Undulating paired disclinations (oily streaks) in lyotropic liquid crystals

Schneider, M. B.; Webb, Watt W.

doi:10.1051/jphys:01984004502027300

Cited by 30 publications

(19 citation statements)

References 30 publications

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“…Conversely, if B is too small, the oily streaks split to focal domains of large eccentricity or do not split at all, showing at most a series of transversal striations as in the model of Schneider and Webb. 27 Likewise, we see an increase in the number of focal conic domains of the second kind, in samples with polymer. Since these defects deform the surrounding region, costing energy of compression given by W, -BR;X(Rl/R, + R,/R1) ( 5 ) it gives a further indication that B decreases.…”

Section: Discussionmentioning

confidence: 89%

Polymer in a lyotropic lamellar system: an experimental study

Singh¹,

Ober²,

Kléman³

1993

J. Phys. Chem.

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We have used a dilute solution in brine of a neutral, flexible polymer, polyacrylamide (nonhydrolyzed), as the solvent of a quasi-ternary system containing CPCl (cetylpyridinium chloride), and hexanol. The phase diagram is not fundamentally modified by the addition of the polymer, but the polymer manifests its presence, specially in the more dilute samples (i-e., samples with higher volume fraction of the solvent), by a strong decrease of the interlamellar spacing of the La phase, as is seen by small-angle X-ray scattering experiments. Microscopic studies of the textures show oily streaks whose typical mode of splitting does not display well-defined elliptical focal domains and also shows an increase in the number of focal conic domains of the second kind, i.e., typical defects with a positive gaussian curvature; both of these facts suggest that the modulus of compressibility, B, and the saddle-splay rigidity, E, are smaller than in samples without polymer. We present qualitative arguments attributing some of these effects and the decrease of the periodicity, to the osmotic pressure gradient introduced by the presence of the polymer molecules which enter the layers in the form of a 2D solution, which causes the membranes to be drawn together. We have no clear understanding up to now of the behavior of K and K.

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Section: Discussionmentioning

confidence: 89%

Polymer in a lyotropic lamellar system: an experimental study

Singh¹,

Ober²,

Kléman³

1993

J. Phys. Chem.

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“…Rather the defects shown in Fig. 3(b) have an appearance similar to oily streaks found in the L a phase of a lyotropic system first discovered by Schneider and Webb [12]. The quiescent zigzag pattern observed by these authors was explained as the transverse undulation of paired 11͞2 disclination cores.…”

mentioning

confidence: 87%

“…Though the defect lines have the appearance of "oily streaks" which are commonly seen in quiescent layered LCs, they were here found to be transient and relaxed shortly after switching off the shear. It can be argued that because of this similarity in structure, the defect lines in our films are a result of a binding transition of dislocation lines [10][11][12] which will be discussed in detail later in this paper.…”

mentioning

confidence: 99%

A Shear-Induced Instability in Freely Suspended Smectic-ALiquid-Crystal Films

Dash¹,

Wu²

1997

Phys. Rev. Lett.

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This experiment examines the stability of a smectic-A liquid-crystal film subject to a uniform shear flow with velocity and velocity gradient in the plane of the smectic layers. The experiment is made possible by virtue of a freely suspended liquid-crystal film which is sheared at its edge by a Couette apparatus. It was found that while there was no shear-induced phase transition, line defects were nucleated in the film when the shear rate ᠨ g attained a certain threshold. The critical shear rate ᠨ g c was measured as a function of temperature and film thickness.[S0031-9007 (97)03898-2] PACS numbers: 61.30.Gd, 47.20.Hw, 61.30.Jf, 64.70.MdElongated molecules are capable of forming various mesophases with different translation and rotation symmetries. Smectic-A (Sm-A) is unique among these phases in that it has a layered structure with molecules aligned parallel to the surface normal. Since there is no translational order, molecules within the layers behave like a twodimensional liquid. The purpose of this investigation is to explore the effects of shear on this simply structured fluid, with emphasis on defect generation and evolution of these defects in the presence and absence of the flow field.That Sm-A is a strongly anisotropic fluid raises interesting questions concerning stability of layer orientations, characterized by their surface normal, with respect to an imposing velocity field. This question was addressed experimentally by Safinya et al. for a sheared thermotropic liquid crystal (LC) using synchrotron x-ray scattering [1]. Specifically, they examined how molecular orientations, which were well known in the nematic phase under shear flow [2], transformed when the smectic phase appeared. It was found that while the average orientation of the molecules, called the directorn, was shear aligned along the velocity direction deep in the nematic phase, it switched to a new configuration withn parallel to the vorticity direction in the vicinity of the nematic to SmA transition. The sudden change took place when the shear rate ᠨ g became comparable to the incipient smectic-order fluctuation time t j of domain size j. More recently simultaneous rheological and structural measurements have been carried out by Panizza et al. [3]. These measurements seemed to suggest that smectic layers could also form multilamellar cylinders oriented along the flow at low shear rates, whereas the layer's normaln became parallel to the vorticity direction at high-shear rates similar to that found by Safinya et al. Though it is intuitively reasonable that this so-called a configuration, i.e., with the velocity and velocity gradient in the plane of smectic layers, has a relatively high stability compared to other configurations due to minimal distortions of the smectic order as well as the director fieldn, it gained theoretical support only recently [4,5].We have observed a striking effect that seems to indicate that even in the a configuration, shear can cause an instability which is not captured by linear theory [4,5]. The a configu...

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“…Oily streaks have been discussed in detail in [12] and [7], and the latter paper provides an explanation of the variety of inner structures observed in these defects. The two main possibilities considered are: (i) the dislocation lines retain a core structure consisting of a pair of disclination lines, with modulations runing along their length, and (ii) they nucleate an array of focal conic domains if the saddle-splay modulus favours large negative Gaussian curvature.…”

Section: Defect Structure Of the Lamellar Phase And Its Evolutimentioning

confidence: 99%

“…Although the shear modulus of an initially highly rigid (and therefore presumably very 4 In the dislocation model [12,7], however, Γ depends on the Burgers-vector content of the line,…”

Section: B Rheometry With Particlesmentioning

confidence: 99%

Structure and rheology of the defect-gel states of pure and particle-dispersed lyotropic lamellar phases

et al. 1999

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We present important new results from light-microscopy and rheometry on a moderately concentrated lyotropic smectic, with and without particulate additives. Shear-treatment aligns the phase rapidly, except for a striking network of oily-streak defects, which anneals out much more slowly. If spherical particles several microns in diameter are dispersed in the lamellar medium, part of the defect network persists under shear-treatment, its nodes anchored on the particles. The sample as prepared has substantial storage and loss moduli, both of which decrease steadily under shear-treatment. Adding particles enhances the moduli and retards their decay under shear. The data for the frequency-dependent storage modulus after various durations of sheartreatment can be scaled to collapse onto a single curve. The elasticity and dissipation in these samples thus arises mainly from the defect network, not directly from the smectic elasticity and hydrodynamics.

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Undulating paired disclinations (oily streaks) in lyotropic liquid crystals

Cited by 30 publications

References 30 publications

Polymer in a lyotropic lamellar system: an experimental study

Polymer in a lyotropic lamellar system: an experimental study

A Shear-Induced Instability in Freely Suspended Smectic-ALiquid-Crystal Films

Structure and rheology of the defect-gel states of pure and particle-dispersed lyotropic lamellar phases

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