Dislocations and metastable chevrons in the electroconvective inplane normal roll state of a bent core nematic liquid crystal

Krishnamurthy, K. S.; Tadapatri, Pramod; Viswanath, P.

doi:10.1039/c4sm00906a

Cited by 6 publications

(5 citation statements)

References 60 publications

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“…The compounds for which the existence of patterns have been reported, can be grouped into three families: (a) hard-core BCNs, HBCN1-HBCN17 (see Figure 1 and Table 1), in which the rigid core determines the opening angle of the arms; (b) soft-core BCNs, SBCN1-SBCN5 (see Figure 2 and Table 2), which are actually dimers of rod-like units connected by an odd-number-length flexible chain, capable to form a twist-bend nematic phase; and (c) dimers DBCN1-DBCN4 (see Figure 3 and Table 3) composed of rod-like and BCN units, connected by a flexible chain. HBCN3 Figure 1(a) Cl -OC10H21 M1118 [32] HBCN4 Figure 1(a) Cl -OC11H23 11Cl [33,34,35] HBCN5 Figure 1(a) Cl -OC12H25 12Cl [36] HBCN6 Figure 1(a) Br -OC12H25 12Br [31] HBCN7 Figure 1(a) CN -OC6H13 6OCN [37] HBCN8 Figure 1(a) CN -OC8H17 1c [38] HBCN9 Figure 1(a) CN -OC9H19 9OCN [13,39] HBCN10 Figure 1…”

Section: Methodsmentioning

confidence: 99%

“…It is q rather than ε that is the key parameter determining the dynamics. (After [39]). Under increasing constraint, the formation of metastable edge dislocations in the prewavy structure is followed by nucleation of loop disclinations that mediate in the development of the final turbulent state [40,13], as in the dynamic scattering mode of standard EC [130].…”

Section: Defects In Patterned Electroconvective States Of Bcnsmentioning

confidence: 99%

“…We may associate a topological charge Q with such a dislocation, defined by where q is the wave vector given by the gradient of the phase variable. The sign of the dislocation may be chosen as positive when, in circumnavigating the defect in an anticlockwise manner, one ends up with an added pair of rolls [39].…”

Section: Defectsmentioning

confidence: 99%

“…By an abrupt large jump in U, a multiple defect state, termed the knitting instability, is obtained; see Figure 12. A later study [39] on HBCN9 analyses the formation, dynamics, and annihilation of edge dislocations of opposite topological charge ±1 in the prewavy state, alternatively described by therein as the inplane normal roll (INR) state. To locate the defect position accurately, the INR pattern is represented by ψ(x, y) = A(x, y) exp (iqx) + c.c., where A(x, y) signifies the complex amplitude field.…”

Section: Defects In Patterned Electroconvective States Of Bcnsmentioning

confidence: 99%

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Electrically driven structures in bent-core nematics

2021

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This review, dedicated to the memory of B. K. Sadashiva, concerns electrically driven patterns and defects in bent core nematics (BCNs) made of hard-core, soft-core or bent-rod type of molecules. It covers (a) spatially periodic patterns associated with electroconvection and flexoelectric deformation, (b) dislocations in these patterned states and (c) various types of defects induced essentially in homogeneous, unpatterned states of BCNs. While in general, the wealth of pattern morphologies is just as rich in BCNs as in rod-like nematics, certain features of electroconvection have so far been observed only in some BCN materials. Some attributes relating to the generation and behaviour of defects in electric field may also be ascribed to material properties specific to BCNs. It is also shown that the electric field induced patterns as well as their modification by light irradiation may serve as optical gratings and thus establishes the background for designing new photonic devices.

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Section: Methodsmentioning

confidence: 99%

Section: Defects In Patterned Electroconvective States Of Bcnsmentioning

confidence: 99%

Section: Defectsmentioning

confidence: 99%

Section: Defects In Patterned Electroconvective States Of Bcnsmentioning

confidence: 99%

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Electrically driven structures in bent-core nematics

2021

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“…The bentcore nematics (BCNs) studied so far mostly exhibit various nonstandard EC patterns [7][8][9][10][11][12][13][14][15][16][17][18][19][20], whose characteristics may substantially differ from those in calamitic LCs. Moreover, there are some unprecedented scenarios, like prewavy patterns at ultrahigh ( / > 50 kHz) frequencies with negative slope of Uc(f), which have been found so far only in BCN compounds [7,[11][12][13], These banana-specific features still wait for explanation.…”

Section: Introductionmentioning

confidence: 99%

Unusual polarity-dependent patterns in a bent-core nematic liquid crystal under low-frequency ac field

Xiang

Zhou

et al. 2015

Phys. Rev. E

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Electric-field-induced patterns of diverse morphology have been observed over a wide frequency range in a recently synthesized bent-core nematic (BCN) liquid crystal. At low frequencies (up to ∼25 Hz), the BCN exhibited unusual polarity-dependent patterns. When the amplitude of the ac field was enhanced, these two time-asymmetrical patterns turned into time-symmetrical prewavylike stripes. At ac frequencies in the middle-frequency range (∼50-3000 Hz), zigzag patterns were detected whose obliqueness varied with the frequency. Finally, if the frequency was increased above 3 kHz, the zigzag pattern was replaced by another, prewavylike pattern, whose threshold voltage depended on the frequency; however, the wave vector did not. For a more complete characterization, material parameters such as elastic constants, dielectric permittivities, and the anisotropy of the diamagnetic susceptibility were also determined.

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Electroconvection of pure nematic liquid crystals without free charge carriers

Lee

Pöschel

2017

Soft Matter

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We consider electroconvection as a response of nematic liquid crystals to an external electric AC field, in the absence of free charge carriers. Previous experimental and theoretical results emphasized charge carriers as a necessary precondition of electroconvection because free-charges in the fluid can respond to an external electric field. Therefore, ionized molecules are considered as responsible for the driving of electroconvective flows. In experiments, finite conductivity is achieved by adding charge-carrying dye molecules or in non-dyed liquid crystals by impurities of the samples. The phenomenon of electroconvection is explained by the Carr-Helfrich theory, supported by numerical simulations. In the present paper, we show that electroconvection may occur also in pure nematic liquid crystals. By means of particle-based numerical simulations we found that bound charges emerge by alignment of polarized liquid crystal molecules in response to the external electric field. In our simulations we could reproduce the characteristic features of electroconvection, such as director-flow patterns, the phase-transition in the voltage-frequency diagram, and dislocation climb/glide motion, which are well known from experiments and hydrodynamic simulations under the assumption of free charge carriers.

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Dislocations and metastable chevrons in the electroconvective inplane normal roll state of a bent core nematic liquid crystal

Cited by 6 publications

References 60 publications

Electrically driven structures in bent-core nematics

Electrically driven structures in bent-core nematics

Unusual polarity-dependent patterns in a bent-core nematic liquid crystal under low-frequency ac field

Electroconvection of pure nematic liquid crystals without free charge carriers

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