A new torsion pendulum technique to measure the twist elastic constant of liquid crystals

Faetti, Sandro; Gatti, M.; Palleschi, V.

doi:10.1051/jphyslet:019850046018088100

Cited by 26 publications

(21 citation statements)

References 8 publications

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“…We have demonstrated the reliability of this new technique by measuring K 22 in 5CB as a function of temperature. 9 In this Letter we investigate the physical behavior of the SiO-nematic interface very close to the clearing temperature T m of 5CB. The anchoring energy exhibits a roughly linear dependence on temperature, greatly decreasing as the phase transition is approached.…”

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confidence: 99%

Almost Critical Behavior of the Anchoring Energy at the Interface between a Nematic Liquid Crystal and a SiO Substrate

et al. 1985

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The anchoring energy at the interface between the nematic liquid crystal 4-n -pentyl-4'cyanobiphenyl (5CB) and a glass plate treated by oblique evaporation of SiO is studied by measurement of the torque exerted on the surface by the nematic subject to an orienting magnetic field. The anchoring energy is sharply reduced as the clearing temperature is approached. The experiment provides the first direct evidence of a strong reduction of the surface order parameter for this system in the anisotropic phase.

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confidence: 99%

Almost Critical Behavior of the Anchoring Energy at the Interface between a Nematic Liquid Crystal and a SiO Substrate

et al. 1985

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“…[23]. Our results imply that the torsion coefficient of the pendulum k (defined as the ratio between the torque τ and the angular rotation ∆θ) is corrected at θ = 0 by an amount ∆k = π…”

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confidence: 58%

Casimir torques between anisotropic boundaries in nematic liquid crystals

Golestanian

Ajdari²,

Fournier³

2001

Phys. Rev. E

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Fluctuation-induced interactions between anisotropic objects immersed in a nematic liquid crystal are shown to depend on the relative orientation of these objects. The resulting long-range "Casimir" torques are explicitely calculated for a simple geometry where elastic effects are absent. Our study generalizes previous discussions restricted to the case of isotropic walls, and leads to new proposals for experimental tests of Casimir forces and torques in nematics.Pacs numbers: 68.60. Bs, 61.30.Hn, 61.30.Dk In the last decade, much theoretical attention has been paid to "Casimir" forces in structured complex fluids [1][2][3]. The pioneering work of Casimir showed that two uncharged conducting plates attract each other in vacuum, due to the modification of the electromagnetic fluctuations imposed by the plates [4]. In complex fluids, similar interactions should exist between embedded objects, as the thermal fluctuations of the medium's elastic distortions are restrained by the boundary conditions imposed by the objects. These interactions are believed to act between bounding surfaces or immersed inclusions in critical fluids or superfluids [5,6], in liquid crystals [7][8][9][10], in bilayer membranes [11][12][13], and also between rodlike polyelectrolytes [14][15][16][17]. Nematic liquid crystals are anisotropic fluids with a quadrupolar longrange order. They are considered as good candidates for the direct observation of "Casimir" interactions in complex fluids. However, clear experimental evidences have to date been scarce [2]. This is probably due to the weakness of fluctuation-induced effects when compared to that of permanent elastic distortions, which are often present.Although nematic liquid crystals are well-known to display orientational effects [18], no "Casimir" interaction directly connected to these orientational properties of nematics have so far been discussed. Here, we demonstrate that thermal fluctuations in nematic liquid crystals can induce torques between bounding surfaces [19]. The existence of "Casimir" torques between objects embedded in complex fluids is usually caused by the anisotropy in the shape of the objects [12,15,17]; here we report on a more subtle effect occurring between infinite plates with translational symmetry. To emphasize the "Casimir" effect, we focus on a situation in which no average elastic distortion is present: two parallel plates with a surface energy favoring an orientation of the local average molecular alignment (director) perpendicular to the surfaces. The ground state is therefore the distorsionless state in which the director is everywhere perpendicular to the boundaries. A "Casimir" torque can arise due to the anisotropy in the rigidity of the surface energy: we assume that deviations from the preferred normal surface orientation is easier in one direction than in the orthogonal one. (This property can be experimentally obtained from a grating surface treated for homeotropic anchoring [20], or by depositing on top of a substrate which is conventionally trea...

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“…I o can be found by measuring I when the LC is under pure water and the bottom alignment layer also produces a planar alignment. For this configuration the birefringence is Δ n eff = n e - n o , which can be independently determined: The value of Δ n eff for 5CB can be found in the literature; 18–21 and it can be found for both 5CB and 5CB/CN6 from our Freedericksz transition measurements. The measured light intensity, obtained by analyzing images at different surfactant concentrations with the software ImageProPlus, was used to calculate an effective birefringence according to Eq.…”

Section: Measurementsmentioning

confidence: 82%

Improving Liquid-Crystal-Based Biosensing in Aqueous Phases

Iglesias

Abbott

Mann

et al. 2012

ACS Appl. Mater. Interfaces

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Liquid crystal (LC)-based biological sensors permit the study of aqueous biological samples without the need for the labeling of biological species with fluorescent dyes (which can be laborious and change the properties of the biological sample under study). To date, studies of LC-based biosensors have explored only a narrow range of the liquid crystal/alignment layer combinations essential to their operation. Here we report a study of the role of LC elastic constants and the surface anchoring energy in determining the sensitivity of LC-based biosensors. By investigating a mixture of rod-shape and bent-shape mesogens, and three different alignment layers, we were able to widen the useful detection range of a LC-based sensor by providing an almost linear mapping of effective birefringence with concentration between 0.05 and 1mM of an anionic surfactant (model target analyte). These studies pave the way for optimization of LC-based biosensors and reveal the importance of the choice of both the LC material and the alignment layer in determining sensor properties.

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A new torsion pendulum technique to measure the twist elastic constant of liquid crystals

Cited by 26 publications

References 8 publications

Almost Critical Behavior of the Anchoring Energy at the Interface between a Nematic Liquid Crystal and a SiO Substrate

Almost Critical Behavior of the Anchoring Energy at the Interface between a Nematic Liquid Crystal and a SiO Substrate

Casimir torques between anisotropic boundaries in nematic liquid crystals

Improving Liquid-Crystal-Based Biosensing in Aqueous Phases

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