Orientational instabilities in nematic liquid crystals with weak anchoring under combined action of steady flow and external fields

Conventional nematic liquid crystal cells are fabricated with small surface pretilt of the director induced by rubbed polymer alignment. Depending on the orientation of the bounding surfaces, this may lead to two slightly different untwisted director configurations, splay and parallel. This small difference leads to remarkably different director profiles during pressure-driven flow, observed here using optical conoscopy. Data show excellent agreement with numerical modeling from Leslie-Ericksen-Parodi theory.

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“…Under pressure-driven flow, a distinctly different, often symmetric about the cell midplane, velocity distribution is observed [15] (see Fig. 2).…”

mentioning

confidence: 94%

Small Surface Pretilt Strikingly Affects the Director Profile during Poiseuille Flow of a Nematic Liquid Crystal

2010

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“…6,7 Such shear flow experiments regularly involve the flowing of a liquid crystal sample between two glass plates by way of dragging one plate relative to the other at a constant velocity, producing a linear velocity distribution across the depth of the cell. In the Poiseuille flow study presented here, a distinctly different flow distribution is observed ͑particularly at low rates͒, 8 a parabolic distribution of flow speeds symmetrical about d / 2 ͓see Fig. 1͑a͔͒, characteristic of any classical pressure gradient flow regime.…”

mentioning

confidence: 99%

Conoscopic observation of director reorientation during Poiseuille flow of a nematic liquid crystal

Holmes

Cornford²,

Sambles

2009

Applied Physics Letters

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Director reorientation under pressure driven ͑Poiseuille͒ flow is observed conoscopically for the liquid crystal 5CB aligned at an azimuthal angle of 45°to the direction of flow. A polyimide surface treatment ͑AL 1254͒ is used to promote planar homogeneous alignment and rubbed to produce an initial azimuthal alignment angle 0 . Conoscopic interference figure rotation is documented as a function of flow rate and compared to that produced from numerical models using Leslie-EricksenParodi theory. Model and data show excellent agreement. © 2009 American Institute of Physics. ͓doi:10.1063/1.3251792͔The viscodynamics of pressure induced ͑Poiseuille͒ flow in nematic liquid crystals are important in many technological applications. These range from the use of low molar mass liquid crystals in the manufacture of liquid crystal displays to liquid crystal polymer ͑LCP͒ injection molding used to spin textile fibers.1 Director orientation and flow channel dimensions for fabrication techniques such as LCP injection molding can often play an important role in the order parameter 2 and the number of defects in finished products. Other studies have examined nematic flow alignment in samples with colloidal dispersions and the topological defects associated with them, as commonly found in foods, paint and drugs. To date, much work has been carried out in examining director reorientation and measurement of various viscoelastic properties of nematic liquid crystals under shear induced ͑Couette͒ flow 4,5 including work involving the shearing of nematic liquid crystals under varying initial azimuthal ͑͒ alignment conditions. 6,7 Such shear flow experiments regularly involve the flowing of a liquid crystal sample between two glass plates by way of dragging one plate relative to the other at a constant velocity, producing a linear velocity distribution across the depth of the cell. In the Poiseuille flow study presented here, a distinctly different flow distribution is observed ͑particularly at low rates͒, 8 a parabolic distribution of flow speeds symmetrical about d / 2 ͓see Fig. 1͑a͔͒, characteristic of any classical pressure gradient flow regime. Such a symmetric flow speed distribution results in differing azimuthal ͑-rotation͒ and tilt ͑-rotation͒ ͓see Fig. 1͑b͔͒ distortions through the depth of the sample compared to that of the linear velocity distribution produced from shear flow.The dynamics of anisotropic fluid flow are inherently complex. This is due to the need for consideration of both the translational and orientational order of the constituent molecules under flow. Liquid crystals undergoing flow are well described by the continuum theory proposed by that of Leslie 9 and Ericksen, 10 with five independent ͑Parodi 11 ͒ viscosity coefficients ͑␣ 1 ...␣ 5 ͒ determining director orientation under perturbation from an external source. Such a theory ͑for the ideal case of no boundary effects͒ proposes that flow aligning liquid crystals ͑␣ 3 / ␣ 2 Ͼ 0͒ rotate to achieve a constant angle L ͑Leslie angle͒ out of the shear plane, a...

