of a nematic system confined in a rectangular cell).…”

Section: Introductionmentioning

confidence: 99%

Electro-Rheological Response of Liquid Crystals under Oscillatory Squeeze Flow

Narumi

See

Yamaguchi

et al. 2005

JSME international journal. Ser. B, Fluids and thermal engineer

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We have investigated the electro-rheological (ER) properties of liquid crystals in an oscillatory squeezing flow. A Micro Fourier Rheometer was utilized to estimate the complex viscosity and phase shift. A clear dip in the phase angle was observed, centred around a frequency which increased with electric field strength. It was found that this critical frequency was proportional to the inverse of the response time required for the liquid crystal molecules to align under the electric field. Similar ER effects to those observed under steady shearing were obtained in the oscillatory flow, but the ER effect at high frequency is smaller than that under no electric field. In other words, a negative ER effect was obtained under low electric fields. A rheological model for the dynamic response was obtained through the control theory and the transfer function thus derived will be useful for estimating the output response in a flow or motion control system.

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Electrorheological effect in the nematic phase of 4-n-pentyl-4′-cyanobiphenyl

Cited by 74 publications

References 26 publications

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Apparent viscosity ofp-methoxybenzylidene-p′-n-butylaniline in the presence of electrohydrodynamic convection

Electro-Rheological Response of Liquid Crystals under Oscillatory Squeeze Flow

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