Critical thickness of a hybrid aligned nematic liquid crystal cell

Barbero, Giovanni; Barberi, R.

doi:10.1051/jphys:01983004405060900

Cited by 144 publications

(150 citation statements)

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“…The parallel correlation length Ell for interface fluctuations, which is in principle measurable by scattering expeiments with momentum transfer parallel to the interface, follows from standard Omstein-Zernike arguments (7, 9) to be given by Ell = [ U~~/ V~(~) ] "~, where the primes denote derivatives with respect 3~eneralizations of [3] and [4] to account for anisotropy of the elastic constants are derived in ref. 17. These exhibit a slightly more complex angle-dependence than the present equations.…”

mentioning

confidence: 56%

On the free energy of nematic wetting layers

Sullivan¹,

Lipowsky²

1988

Can. J. Chem.

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. J. Chem. 66,553 (1988).The contributions to the free energy of a nematic wetting layer as a function of its thickness 1 are analyzed. The longest-range contribution is due to distortion of the nematic director across the film, resulting from different preferred molecular orientations at the two interfaces bounding the film. Van der Waals forces as well as the decaying tails of the interfacial order-parameter profiles yield contributions to the free energy of successively shorter range. These effects lead to crossovers between different scaling rtgimes for variation of the mean wetting-layer thickness with temperature. Experimental implications of the results are described. DONALD E. SULLIVAN, et REINHARD LIPOWSKY. Can. J. Chem. 66,553 (1988).On a ttudit l'influence de l'tpaisseur (I) de couches ntmatiques mouillantes sur l'tnergie libre. La contribution qui se fait sentir a la plus grande distance est d i e a une distorsion du directeur ntmatique a travers le film; cette distorsion rtsulte elle-m&me d'orientations moltculaires prtftrentielles difftrentes aux deux interfaces qui lient le film. Utilisant les forces de van der Waals ainsi que les profils de l'ordre interfaciallparam2tres, on a pu dtduire les contributions a l'tnergie libre pour des distances de plus en plus courtes. Ces effets conduisent a des croisements entre divers rCgimes dlCchelles pou la variation de l'tpaisseur moyenne de la couche mouillante avec la temperature. On dtcrit les implications exptrimentales de ces rtsultats.[Traduit par la revue]Consider a liquid crystalline material at a temperature T slightly above the transition temperature TNI for the onset of nematic (N) ordering. While the bulk of the material then exists as an isotropic (I) liquid phase with no long-range orientational order, some degree of local nematic order may be spontaneously induced at the interface with another phase a such as a solid substrate or the vapour in coexistence with the liquid. If, on lowering the temperature, the mean thickness i of that nematic layer diverges over microscopic length scales as T + TNI, then we say that the nematic phase completely wets the a-I interface. Experimental evidence of this effect has been seen for a number of nematic substances, at both solid-liquid (1-3) and liquidvapour (3-5) interface^.^ Entirely analogous wetting phenomena involving three distinct thermodynamic phases have recently come under extensive study for a variety of physical systems (7).In this note we draw attention to some novel critical properties characterizing the approach to complete wetting in the case of a nematic layer. In particular, we focus on the asymptotic scaling laws describing the divergence of the mean wetting-layer thickness in the limit that t = (T -TNI)/TNI goes to zero. Our approach is similar to ones used successfully to describe wetting in other systems (7-lo), based on analysis of the free energy u(1) per unit area of the layer as a function of its thickness 1, when the latter is viewed as a thermodynamic order parameter. This approac...

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mentioning

confidence: 56%

On the free energy of nematic wetting layers

Sullivan¹,

Lipowsky²

1988

Can. J. Chem.

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“…The director n is subjected to antagonistic boundary conditions n(x 1 , x 2 , 0) = ±e 1 , n(x 1 , x 2 , δ) = ±e 3 on the plates, and periodic boundary conditions n(0, x 2 , x 3 ) = n(l 1 , x 2 , x 3 ), n(x 1 , 0, x 3 ) = n(x 1 , l 2 , x 3 ) on the other faces. Similar problems have been considered by many authors using a variety of models (see, for example, [2,13,14,23,30,61,76]). In [6] it is explained how using a Landau -de Gennes model, or molecular dynamics simulations [77], leads for sufficiently small plate separation δ to a jump in the director (defined as in Section 3.2 as the eigenvector of Q corresponding to it largest eigenvalue).…”

Section: Order Reconstructionmentioning

confidence: 98%

Liquid Crystals and Their Defects

Ball

2017

Mathematical Thermodynamics of Complex Fluids

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“…In the limit of strong anchoring, the extrapolation length is of the order of few nanometers. However, for surfaces possessing weak anchoring, this length may go up to few micrometers [69].…”

Section: 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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Critical thickness of a hybrid aligned nematic liquid crystal cell

Cited by 144 publications

References 7 publications

On the free energy of nematic wetting layers

On the free energy of nematic wetting layers

Liquid Crystals and Their Defects

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

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