The determination of the elastic constants of a series of <i>n</i>-alkylcyanobiphenyls by anisotropy of turbidity

Hakemi, H.; Jagodzinski, E. F.; DuPré, Donald B.

doi:10.1063/1.444840

Cited by 42 publications

(14 citation statements)

References 20 publications

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“…Three of the bulk nematic LC deformation modes, splay, twist and bend, are well known and have associated elastic moduli K 1 , K 2 and K 3 , respectively. These moduli have been intensely studied because they are easy to visualize, and because it is possible to independently excite the modes via clever usage of sample geometry [10][11][12], LC boundary conditions [13,14], and external fields [15,16]. As a result, these moduli have been measured for a variety of thermotropic and lyotropic LCs [12,[16][17][18][19][20].…”

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Chiral structures and defects of lyotropic chromonic liquid crystals induced by saddle-splay elasticity

et al. 2015

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An experimental and theoretical study of lyotropic chromonic liquid crystals (LCLCs) confined in cylinders with degenerate planar boundary conditions elucidates LCLC director configurations. When the Frank saddlesplay modulus is more than twice the twist modulus, the ground state adopts an inhomogeneous escapedtwisted configuration. Analysis of the configuration yields a large saddle-splay modulus, which violates Ericksen inequalities but not thermodynamic stability. Lastly, we observe point defects between opposite-handed domains, and we explain a preference for point defects over domain walls. The elastic properties of nematic liquid crystals (LCs) are crucial for liquid crystal display applications [1,2], and they continue to give rise to unanticipated fundamental phenomena [3][4][5][6][7][8][9]. Three of the bulk nematic LC deformation modes-splay, twist, and bend-are well known and have associated elastic moduli K 1 , K 2 , and K 3 , respectively. These moduli have been studied intensely because they are easy to visualize, and because it is possible to independently excite the modes via clever usage of sample geometry [10][11][12], LC boundary conditions [13,14], and external fields [15,16]. As a result, these moduli have been measured for a variety of thermotropic and lyotropic LCs [12,[16][17][18][19][20]. By contrast, a much less studied fourth independent mode [21-23] of elastic deformation in nematic LCs can exist; it is called saddle-splay. Saddle-splay is hard to visualize and to independently excite [23,24]. Moreover, the energy of this deformation class can be integrated to the boundary, so that the mode does not appear in the Euler-Lagrange equations, and with fixed boundary conditions the saddle-splay energy will have no effect on the LC director configuration. Even with free boundary conditions, the saddle-splay energy will not affect the bulk LC configuration unless the principal curvatures of the surface are different, i.e., saddle-splay effects are not expected for spherical or flat surfaces. Thus, although much progress in understanding saddle-splay has been made [25,26], especially with thermotropic nematic LCs, unambiguous determination of saddle-splay energy effects on liquid crystal configurations and measurement of the saddle-splay elastic modulus, K 24 , remain difficult [27].While the bulk elastic constants described above strongly influence LC director configurations, LC boundary conditions at material interfaces also influence bulk structure. Indeed, considerable effort has gone into development of surface preparation techniques to produce particular bulk director configurations [13,[28][29][30][31][32][33]. The saddle-splay term integrates to the boundary and effectively imposes boundary conditions at free surfaces favoring director alignment along the direction of highest surface curvature for positive K 24 [34] and outwardly * jjeong@unist.ac.kr pointing surface normals. For this effect to be present, the director cannot be held perpendicular to the surface, as was the case in ou...

