High-resolution x-ray and light-scattering study of critical behavior associated with the nematic—smectic-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>transition in 4-cyano-4′-octylbiphenyl

Davidov, D.; Safinya, Cyrus R.; Kaplan, M. L.; Dana, S. S.; Schaetzing, Ralph; Birgeneau, R. J.; Litster, J. D.

doi:10.1103/physrevb.19.1657

Cited by 155 publications

(69 citation statements)

References 23 publications

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“…During sample loading, the microchannel substrate was heated to a temperature above the nematic-isotropic transition temperature (40.5°C) to facilitate the flow of 8CB into the channels. The substrate was then fast-cooled to room temperature at which the 8CB is in the Sm phase (T nematic-smectic A ϭ 33.5°C) (10).…”

Section: Methodsmentioning

confidence: 99%

Ordered patterns of liquid crystal toroidal defects by microchannel confinement

Choi

Pfohl

Zhang

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

105

113

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In this article we present experimental results demonstrating an approach to controlling the size and spatial patterning of defect domains in a smectic liquid crystal (LC) by geometric confinement in surface-modified microchannels. By confining the LC 4-octyl-4-cyanobiphenyl in m-sized rectangular channels with controlled surface polarity, we were able to generate defect domains that are not only nearly uniform in size but also arranged in quasi-2D ordered patterns. Atomic force microscopy measurements revealed that the defects have a toroidal topology, which we argue is dictated by the boundary conditions imposed by the walls of the microchannel. We show that the defects can be considered to be colloidal objects, which interact with each other to form ordered patterns. This method opens the possibility for exploiting the unique optical and rheological properties associated with LC defects to making new materials. For example, the control of the shape, size, and spatial arrangement of the defects at the mesoscale suggests applications in patterning, templating, and when extended to lyotropic LCs, a process leading to uniform-sized spherical particles for chemical encapsulation and delivery. S mectic (Sm) liquid crystals (LCs) are composed of elongated molecules that are aligned and arranged in layers. Local disruptions in the orientational, positional (layering), or morphological (bending) order in a Sm LC result in defects, which, when observed under crossed polarizers, exhibit complex textural patterns. The texture, which results from the highly anisotropic polarization of the molecules, has been extensively characterized and categorized based on the type of the LC structure (1-5). Understanding and controlling defects in LCs are important to many technological applications (e.g., displays and optical switches). Because the defects are considered to be anomalies in the LC structure, general efforts have been directed at minimizing or annealing the defects, including by using microstructures (grooves and gratings) (6-8). However, defect domains possess unique rheological and optical properties, which could potentially be exploited to make novel materials. The prerequisite for doing so would be the control of the type, size, and spatial distribution of the defects, which had not been well addressed in previous studies.In this article we present experimental results demonstrating the control of the size and spatial distribution of toroidal defects in the Sm LC 4Ј-octyl-4-cyanobiphenyl (8CB) by using surfacemodified microchannels. By changing the depth and width of the m-scale channels with a controlled surface polarity, we were able to generate focal conic defects that are not only nearly uniform in size but also arranged in 2D ordered patterns. This process is analogous to colloidal crystallization, except here the ''colloids'' are the ''soft'' m-sized defects within the LC. In addition, whereas the ordering in colloids is caused by interparticle interactions, here the defect ordering results from the competitio...

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Section: Methodsmentioning

confidence: 99%

Ordered patterns of liquid crystal toroidal defects by microchannel confinement

Choi

Pfohl

Zhang

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

105

113

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“…1, and convolve with the instrumental resolution. In this analysis ξ 1 and ξ 1⊥ are treated as independent parameters, while the amplitude of the fourth-order correction term, c, is treated as a function of ξ 1⊥ , with c(ξ 1⊥ ) set by the behavior in pure 8CB [35,36,37]. (Allowing c to vary as an additional free parameter leads to significant scatter in the fit parameters.)…”