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“…The theoretical description taking into account weak anchoring was developed in the past decade [69,[77][78][79][80][81][82]. It is obvious that the effects of weak anchoring (see Chapter 5) have to modify both static and dynamic behavior of liquid crystals as in the case of electric fields.…”

Section: Shear Flows At Weak Anchoringmentioning

confidence: 99%

“…Recently, the influence of weak anchoring on nonlinear phenomena induced by shear flows in nematic liquid crystals was considered in some publications [80][81][82]. The developed theoretical description is restricted to the simplest type of instabilities arising under steady flows (plane Couette or Poiseuille flows) in the samples with the initial planar orientation normal to the flow plane (see Sections 3.3.1 and 3.3.2).…”

Section: Hydrodynamic Instabilities At Weak Anchoringmentioning

confidence: 99%

“…The solution of the nematodynamic equations is essentially simplified for this case due to a homogeneous orientation and linear (Couette flow) or parabolic (Poiseuille flow) velocity profile corresponding to the linear basic state. A detailed analysis of the problem in the case of additional action of magnetic (electric) fields can be found in [82]. Moreover, in the case of Poiseuille flow, the variation of the anchoring strength can produce a crossover between homogeneous or spatially periodic (roll) instabilities, at least for MBBA.…”

Section: Hydrodynamic Instabilities At Weak Anchoringmentioning

confidence: 99%

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Flows of Anisotropic Liquids

2009

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j45 crystals, presented in Chapter 2. The connection between translation motions and the orientational ones described accordingly in terms of a velocity field v(r, t) and a director field n(r, t) can be considered as a fundamental physical property of liquid crystals. It is responsible for a number of specific phenomena existing in liquid crystal media. Some of them, such as various instabilities induced by electric fields in thin layers of nematic liquid crystals or backflow effects arising at the turning off electric fields are of great practical importance (Chapter 6).The connection between v(r, t) and n(r, t) also results in a number of linear and nonlinear phenomena in flows of liquid crystals. In general, liquid crystals show non-Newtonian rheological behavior. It means, for example, that apparent shear viscosity depends on the shear rate. At the same time, strong magnetic (electric) fields allow to stabilize orientational structure. In thin layers of LC, the proper surface treatment can also provide the given orientation (e.g., planar or homeotropic, see Chapter 2). Liquid crystal with a stabilized orientational structure can be considered a conventional Newtonian fluid with shear viscosity determined by a field direction [1] or by a surfaceinduced direction. Such rheological behavior has to be taken into account at viscometric studies of liquid crystals. It will be described in detail in Chapter 5.A number of interesting flow-induced phenomena in smectic A and C phases are out of scope of this book. A reader can find description of such effects in Oswald and Pieranski [2] and in original papers [3,4]. We also do not consider rheological behavior of polymeric liquid crystals, lyotropic liquid crystals, and active liquid crystals. The last two classes of LC materials are of interest for understanding very complicated phenomena existing in living nature. In particular, active liquid crystals show unusual rheological properties that demand a special consideration [5][6][7].

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Orientational instabilities in nematic liquid crystals with weak anchoring under combined action of steady flow and external fields

Cited by 5 publications

References 15 publications

Small Surface Pretilt Strikingly Affects the Director Profile during Poiseuille Flow of a Nematic Liquid Crystal

Small Surface Pretilt Strikingly Affects the Director Profile during Poiseuille Flow of a Nematic Liquid Crystal

Conoscopic observation of director reorientation during Poiseuille flow of a nematic liquid crystal

Flows of Anisotropic Liquids

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