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Chiral structures and defects of lyotropic chromonic liquid crystals induced by saddle-splay elasticity

et al. 2015

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“…3 yields a factor of 6.05 nm (5CB) and 7.03 nm (6CB), respectively. With the typical correlation length ξ * ∼ 10-15 nm, elastic constants k = 7.7 × 10 −12 N (5CB) and k = 8.0 × 10 −12 N (6CB) [29], and Q 0 ∼ 1/2, one yields an anchoring strength W n1 in the range 50-100 μJ/m 2 . These values are of the same order of magnitude as obtained for 8CB in random pore networks of controlled pore glass [8], i.e., the interaction of 5CB or 6CB molecules with SiO 2 surface corresponds to a regime of weak anchoring.…”

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Influence of nanoconfinement on the nematic behavior of liquid crystals

et al. 2012

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We explore the nematic ordering of the rodlike liquid crystals 5CB and 6CB, embedded into parallel-aligned nanochannels in mesoporous silicon and silica membranes as a function of mean channel radius (4.7 ≤ R ≤ .3 nm), and, thus, geometrical confinement strength, by optical birefringence measurements in the infrared region. The orientational order inside the nanochannels results in an excess birefringence, which is proportional to the nematic order parameter. It evolves continuously on cooling with a precursor behavior, typical of a paranematic state at high temperatures. These observations are compared with the bulk behavior and analyzed within a phenomenological model. Such an approach indicates that the strength of the nematic ordering fields σ is beyond a critical threshold σ(c) = 1/2 that separates discontinuous from continuous paranematic-to-nematic behavior. In agreement with the predictions of the phenomenological approach, a linear dependency of σ on the inverse channel radius is found and we can infer therefrom the critical channel radii, R(c) separating continuous from discontinuous paranematic-to-isotropic behavior, for 5CB (12.1 nm) and 6CB (14.0 nm). Our analysis suggests that the tangential anchoring at the channel walls is of similar strength in mesoporous silicon and mesoporous silica membranes. A comparison with the bulk phase behavior reveals that the nematic order in nanoconfinement is significantly affected by channel wall roughness, leading to a reduction of the effective nematic ordering.

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“…7(a) and 7(b) yields ̺-factor which varies from 5 to 7.5 nm. With the typical correlation length ξ * ∼10-15 nm, elastic constant k =(8.0 ÷ 9.0)·10 −12 N, [45] and Q 0 ∼1/2 one yields an anchoring strength W n1 in the range 50-100 µJ/m 2 which corresponds to a regime of weak anchoring.…”

Section: Fig 6: (Color Online) Nematic Ordering Field σ Vs Mole Framentioning

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Paranematic-to-nematic ordering of a binary mixture of rodlike liquid crystals confined in cylindrical nanochannels

et al. 2014

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We explore the optical birefringence of the nematic binary mixtures 6CB1−x7CBx (0 ≤ x ≤ 1) imbibed into parallel-aligned nanochannels of mesoporous alumina and silica membranes for channel radii of 3.4 ≤ R ≤ 21.0 nm. The results are compared with the bulk behavior and analyzed with a Landau-de-Gennes model. Depending on the channel radius the nematic ordering in the cylindrical nanochannels evolves either discontinuously (subcritical regime, nematic ordering field σ < 1/2) or continuously (overcritical regime, σ > 1/2), but in both cases with a characteristic paranematic precursor behavior. The strength of the ordering field, imposed by the channel walls, and the magnitude of quenched disorder varies linearly with the mole fraction x and scales inversely proportional with R for channel radii larger than 4 nm. The critical pore radius, Rc, separating a continuous from a discontinuous paranematic-to-nematic evolution, varies linearly with x and differs negligibly between the silica and alumina membranes. We find no hints of preferred adsorption of one species at the channels walls. By contrast, a linear variation of the nematic-to-paranematic transition point TPN and of the nematic ordering field σ vs. x suggest that the binary mixtures of cyanobiphenyls 6CB and 7CB keep their homogeneous bulk stoichiometry also in nanoconfinement, at least for channel diameters larger than ∼7 nm.

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The determination of the elastic constants of a series of n-alkylcyanobiphenyls by anisotropy of turbidity

Cited by 42 publications

References 20 publications

Chiral structures and defects of lyotropic chromonic liquid crystals induced by saddle-splay elasticity

Chiral structures and defects of lyotropic chromonic liquid crystals induced by saddle-splay elasticity

Influence of nanoconfinement on the nematic behavior of liquid crystals

Paranematic-to-nematic ordering of a binary mixture of rodlike liquid crystals confined in cylindrical nanochannels

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