Section: Smectic Correlations: Evidence For An Xy Bragg Glassmentioning

confidence: 99%

Smectic liquid crystals in an anisotropic random environment

Liang

Leheny

2007

Phys. Rev. E

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We report a high-resolution x-ray scattering study of the smectic liquid crystal octylcyanobiphenyl (8CB) confined to aligned colloidal aerosil gels. The aligned gels introduce orientational fields that promote long-range nematic order while imposing positional random fields that couple to the smectic density wave and disrupt the formation of an ordered smectic phase. At low densities of aerosil, the low-temperature scattering intensity is consistent with the presence of a topologically ordered XY Bragg glass phase that is predicted to form in response to such anisotropic quenched disorder. The observed features of the phase include an algebraic decay of the smectic correlations, which is truncated at large length scales due to the imperfect nematic order, and a power-law exponent that agrees closely with the universal value predicted for the XY Bragg glass. At higher aerosil densities, deviations from the XY Bragg glass form are apparent. At high temperature, the scattering intensity displays pre-transitional dynamic fluctuations associated with the destroyed nematic to smectic-A transition. The fluctuations obey quasi-critical behavior over an extended range of reduced temperature. The effective critical exponents for the correlation lengths and smectic susceptibility differ systematically from those of pure 8CB, indicating that coupling of the nematic order to the gel suppresses its role in the smectic critical behavior.

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“…The second group includes theories based on the de Gennes model [4] and on the dislocation-mediated melting mechanism [5] which predict anisotropic divergence with v\\ =s 2v±. On the experimental front, this transition has been studied by several experimental techniques including x-ray diffraction [6,7], quasielastic light scattering [8,9], and heat capacity [10,11]. Anisotropic critical fluctuations at this transition are a common empirical feature among all materials.…”

mentioning

confidence: 99%

Critical behavior at the nematic–to–smectic-A transition in a nonpolar liquid crystal with wide nematic range

et al. 1991

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High-resolution x-ray scattering experiments to study the critical smectic-^ fluctuations in the nematic phase have been performed on a mixture of two nonpolar, two-benzene-ring compounds. The ratio TNA/THI (-0.885) for this mixture is much smaller than for previously studied similar materials. The smectic correlation lengths parallel and perpendicular to the smectic-layer normal are found to diverge anisotropically with characteristic exponents v« =0.75 ±0.03, v± =0.65 ± 0.03, and the susceptibility with /=1.24 ±0.05. PACS numbers: 64.70.Md, 61.10.H, 6l.30.Eb, 64.60.Fr Since the pioneering work of McMillan [l], the nematic-to-smectic-/4 (NA) phase transition in thermotropic liquid crystals has been a subject of extensive theoretical and experimental studies. A convenient physical realization of the simplest freezing transition in nature, the NA transition is of fundamental scientific significance; however, the detailed nature of this apparently simple transition remains poorly understood. Theoretical attempts to understand the NA transition can be divided into two groups. The first group predicts isotropic divergence of the size of a correlated region of critical smectic-v4 fluctuations in the nematic phase. The values of the characteristic exponents v\\ and V_L, describing the divergence parallel and perpendicular to the smectic density wave, are predicted to be j by the mean-field theories [1,2], and y by de Gennes's analogy [3] to critical behavior of superfluid He. The second group includes theories based on the de Gennes model [4] and on the dislocation-mediated melting mechanism [5] which predict anisotropic divergence with v\\ =s 2v±. On the experimental front, this transition has been studied by several experimental techniques including x-ray diffraction [6,7], quasielastic light scattering [8,9], and heat capacity [10,11]. Anisotropic critical fluctuations at this transition are a common empirical feature among all materials. However, the values of critical exponents are material dependent and the ratio of the correlation-length exponents determined by x-ray scattering lies between 1.2 and 1.5 [8][9][10][11]. The value of the susceptibility exponent y lies between 1.10 and 1.53, and for several materials agrees with the predictions based on the He analogy within experimental uncertainties. Thus, some aspects of different models appear to be correct while none of them is right in its entirety. The situation is not hopeless. The hyperscaling relation, 2V_L + v\\ =2 -a, where a is the heat-capacity exponent, holds for most of the materials, suggesting some underlying universal behavior.This complex situation can, to some extent, be ascribed to a number of factors. Ideally, one would like to study systems in which only the smectic order parameter is important near the NA transition. Unfortunately, the coupling between the smectic and nematic order parameters is significant. In materials exhibiting a narrow nematic range, the nematic order parameter does not saturate before the NA transition occurs. As th...

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High-resolution x-ray and light-scattering study of critical behavior associated with the nematic—smectic-Atransition in 4-cyano-4′-octylbiphenyl

Cited by 155 publications

References 23 publications

Ordered patterns of liquid crystal toroidal defects by microchannel confinement

Ordered patterns of liquid crystal toroidal defects by microchannel confinement

Smectic liquid crystals in an anisotropic random environment

Critical behavior at the nematic–to–smectic-A transition in a nonpolar liquid crystal with wide nematic range

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